[PATCH 0/2] drm/hyperv: harden VMBus message parser input validation

[PATCH 0/2] drm/hyperv: harden VMBus message parser input validation

[Berkant Koc]

The hyperv synthetic video driver parses VMBus messages from the host
without bounding two host-controlled values that feed into fixed-size
buffers. Both items are input validation, not security bugs: the
Hyper-V host sits inside the trusted compute base under the default
Hyper-V threat-model. The patches still trim the inputs the driver
accepts at face value, matching the trajectory drivers/hv/ has
followed for Confidential-VMBus work where the host is no longer
fully trusted.

Patch 1 bounds resolution_count against
supported_resolution[SYNTHVID_MAX_RESOLUTION_COUNT]; the existing
default_resolution_index check is bypassable when both values
exceed 64.

Patch 2 forwards bytes_recvd from vmbus_recvpacket() into the
sub-handler so that vid_hdr.type and feature_chg.is_dirt_needed
are only read once the host actually delivered enough bytes, and
so that the init_buf memcpy uses the received length.

Sending as a plain patch series, not a security disclosure.
Compile-tested against drm-fixes (6916d5703ddf), static-only.

Berkant Koc (2):
  drm/hyperv: validate resolution_count from host VMBus message
  drm/hyperv: validate VMBus packet size in receive callback

 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)


base-commit: 6916d5703ddf9a38f1f6c2cc793381a24ee914c6
-- 
2.47.3

[PATCH 1/2] drm/hyperv: validate resolution_count from host VMBus message

[Berkant Koc]

The synthetic video device receives a SYNTHVID_RESOLUTION_RESPONSE
containing a u8 resolution_count and a u8 default_resolution_index
from the host. The existing check rejects resolution_count == 0 and
rejects an index that is greater or equal to resolution_count, but
does not bound resolution_count itself against the fixed
supported_resolution[SYNTHVID_MAX_RESOLUTION_COUNT] array. A host
that returns resolution_count > 64 together with an in-range
default_resolution_index causes the subsequent loop to read past
the array.

Reject any resolution_count that exceeds SYNTHVID_MAX_RESOLUTION_COUNT,
folded into the existing zero-check for one log entry per failure.
This matches the input-validation pattern used by other VMBus parsers
under drivers/hv/ and trims one host-controlled value from the trusted
path.

Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Berkant Koc <me@berkoc.com>
---
 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
index 051ecc526832..003bb118d64c 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
@@ -391,8 +391,11 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
 		return -ETIMEDOUT;
 	}
 
-	if (msg->resolution_resp.resolution_count == 0) {
-		drm_err(dev, "No supported resolutions\n");
+	if (msg->resolution_resp.resolution_count == 0 ||
+	    msg->resolution_resp.resolution_count >
+	    SYNTHVID_MAX_RESOLUTION_COUNT) {
+		drm_err(dev, "Invalid resolution count: %d\n",
+			msg->resolution_resp.resolution_count);
 		return -ENODEV;
 	}
 
-- 
2.47.3

[PATCH 2/2] drm/hyperv: validate VMBus packet size in receive callback

[Berkant Koc]

hyperv_receive() reads bytes_recvd from vmbus_recvpacket() but does not
forward that value to hyperv_receive_sub(). The sub-handler reads
msg->vid_hdr.type and msg->feature_chg.is_dirt_needed without knowing
how many bytes the host actually wrote, so a short packet leaves the
parser reading bytes that the host did not write in this packet. The
unconditional memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE) on the
wait-completion path also copies the full 16 KiB recv_buf regardless
of bytes_recvd, including any residue from the prior message.

Pass bytes_recvd into hyperv_receive_sub() and reject any packet shorter
than the pipe + synthvid header. The dirt-feature branch additionally
requires the feature_change payload size before reading is_dirt_needed.
The init_buf copy now uses bytes_recvd as the length argument, which
keeps it bounded by VMBUS_MAX_PACKET_SIZE through the new upper check.

Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Berkant Koc <me@berkoc.com>
---
 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
index 003bb118d64c..0d32d9944c43 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
@@ -420,26 +420,35 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
 	return 0;
 }
 
-static void hyperv_receive_sub(struct hv_device *hdev)
+static void hyperv_receive_sub(struct hv_device *hdev, u32 bytes_recvd)
 {
 	struct hyperv_drm_device *hv = hv_get_drvdata(hdev);
 	struct synthvid_msg *msg;
+	size_t hdr_size;
 
 	if (!hv)
 		return;
 
+	hdr_size = sizeof(struct pipe_msg_hdr) +
+		   sizeof(struct synthvid_msg_hdr);
+	if (bytes_recvd < hdr_size || bytes_recvd > VMBUS_MAX_PACKET_SIZE)
+		return;
+
 	msg = (struct synthvid_msg *)hv->recv_buf;
 
 	/* Complete the wait event */
 	if (msg->vid_hdr.type == SYNTHVID_VERSION_RESPONSE ||
 	    msg->vid_hdr.type == SYNTHVID_RESOLUTION_RESPONSE ||
 	    msg->vid_hdr.type == SYNTHVID_VRAM_LOCATION_ACK) {
-		memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE);
+		memcpy(hv->init_buf, msg, bytes_recvd);
 		complete(&hv->wait);
 		return;
 	}
 
 	if (msg->vid_hdr.type == SYNTHVID_FEATURE_CHANGE) {
+		if (bytes_recvd < hdr_size +
+		    sizeof(struct synthvid_feature_change))
+			return;
 		hv->dirt_needed = msg->feature_chg.is_dirt_needed;
 		if (hv->dirt_needed)
 			hyperv_hide_hw_ptr(hv->hdev);
@@ -466,7 +475,7 @@ static void hyperv_receive(void *ctx)
 				       &bytes_recvd, &req_id);
 		if (bytes_recvd > 0 &&
 		    recv_buf->pipe_hdr.type == PIPE_MSG_DATA)
-			hyperv_receive_sub(hdev);
+			hyperv_receive_sub(hdev, bytes_recvd);
 	} while (bytes_recvd > 0 && ret == 0);
 }
 
-- 
2.47.3

Re: [PATCH 1/2] drm/hyperv: validate resolution_count from host VMBus message

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] Uninitialized State on Error Path Leaves Display Unusable
- [High] Missing Completion Re-Initialization Breaks Driver Synchronization
--

commit 94c5684ae582179bbf9c456d94f6a0bd0f92785f
Author: Berkant Koc <me@berkoc.com>
Subject: drm/hyperv: validate resolution_count from host VMBus message

This patch adds validation to ensure the host-provided resolution_count does
not exceed SYNTHVID_MAX_RESOLUTION_COUNT. It folds this check into the
existing zero-count check to prevent reading past the end of the supported
resolution array.

> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> index 051ecc5268321..003bb118d64c9 100644
> --- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> @@ -391,8 +391,11 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  		return -ETIMEDOUT;
>  	}

This isn't a bug in your patch, but looking at the wait_for_completion_timeout
call just before this hunk, does the driver reuse the hv->wait completion
structure for multiple sequential VMBus requests without calling
reinit_completion()?

If a previous request times out but the host sends a delayed response, the
receive callback might execute complete(&hv->wait). Could a subsequent
request's wait_for_completion_timeout incorrectly consume this stray
completion, return immediately, and parse stale data from hv->init_buf?

>  
> -	if (msg->resolution_resp.resolution_count == 0) {
> -		drm_err(dev, "No supported resolutions\n");
> +	if (msg->resolution_resp.resolution_count == 0 ||
> +	    msg->resolution_resp.resolution_count >
> +	    SYNTHVID_MAX_RESOLUTION_COUNT) {
> +		drm_err(dev, "Invalid resolution count: %d\n",
> +			msg->resolution_resp.resolution_count);
>  		return -ENODEV;

This is also a pre-existing issue, but I noticed that returning an error
here can leave the display unusable.

When hyperv_get_supported_resolution() fails due to this new bounds check,
the caller hyperv_connect_vsp() logs a warning to use defaults, but it
skips setting up the resolution limits. As a result, hv->screen_width_max
and hv->screen_height_max remain 0.

Later, hyperv_mode_config_init() sets dev->mode_config.max_width and
max_height to 0. Since the DRM core's drm_internal_framebuffer_create()
strictly bounds framebuffer dimensions to dev->mode_config.max_width,
would this cause all userspace attempts to create a framebuffer to fail
with -EINVAL?

>  	}
>

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260517-drm-hyperv-cover@berkoc.com?part=1

Re: [PATCH 2/2] drm/hyperv: validate VMBus packet size in receive callback

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Incomplete payload size validation causes wait-completion handlers to read stale request data, allowing response spoofing and invalid state.
--

commit ffe1d9057aa46275f644c2e152d0e7a9e8837c3b
Author: Berkant Koc <me@berkoc.com>

drm/hyperv: validate VMBus packet size in receive callback

This commit introduces bytes_recvd into hyperv_receive_sub() to validate
packet sizes. It restricts the memcpy length to bytes_recvd and checks
payload size for feature change messages.

Cc: stable@vger.kernel.org # 5.14+

> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> --- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c

[ ... ]

> @@ -420,26 +420,35 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  static void hyperv_receive_sub(struct hv_device *hdev, u32 bytes_recvd)
>  {
>  	struct hyperv_drm_device *hv = hv_get_drvdata(hdev);
>  	struct synthvid_msg *msg;
> +	size_t hdr_size;
>  
>  	if (!hv)
>  		return;
>  
> +	hdr_size = sizeof(struct pipe_msg_hdr) +
> +		   sizeof(struct synthvid_msg_hdr);
> +	if (bytes_recvd < hdr_size || bytes_recvd > VMBUS_MAX_PACKET_SIZE)
> +		return;
> +
>  	msg = (struct synthvid_msg *)hv->recv_buf;
>  
>  	/* Complete the wait event */
>  	if (msg->vid_hdr.type == SYNTHVID_VERSION_RESPONSE ||
>  	    msg->vid_hdr.type == SYNTHVID_RESOLUTION_RESPONSE ||
>  	    msg->vid_hdr.type == SYNTHVID_VRAM_LOCATION_ACK) {
> -		memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE);
> +		memcpy(hv->init_buf, msg, bytes_recvd);

Could this change introduce a regression where a short packet leaves stale
request data in hv->init_buf?

The patch bounds the memcpy to bytes_recvd, but it doesn't appear to validate
that bytes_recvd is large enough to contain the expected response payloads
before calling complete(&hv->wait).

Because hv->init_buf is used for both sending requests and receiving responses,
a short response might only partially overwrite hv->init_buf. The payload
portion would retain the outgoing request's data or zeroes from the pre-send
initialization.

For example, when sending a VRAM location update, the driver sets
msg->vram.user_ctx = vram_pp. If a short response arrives, the payload is not
overwritten. The waiter checks if (msg->vram_ack.user_ctx != vram_pp). Since
vram and vram_ack share a union in struct synthvid_msg, this check might pass
against the driver's own request data, allowing a truncated packet to spoof a
successful ACK.

Similarly, in hyperv_get_supported_resolution(), if a truncated response
includes the resolution_count but omits the supported_resolution array, the
uncopied array memory remains zero. The driver could parse zeroed dimensions,
setting hv->preferred_width and height to 0, which could propagate to the DRM
core.

Would it be safer to have the receive handler validate the specific payload
sizes for these wait-completion response types, similar to how it validates
SYNTHVID_FEATURE_CHANGE below?

>  		complete(&hv->wait);
>  		return;
>  	}
>  
>  	if (msg->vid_hdr.type == SYNTHVID_FEATURE_CHANGE) {
> +		if (bytes_recvd < hdr_size +
> +		    sizeof(struct synthvid_feature_change))
> +			return;
>  		hv->dirt_needed = msg->feature_chg.is_dirt_needed;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260517-drm-hyperv-cover@berkoc.com?part=2

[PATCH v2 0/2] drm/hyperv: harden VMBus message parser input validation

[Berkant Koc]

v2 folds two further issues into patch 1 that the sashiko-bot review
pointed out on v1:

  1. The resolution_count bounds check in v1 returned -ENODEV, but
     hyperv_connect_vsp() only logged a warning and continued without
     setting hv->screen_width_max / height_max / preferred_*. That
     left dev->mode_config.max_width and max_height at 0, which made
     drm_internal_framebuffer_create() reject every userspace
     framebuffer with -EINVAL. v2 falls back to the WIN8 defaults on
     that error path, matching the pre-WIN10 branch.

  2. The three sequential VSP requests in hyperv_connect_vsp()
     (negotiate version, update VRAM location, get supported
     resolution) all wait on the same hv->wait completion without
     calling reinit_completion() between requests. A delayed
     complete() after a wait_for_completion_timeout() can leak into
     the next request and let it parse stale data out of
     hv->init_buf. v2 calls reinit_completion() before each send.

Patch 2 is unchanged from v1.

v1: https://lore.kernel.org/r/20260517-drm-hyperv-cover@berkoc.com

Berkant Koc (2):
  drm/hyperv: validate resolution_count and harden VSP request paths
  drm/hyperv: validate VMBus packet size in receive callback

 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 32 ++++++++++++++++++-----
 1 file changed, 26 insertions(+), 6 deletions(-)


base-commit: 6916d5703ddf9a38f1f6c2cc793381a24ee914c6
-- 
2.47.3

[PATCH v2 1/2] drm/hyperv: validate resolution_count and harden VSP request paths

[Berkant Koc]

The synthetic video device parses a SYNTHVID_RESOLUTION_RESPONSE that
contains a u8 resolution_count and a u8 default_resolution_index. The
existing check rejects resolution_count == 0 and an index greater or
equal to resolution_count, but does not bound resolution_count itself
against the fixed supported_resolution[SYNTHVID_MAX_RESOLUTION_COUNT]
array. A host that returns resolution_count > 64 together with an
in-range default_resolution_index causes the subsequent loop to read
past the array. Reject any resolution_count that exceeds the array
bound, folded into the existing zero-check so a single log entry
remains per failure.

When that bounds check (or any later failure in
hyperv_get_supported_resolution()) returns an error, the caller in
hyperv_connect_vsp() previously logged a warning and continued without
populating hv->screen_width_max / hv->screen_height_max / preferred_*.
hyperv_mode_config_init() then set dev->mode_config.max_width and
max_height to 0, which makes drm_internal_framebuffer_create() reject
every userspace framebuffer with -EINVAL. Populate the fields with the
WIN8 defaults that the pre-WIN10 branch already uses so a failed
resolution probe degrades to a usable display instead of disabling it.

The driver also issues three sequential VSP requests (negotiate
version, update VRAM location, get supported resolution) that share a
single hv->wait completion. None of the call sites call
reinit_completion() between requests. If wait_for_completion_timeout()
returns 0 but a delayed response later triggers complete(&hv->wait) in
the receive callback, the next request's wait can consume that stale
completion, return immediately, and parse stale data out of
hv->init_buf. Call reinit_completion() before each send so every
request waits for its own response.

Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Berkant Koc <me@berkoc.com>
---
 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
index 051ecc526832..3b5065fe06e4 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
@@ -223,6 +223,7 @@ static int hyperv_negotiate_version(struct hv_device *hdev, u32 ver)
 	msg->vid_hdr.size = sizeof(struct synthvid_msg_hdr) +
 		sizeof(struct synthvid_version_req);
 	msg->ver_req.version = ver;
+	reinit_completion(&hv->wait);
 	hyperv_sendpacket(hdev, msg);
 
 	t = wait_for_completion_timeout(&hv->wait, VMBUS_VSP_TIMEOUT);
@@ -257,6 +258,7 @@ int hyperv_update_vram_location(struct hv_device *hdev, phys_addr_t vram_pp)
 	msg->vram.user_ctx = vram_pp;
 	msg->vram.vram_gpa = vram_pp;
 	msg->vram.is_vram_gpa_specified = 1;
+	reinit_completion(&hv->wait);
 	hyperv_sendpacket(hdev, msg);
 
 	t = wait_for_completion_timeout(&hv->wait, VMBUS_VSP_TIMEOUT);
@@ -383,6 +385,7 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
 		sizeof(struct synthvid_supported_resolution_req);
 	msg->resolution_req.maximum_resolution_count =
 		SYNTHVID_MAX_RESOLUTION_COUNT;
+	reinit_completion(&hv->wait);
 	hyperv_sendpacket(hdev, msg);
 
 	t = wait_for_completion_timeout(&hv->wait, VMBUS_VSP_TIMEOUT);
@@ -391,8 +394,11 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
 		return -ETIMEDOUT;
 	}
 
-	if (msg->resolution_resp.resolution_count == 0) {
-		drm_err(dev, "No supported resolutions\n");
+	if (msg->resolution_resp.resolution_count == 0 ||
+	    msg->resolution_resp.resolution_count >
+	    SYNTHVID_MAX_RESOLUTION_COUNT) {
+		drm_err(dev, "Invalid resolution count: %d\n",
+			msg->resolution_resp.resolution_count);
 		return -ENODEV;
 	}
 
@@ -506,8 +512,13 @@ int hyperv_connect_vsp(struct hv_device *hdev)
 
 	if (hyperv_version_ge(hv->synthvid_version, SYNTHVID_VERSION_WIN10)) {
 		ret = hyperv_get_supported_resolution(hdev);
-		if (ret)
+		if (ret) {
 			drm_err(dev, "Failed to get supported resolution from host, use default\n");
+			hv->screen_width_max = SYNTHVID_WIDTH_WIN8;
+			hv->screen_height_max = SYNTHVID_HEIGHT_WIN8;
+			hv->preferred_width = SYNTHVID_WIDTH_WIN8;
+			hv->preferred_height = SYNTHVID_HEIGHT_WIN8;
+		}
 	} else {
 		hv->screen_width_max = SYNTHVID_WIDTH_WIN8;
 		hv->screen_height_max = SYNTHVID_HEIGHT_WIN8;
-- 
2.47.3

[PATCH v2 2/2] drm/hyperv: validate VMBus packet size in receive callback

[Berkant Koc]

hyperv_receive() reads bytes_recvd from vmbus_recvpacket() but does not
forward that value to hyperv_receive_sub(). The sub-handler reads
msg->vid_hdr.type and msg->feature_chg.is_dirt_needed without knowing
how many bytes the host actually wrote, so a short packet leaves the
parser reading bytes that the host did not write in this packet. The
unconditional memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE) on the
wait-completion path also copies the full 16 KiB recv_buf regardless
of bytes_recvd, including any residue from the prior message.

Pass bytes_recvd into hyperv_receive_sub() and reject any packet shorter
than the pipe + synthvid header. The dirt-feature branch additionally
requires the feature_change payload size before reading is_dirt_needed.
The init_buf copy now uses bytes_recvd as the length argument, which
keeps it bounded by VMBUS_MAX_PACKET_SIZE through the new upper check.

Unchanged from v1.

Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Berkant Koc <me@berkoc.com>
---
 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
index 3b5065fe06e4..cdab4895dd40 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
@@ -423,26 +423,35 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
 	return 0;
 }
 
-static void hyperv_receive_sub(struct hv_device *hdev)
+static void hyperv_receive_sub(struct hv_device *hdev, u32 bytes_recvd)
 {
 	struct hyperv_drm_device *hv = hv_get_drvdata(hdev);
 	struct synthvid_msg *msg;
+	size_t hdr_size;
 
 	if (!hv)
 		return;
 
+	hdr_size = sizeof(struct pipe_msg_hdr) +
+		   sizeof(struct synthvid_msg_hdr);
+	if (bytes_recvd < hdr_size || bytes_recvd > VMBUS_MAX_PACKET_SIZE)
+		return;
+
 	msg = (struct synthvid_msg *)hv->recv_buf;
 
 	/* Complete the wait event */
 	if (msg->vid_hdr.type == SYNTHVID_VERSION_RESPONSE ||
 	    msg->vid_hdr.type == SYNTHVID_RESOLUTION_RESPONSE ||
 	    msg->vid_hdr.type == SYNTHVID_VRAM_LOCATION_ACK) {
-		memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE);
+		memcpy(hv->init_buf, msg, bytes_recvd);
 		complete(&hv->wait);
 		return;
 	}
 
 	if (msg->vid_hdr.type == SYNTHVID_FEATURE_CHANGE) {
+		if (bytes_recvd < hdr_size +
+		    sizeof(struct synthvid_feature_change))
+			return;
 		hv->dirt_needed = msg->feature_chg.is_dirt_needed;
 		if (hv->dirt_needed)
 			hyperv_hide_hw_ptr(hv->hdev);
@@ -469,7 +478,7 @@ static void hyperv_receive(void *ctx)
 				       &bytes_recvd, &req_id);
 		if (bytes_recvd > 0 &&
 		    recv_buf->pipe_hdr.type == PIPE_MSG_DATA)
-			hyperv_receive_sub(hdev);
+			hyperv_receive_sub(hdev, bytes_recvd);
 	} while (bytes_recvd > 0 && ret == 0);
 }
 
-- 
2.47.3

Re: [PATCH v2 1/2] drm/hyperv: validate resolution_count and harden VSP request paths

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Critical] The fix for delayed VSP responses is incomplete, leaving a critical data race on the shared `hv->init_buf` that allows type confusion and out-of-bounds memory reads.
--

commit 3d049f39a946f7898e25e92a422ae95b066e9989
Author: Berkant Koc <me@berkoc.com>

drm/hyperv: validate resolution_count and harden VSP request paths

This commit adds bounds checking for resolution_count when parsing
SYNTHVID_RESOLUTION_RESPONSE. It degrades gracefully to default
resolutions on failure, and adds reinit_completion() before VSP requests
to prevent consuming stale completions.

> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> --- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
[ ... ]
> @@ -383,6 +385,7 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  		sizeof(struct synthvid_supported_resolution_req);
>  	msg->resolution_req.maximum_resolution_count =
>  		SYNTHVID_MAX_RESOLUTION_COUNT;
> +	reinit_completion(&hv->wait);
>  	hyperv_sendpacket(hdev, msg);

Does this reinit_completion() prevent a data race on the shared hv->init_buf?

If a timed-out request's delayed response arrives while a new request is
being constructed, could hyperv_receive_sub() concurrently overwrite
hv->init_buf without holding any locks?

Since the buffer is used for both sending and receiving, if a delayed
response arrives exactly between the memset(msg, 0) and
hyperv_sendpacket(hdev, msg), could msg->vid_hdr.size be overwritten by
the host's response?

If a host provides a crafted size, could this cause an integer overflow in
totalbytes_towrite within hv_ringbuffer_write(), bypassing ring buffer
free-space checks and causing an out-of-bounds read of guest memory into
the VMBus ring buffer?

Additionally, if the delayed response triggers complete(&hv->wait), is
there a chance the driver parses a mismatched response because it does not
validate that msg->vid_hdr.type matches the expected response?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260517-drm-hyperv-cover-v2@berkoc.com?part=1

Re: [PATCH v2 2/2] drm/hyperv: validate VMBus packet size in receive callback

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Incomplete validation of synchronous response payload sizes causes state leakage and deterministic false success.
--

commit 31b0cdbb563323a78bd6df8c18d507462a03a084
Author: Berkant Koc <me@berkoc.com>

drm/hyperv: validate VMBus packet size in receive callback

This commit updates the hyperv_receive callback to validate the received
packet size before processing it. It ensures that the packet contains at
least the message headers, passes bytes_recvd to the sub-handler, and
uses it to bound the memcpy for synchronous responses.

> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> index 3b5065fe06e46..cdab4895dd404 100644
> --- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
[ ... ]
> @@ -423,26 +423,35 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  	return 0;
>  }
>  
> -static void hyperv_receive_sub(struct hv_device *hdev)
> +static void hyperv_receive_sub(struct hv_device *hdev, u32 bytes_recvd)
>  {
>  	struct hyperv_drm_device *hv = hv_get_drvdata(hdev);
>  	struct synthvid_msg *msg;
> +	size_t hdr_size;
>  
>  	if (!hv)
>  		return;
>  
> +	hdr_size = sizeof(struct pipe_msg_hdr) +
> +		   sizeof(struct synthvid_msg_hdr);
> +	if (bytes_recvd < hdr_size || bytes_recvd > VMBUS_MAX_PACKET_SIZE)
> +		return;
> +
>  	msg = (struct synthvid_msg *)hv->recv_buf;
>  
>  	/* Complete the wait event */
>  	if (msg->vid_hdr.type == SYNTHVID_VERSION_RESPONSE ||
>  	    msg->vid_hdr.type == SYNTHVID_RESOLUTION_RESPONSE ||
>  	    msg->vid_hdr.type == SYNTHVID_VRAM_LOCATION_ACK) {
> -		memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE);
> +		memcpy(hv->init_buf, msg, bytes_recvd);

Is it possible for a truncated response to incorrectly report success here?

Since hv->init_buf is reused for both sending requests and receiving responses,
and their structures alias each other in the synthvid_msg union, a short packet
would leave the payload section of hv->init_buf unmodified.

For example, if a truncated SYNTHVID_VRAM_LOCATION_ACK packet contains only
headers, memcpy() will only copy the headers. The request's vram.user_ctx and
the response's vram_ack.user_ctx reside at the exact same memory offset in the
union. Because the payload in hv->init_buf is unmodified, msg->vram_ack.user_ctx
will perfectly retain the request's vram_pp value.

The waiting thread checks this with:

	if (msg->vram_ack.user_ctx != vram_pp)

Because the memory was unchanged, this check will deterministically evaluate
to false, causing the driver to proceed as if the VRAM location was
successfully acknowledged despite the malformed packet.

Should this code validate that bytes_recvd is large enough to contain the
specific response type's payload before copying and calling complete()?

>  		complete(&hv->wait);
>  		return;
>  	}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260517-drm-hyperv-cover-v2@berkoc.com?part=2

RE: [PATCH v2 1/2] drm/hyperv: validate resolution_count and harden VSP request paths

[Michael Kelley]

From: Berkant Koc <me@berkoc.com> Sent: Sunday, May 17, 2026 7:25 AM
> 
> The synthetic video device parses a SYNTHVID_RESOLUTION_RESPONSE that
> contains a u8 resolution_count and a u8 default_resolution_index. The
> existing check rejects resolution_count == 0 and an index greater or
> equal to resolution_count, but does not bound resolution_count itself
> against the fixed supported_resolution[SYNTHVID_MAX_RESOLUTION_COUNT]
> array. A host that returns resolution_count > 64 together with an
> in-range default_resolution_index causes the subsequent loop to read
> past the array. Reject any resolution_count that exceeds the array
> bound, folded into the existing zero-check so a single log entry
> remains per failure.

I think this is a good fix, but let me provide some background. A core
assumption has been that the Hyper-V host is not malicious, and data from
the host can be trusted to be well-formed and valid. Linux code interacting
with the host does *not* take a paranoid approach and does *not* validate
every value received from the host. Existing validation is ad hoc and
probably not comprehensive.

A few years ago with the introduction of Confidential Computing and
"CoCo VMs" in in Linux, the assumption changed. (Hyper-V calls these
"Confidential VMs", or sometimes "Isolated VMs" -- unfortunately, the
terminology is a bit scattered.) The host is treated as potentially malicious
and Linux code was hardened against bad values passed from the host. But
not all drivers for Hyper-V synthetic devices were hardened because CoCo VMs
on Hyper-V don't allow all the possible synthetic devices. The Hyper-V synthetic
frame buffer is one such disallowed device, so the Hyper-V DRM driver was not
hardened. The allowed devices can be seen in the vmbus_devs[] array
where .allowed_in_isolation is "true".

All that said, I'm in favor of adding hardening, even if it is done piecemeal.
There won't likely be a comprehensive effort to harden the Hyper-V DRM
driver unless the frame buffer device becomes available in a CoCo VM.

> 
> When that bounds check (or any later failure in
> hyperv_get_supported_resolution()) returns an error, the caller in
> hyperv_connect_vsp() previously logged a warning and continued without
> populating hv->screen_width_max / hv->screen_height_max / preferred_*.
> hyperv_mode_config_init() then set dev->mode_config.max_width and
> max_height to 0, which makes drm_internal_framebuffer_create() reject
> every userspace framebuffer with -EINVAL. Populate the fields with the
> WIN8 defaults that the pre-WIN10 branch already uses so a failed
> resolution probe degrades to a usable display instead of disabling it.

Yes, this also makes sense.

> 
> The driver also issues three sequential VSP requests (negotiate
> version, update VRAM location, get supported resolution) that share a
> single hv->wait completion. None of the call sites call
> reinit_completion() between requests. If wait_for_completion_timeout()
> returns 0 but a delayed response later triggers complete(&hv->wait) in
> the receive callback, the next request's wait can consume that stale
> completion, return immediately, and parse stale data out of
> hv->init_buf. Call reinit_completion() before each send so every
> request waits for its own response.

This change should probably be a separate patch, as it's not really
related to the bounds checking issue. You could even argue that the
bounds check and using the default size values in case of an error
should be separate patches, but it's a judgment call and I could go
either way. Combining the first two works for me, though someone
else might disagree.

All that notwithstanding, I don't think your fix is needed in its current
form. If wait_for_completion_timeout() times out in "negotiate version"
or "update VRAM location", the code returns -ETIMEDOUT. The caller
then bails out and does not send the next message in the sequence.
Eventually hyperv_vmbus_probe() fails and the struct hyperv_drm_device
memory is freed. A similar thing could happen with hyperv_vmbus_resume().
The memory isn't freed, but if the resume fails and is tried again later, it
should be with a fresh copy of the saved memory image. In that case, the
completion should be clean for the retry.

The problem case is "get supported resolution", which would leave the
completion in a pending state if it times out. This could later affect
hyperv_vmbus_resume().  I could see reinit'ing the completion in
hyperv_vmbus_resume() to handle this case.

But double-check my thinking on all this. Maybe I'm missing something.

One other comment:  In my view, your commit message is a bit too
detailed. A commit message should focus on "why" the change, and
less on the details of the implementation. Explaining the failure case
is fine, but don't recapitulate code that is straightforward and obvious.

> 
> Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
> Cc: stable@vger.kernel.org # 5.14+
> Signed-off-by: Berkant Koc <me@berkoc.com>
> ---
>  drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> index 051ecc526832..3b5065fe06e4 100644
> --- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> @@ -223,6 +223,7 @@ static int hyperv_negotiate_version(struct hv_device *hdev, u32 ver)
>  	msg->vid_hdr.size = sizeof(struct synthvid_msg_hdr) +
>  		sizeof(struct synthvid_version_req);
>  	msg->ver_req.version = ver;
> +	reinit_completion(&hv->wait);
>  	hyperv_sendpacket(hdev, msg);
> 
>  	t = wait_for_completion_timeout(&hv->wait, VMBUS_VSP_TIMEOUT);
> @@ -257,6 +258,7 @@ int hyperv_update_vram_location(struct hv_device *hdev, phys_addr_t vram_pp)
>  	msg->vram.user_ctx = vram_pp;
>  	msg->vram.vram_gpa = vram_pp;
>  	msg->vram.is_vram_gpa_specified = 1;
> +	reinit_completion(&hv->wait);
>  	hyperv_sendpacket(hdev, msg);
> 
>  	t = wait_for_completion_timeout(&hv->wait, VMBUS_VSP_TIMEOUT);
> @@ -383,6 +385,7 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  		sizeof(struct synthvid_supported_resolution_req);
>  	msg->resolution_req.maximum_resolution_count =
>  		SYNTHVID_MAX_RESOLUTION_COUNT;
> +	reinit_completion(&hv->wait);
>  	hyperv_sendpacket(hdev, msg);
> 
>  	t = wait_for_completion_timeout(&hv->wait, VMBUS_VSP_TIMEOUT);
> @@ -391,8 +394,11 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  		return -ETIMEDOUT;
>  	}
> 
> -	if (msg->resolution_resp.resolution_count == 0) {
> -		drm_err(dev, "No supported resolutions\n");
> +	if (msg->resolution_resp.resolution_count == 0 ||
> +	    msg->resolution_resp.resolution_count >
> +	    SYNTHVID_MAX_RESOLUTION_COUNT) {
> +		drm_err(dev, "Invalid resolution count: %d\n",
> +			msg->resolution_resp.resolution_count);
>  		return -ENODEV;
>  	}
> 
> @@ -506,8 +512,13 @@ int hyperv_connect_vsp(struct hv_device *hdev)
> 
>  	if (hyperv_version_ge(hv->synthvid_version, SYNTHVID_VERSION_WIN10)) {
>  		ret = hyperv_get_supported_resolution(hdev);
> -		if (ret)
> +		if (ret) {
>  			drm_err(dev, "Failed to get supported resolution from host, use default\n");
> +			hv->screen_width_max = SYNTHVID_WIDTH_WIN8;
> +			hv->screen_height_max = SYNTHVID_HEIGHT_WIN8;
> +			hv->preferred_width = SYNTHVID_WIDTH_WIN8;
> +			hv->preferred_height = SYNTHVID_HEIGHT_WIN8;
> +		}
>  	} else {
>  		hv->screen_width_max = SYNTHVID_WIDTH_WIN8;
>  		hv->screen_height_max = SYNTHVID_HEIGHT_WIN8;

Is there a separate problem here in that preferred_width and preferred_height
are not set in the pre-WIN10 case? I'm not familiar enough with DRM driver code
to know how these preferred values are used. It looks inconsistent to have the
two fallback cases be different.

Also, having to duplicate the fallback code is distasteful. Instead of having an
"else" clause, maybe have a follow-up test for screen_width_max (or any of the
values) being zero as an indicator that they haven't been set, and apply the defaults in
that case?

Michael

RE: [PATCH v2 2/2] drm/hyperv: validate VMBus packet size in receive callback

[Michael Kelley]

From: Berkant Koc <me@berkoc.com> Sent: Sunday, May 17, 2026 7:25 AM
> 
> hyperv_receive() reads bytes_recvd from vmbus_recvpacket() but does not
> forward that value to hyperv_receive_sub(). The sub-handler reads
> msg->vid_hdr.type and msg->feature_chg.is_dirt_needed without knowing
> how many bytes the host actually wrote, so a short packet leaves the
> parser reading bytes that the host did not write in this packet.

This is a valid concern. Similar patterns have hopefully been fixed in other
drivers that were hardened for CoCo VMs.

> The unconditional memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE) on the
> wait-completion path also copies the full 16 KiB recv_buf regardless
> of bytes_recvd, including any residue from the prior message.

Does copying the full 16 KiB break anything? Or are you flagging as just
wasteful activity? This gets to what I previously mentioned about commit
messages -- explain "why".  Being wasteful is a valid reason, but it can be
helpful to future readers to be explicit about the "why".

> 
> Pass bytes_recvd into hyperv_receive_sub() and reject any packet shorter
> than the pipe + synthvid header. The dirt-feature branch additionally
> requires the feature_change payload size before reading is_dirt_needed.
> The init_buf copy now uses bytes_recvd as the length argument, which
> keeps it bounded by VMBUS_MAX_PACKET_SIZE through the new upper check.
> 
> Unchanged from v1.

Version related comments should go below the "---" following the Signed-off
line. They will be visible in the patch emails, but won't get put into the
git log when the patch is finally accepted. The Linux kernel doesn't want
to clutter the git log with intermediate version information that accumulates
as the patch is reviewed and revised.

> 
> Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
> Cc: stable@vger.kernel.org # 5.14+
> Signed-off-by: Berkant Koc <me@berkoc.com>
> ---
>  drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> index 3b5065fe06e4..cdab4895dd40 100644
> --- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> @@ -423,26 +423,35 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  	return 0;
>  }
> 
> -static void hyperv_receive_sub(struct hv_device *hdev)
> +static void hyperv_receive_sub(struct hv_device *hdev, u32 bytes_recvd)
>  {
>  	struct hyperv_drm_device *hv = hv_get_drvdata(hdev);
>  	struct synthvid_msg *msg;
> +	size_t hdr_size;
> 
>  	if (!hv)
>  		return;
> 
> +	hdr_size = sizeof(struct pipe_msg_hdr) +
> +		   sizeof(struct synthvid_msg_hdr);
> +	if (bytes_recvd < hdr_size || bytes_recvd > VMBUS_MAX_PACKET_SIZE)

The check against VMBUS_MAX_PACKET_SIZE shouldn't be needed.
vmbus_recvpacket() takes VMBUS_MAX_PACKET_SIZE as the max
receive size and won't return a bytes_recvd value that is larger.

> +		return;

In similar cases in other drivers that have been hardened for CoCo VMs, the
code outputs a rate limited error message. That should be done here so the
bad received message isn't just ignored. See hv_kvp_onchannelcallback() for
example.

> +
>  	msg = (struct synthvid_msg *)hv->recv_buf;
> 
>  	/* Complete the wait event */
>  	if (msg->vid_hdr.type == SYNTHVID_VERSION_RESPONSE ||
>  	    msg->vid_hdr.type == SYNTHVID_RESOLUTION_RESPONSE ||
>  	    msg->vid_hdr.type == SYNTHVID_VRAM_LOCATION_ACK) {
> -		memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE);
> +		memcpy(hv->init_buf, msg, bytes_recvd);

Additional logic is needed here. Each of the three message types
in the "if" statement has data beyond just the header. Before doing
the memcpy() and complete(), the code should validate that the msg
is big enough to contain that expected data.  I.e., it needs to do the
same as you have done for SYNTHVID_FEATURE_CHANGE.
Otherwise, the code that wakes up from the completion
will think that it has valid data to work with, and it may not.

Of course, the problem already exists in the upstream code. But if you
are going to make changes to fix the problem, the fix should fully
handle situation and not just be a partial fix.

>  		complete(&hv->wait);
>  		return;
>  	}
> 
>  	if (msg->vid_hdr.type == SYNTHVID_FEATURE_CHANGE) {
> +		if (bytes_recvd < hdr_size +
> +		    sizeof(struct synthvid_feature_change))
> +			return;

Same here.  Output a rate limited error message.

>  		hv->dirt_needed = msg->feature_chg.is_dirt_needed;
>  		if (hv->dirt_needed)
>  			hyperv_hide_hw_ptr(hv->hdev);
> @@ -469,7 +478,7 @@ static void hyperv_receive(void *ctx)
>  				       &bytes_recvd, &req_id);
>  		if (bytes_recvd > 0 &&
>  		    recv_buf->pipe_hdr.type == PIPE_MSG_DATA)
> -			hyperv_receive_sub(hdev);
> +			hyperv_receive_sub(hdev, bytes_recvd);
>  	} while (bytes_recvd > 0 && ret == 0);
>  }
> 
> --
> 2.47.3
> 

[PATCH v3 1/2] drm/hyperv: validate resolution_count and fix WIN8 fallback

[Berkant Koc]

A SYNTHVID_RESOLUTION_RESPONSE with resolution_count > 64 walks past
the supported_resolution[SYNTHVID_MAX_RESOLUTION_COUNT] array in the
parse loop. Bound resolution_count against the array size, folded
into the existing zero-check.

When the WIN10 resolution probe fails, the caller in
hyperv_connect_vsp() left hv->screen_*_max / preferred_* unpopulated,
which sets mode_config.max_width / max_height to 0 and makes
drm_internal_framebuffer_create() reject every userspace framebuffer
with -EINVAL. The pre-WIN10 branch had the same gap for
preferred_width / preferred_height. Use a single post-probe fallback
guarded by screen_width_max == 0 so both paths converge on the WIN8
defaults.

Signed-off-by: Berkant Koc <me@berkoc.com>
Assisted-by: Claude:claude-opus-4-7 berkoc-pipeline
Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
---
 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
index 051ecc526..c3d0ff229 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
@@ -391,8 +391,11 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
 		return -ETIMEDOUT;
 	}
 
-	if (msg->resolution_resp.resolution_count == 0) {
-		drm_err(dev, "No supported resolutions\n");
+	if (msg->resolution_resp.resolution_count == 0 ||
+	    msg->resolution_resp.resolution_count >
+	    SYNTHVID_MAX_RESOLUTION_COUNT) {
+		drm_err(dev, "Invalid resolution count: %d\n",
+			msg->resolution_resp.resolution_count);
 		return -ENODEV;
 	}
 
@@ -508,9 +511,13 @@ int hyperv_connect_vsp(struct hv_device *hdev)
 		ret = hyperv_get_supported_resolution(hdev);
 		if (ret)
 			drm_err(dev, "Failed to get supported resolution from host, use default\n");
-	} else {
+	}
+
+	if (!hv->screen_width_max) {
 		hv->screen_width_max = SYNTHVID_WIDTH_WIN8;
 		hv->screen_height_max = SYNTHVID_HEIGHT_WIN8;
+		hv->preferred_width = SYNTHVID_WIDTH_WIN8;
+		hv->preferred_height = SYNTHVID_HEIGHT_WIN8;
 	}
 
 	hv->mmio_megabytes = hdev->channel->offermsg.offer.mmio_megabytes;
-- 
2.47.3

[PATCH v3 2/2] drm/hyperv: validate VMBus packet size in receive callback

[Berkant Koc]

hyperv_receive_sub() reads msg->vid_hdr.type and dispatches into one
of four message-type branches without knowing how many bytes the host
wrote into hv->recv_buf. The completion path then runs
memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE), so the consumer
that wakes on wait_for_completion_timeout() can read up to 16 KiB of
residue from a prior message as if it were the response payload.

Pass bytes_recvd into hyperv_receive_sub() and reject any packet that
does not cover the pipe + synthvid header. For each of the three
completion-driving types (SYNTHVID_VERSION_RESPONSE,
SYNTHVID_RESOLUTION_RESPONSE, SYNTHVID_VRAM_LOCATION_ACK) also
require the type-specific payload before memcpy/complete, and apply
the same rule to SYNTHVID_FEATURE_CHANGE before reading is_dirt_needed.
The memcpy then uses bytes_recvd, which is bounded by
VMBUS_MAX_PACKET_SIZE through the call to vmbus_recvpacket().

Rejected packets are reported via drm_err_ratelimited() rather than
silently dropped, matching the CoCo-hardened pattern in
hv_kvp_onchannelcallback().

Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Berkant Koc <me@berkoc.com>
Assisted-by: Claude:claude-opus-4-7 berkoc-pipeline
---
 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 42 +++++++++++++++++++++--
 1 file changed, 39 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
index c3d0ff229..12d3feb4f 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
@@ -420,26 +420,62 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
 	return 0;
 }
 
-static void hyperv_receive_sub(struct hv_device *hdev)
+static void hyperv_receive_sub(struct hv_device *hdev, u32 bytes_recvd)
 {
 	struct hyperv_drm_device *hv = hv_get_drvdata(hdev);
 	struct synthvid_msg *msg;
+	size_t hdr_size;
 
 	if (!hv)
 		return;
 
+	hdr_size = sizeof(struct pipe_msg_hdr) +
+		   sizeof(struct synthvid_msg_hdr);
+	if (bytes_recvd < hdr_size) {
+		drm_err_ratelimited(&hv->dev,
+				    "synthvid packet too small for header: %u\n",
+				    bytes_recvd);
+		return;
+	}
+
 	msg = (struct synthvid_msg *)hv->recv_buf;
 
 	/* Complete the wait event */
 	if (msg->vid_hdr.type == SYNTHVID_VERSION_RESPONSE ||
 	    msg->vid_hdr.type == SYNTHVID_RESOLUTION_RESPONSE ||
 	    msg->vid_hdr.type == SYNTHVID_VRAM_LOCATION_ACK) {
-		memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE);
+		size_t need = hdr_size;
+
+		switch (msg->vid_hdr.type) {
+		case SYNTHVID_VERSION_RESPONSE:
+			need += sizeof(struct synthvid_version_resp);
+			break;
+		case SYNTHVID_RESOLUTION_RESPONSE:
+			need += sizeof(struct synthvid_supported_resolution_resp);
+			break;
+		case SYNTHVID_VRAM_LOCATION_ACK:
+			need += sizeof(struct synthvid_vram_location_ack);
+			break;
+		}
+		if (bytes_recvd < need) {
+			drm_err_ratelimited(&hv->dev,
+					    "synthvid packet too small for type %u: %u < %zu\n",
+					    msg->vid_hdr.type, bytes_recvd, need);
+			return;
+		}
+		memcpy(hv->init_buf, msg, bytes_recvd);
 		complete(&hv->wait);
 		return;
 	}
 
 	if (msg->vid_hdr.type == SYNTHVID_FEATURE_CHANGE) {
+		if (bytes_recvd < hdr_size +
+		    sizeof(struct synthvid_feature_change)) {
+			drm_err_ratelimited(&hv->dev,
+					    "synthvid feature change packet too small: %u\n",
+					    bytes_recvd);
+			return;
+		}
 		hv->dirt_needed = msg->feature_chg.is_dirt_needed;
 		if (hv->dirt_needed)
 			hyperv_hide_hw_ptr(hv->hdev);
@@ -466,7 +502,7 @@ static void hyperv_receive(void *ctx)
 				       &bytes_recvd, &req_id);
 		if (bytes_recvd > 0 &&
 		    recv_buf->pipe_hdr.type == PIPE_MSG_DATA)
-			hyperv_receive_sub(hdev);
+			hyperv_receive_sub(hdev, bytes_recvd);
 	} while (bytes_recvd > 0 && ret == 0);
 }
 
-- 
2.47.3

[PATCH v3 0/2] drm/hyperv: harden host message parsing

[Berkant Koc]

Two independent issues in the synthetic video driver that both stem
from trusting unvalidated host data.

1/2 bounds resolution_count from SYNTHVID_RESOLUTION_RESPONSE against
the supported_resolution[] array, and populates WIN8 defaults for
hv->screen_*_max / hv->preferred_* in both the WIN10-probe-failure
path and the pre-WIN10 path, so a failed or pre-WIN10 probe yields
a usable display instead of having drm_internal_framebuffer_create()
reject every userspace framebuffer with -EINVAL.

2/2 forwards bytes_recvd from vmbus_recvpacket() into the sub-handler,
rejects packets that do not cover the synthvid header, and additionally
requires the type-specific payload size before memcpy/complete or
before reading the feature-change byte. Rejected packets are logged
via drm_err_ratelimited() instead of being silently dropped, matching
the CoCo-hardened pattern in hv_kvp_onchannelcallback().

Changes since v2 (per review by Michael Kelley):

  1/2: dropped the reinit_completion() change. Kelley pointed out that
       the negotiate-version and update-vram-location timeouts cause
       hyperv_vmbus_probe() to fail and free the device, so the stale
       completion can only outlive its request in hyperv_vmbus_resume()
       after a get_supported_resolution() timeout. That is a narrower
       fix and belongs in a separate patch against the resume path.
       Subject and commit message rewritten to reflect that this patch
       is now bounds-check + WIN8 fallback only. Pre-WIN10 branch now
       also populates hv->preferred_* (Kelley spotted the gap).
       Followed the post-probe-test refactor Kelley suggested: the else
       branch is gone, a single screen_width_max == 0 check covers
       both the pre-WIN10 case and a failed WIN10 probe.

  2/2: dropped the redundant upper bound on bytes_recvd. Added a
       per-type switch for the three completion-driving message types
       (SYNTHVID_VERSION_RESPONSE, SYNTHVID_RESOLUTION_RESPONSE,
       SYNTHVID_VRAM_LOCATION_ACK) so the wait-completion path
       validates payload size before memcpy/complete. Every reject
       path now emits drm_err_ratelimited() rather than returning
       silently. Commit message rewritten to lead with the residue
       read, with "wasteful copy" reframed as the secondary observation.

Changes since v1:
  1/2: bound resolution_count check folded into the existing zero
       check; populate WIN8 defaults when hyperv_get_supported_resolution()
       fails.
  2/2: forward bytes_recvd into hyperv_receive_sub(); enforce the
       pipe + synthvid header minimum; check synthvid_feature_change
       payload size before reading is_dirt_needed.

Both patches carry an Assisted-by: Claude:claude-opus-4-7 berkoc-pipeline
trailer per the kernel coding-assistants policy. Code, analysis and
review responses are mine; the model is used as a structured reviewer
under human verification.

base-commit: 4bf5d3da79c48e1df4bab82c9680c53adeff7820

Berkant Koc (2):
  drm/hyperv: validate resolution_count and fix WIN8 fallback
  drm/hyperv: validate VMBus packet size in receive callback

 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 58 ++++++++++++++++++++---
 1 file changed, 52 insertions(+), 6 deletions(-)

--
2.47.3

Re: [PATCH v2 1/2] drm/hyperv: validate resolution_count and harden VSP request paths

[Berkant Koc]

Hello Michael,

Thanks for the CoCo context, that lines up with what is in
vmbus_devs[] for the framebuffer device. The piecemeal approach is
what I am aiming for here.

v3 is on the list and addresses your three concrete points:

> This change should probably be a separate patch, as it's not really
> related to the bounds checking issue.
> [...]
> All that notwithstanding, I don't think your fix is needed in its
> current form.

Dropped from v3. You are right that the negotiate-version and
update-vram-location timeouts let hyperv_vmbus_probe() bail out and
free the device, so the stale-completion path is only reachable from
hyperv_vmbus_resume() after a get_supported_resolution() timeout.
That is a narrower fix and belongs in a separate patch against the
resume path, which I will send afterwards.

> Is there a separate problem here in that preferred_width and
> preferred_height are not set in the pre-WIN10 case?

Yes, separate problem, and I missed it in v2. The pre-WIN10 branch
in hyperv_connect_vsp() sets only screen_*_max and leaves preferred_*
at zero, which is inconsistent with the WIN10-failure path.

> Also, having to duplicate the fallback code is distasteful. Instead
> of having an "else" clause, maybe have a follow-up test for
> screen_width_max [...] being zero as an indicator [...]

Adopted in v3. The else branch is gone; the WIN10 path runs the probe
and the post-probe block applies the WIN8 defaults whenever
screen_width_max is still zero. One source of truth, both paths
converge on it.

> In my view, your commit message is a bit too detailed.

Rewritten in v3. The bounds check and the WIN8 fallback are now one
short paragraph each, focused on the "why".

Series: <cover.1779221339.git.me@berkoc.com>

The v3 patches carry an `Assisted-by: Claude:claude-opus-4-7
berkoc-pipeline` trailer per the kernel coding-assistants policy.
Code, analysis and review responses are mine; the model is used as
a structured reviewer under human verification.

Thanks,
Berkant

Re: [PATCH v2 2/2] drm/hyperv: validate VMBus packet size in receive callback

[Berkant Koc]

Hello Michael,

Thanks for the thorough review. v3 is on the list and addresses each
point:

> Does copying the full 16 KiB break anything? Or are you flagging as just
> wasteful activity?

It is the residue read that is the actual hazard, not the copy cost:
the consumer that wakes on complete() then reads up to 16 KiB of bytes
the host did not write in this packet as if it were the response
payload. The v3 commit message now leads with that and treats
"wasteful" as a secondary observation.

> Version related comments should go below the "---" following the
> Signed-off line.

Moved into the cover letter changelog in v3 so it stays out of git
log.

> The check against VMBUS_MAX_PACKET_SIZE shouldn't be needed.

Dropped. The v3 check is bytes_recvd < hdr_size only.

> In similar cases in other drivers that have been hardened for CoCo VMs,
> the code outputs a rate limited error message. [...] See
> hv_kvp_onchannelcallback() for example.

Done in v3 via drm_err_ratelimited() on every short-packet path
(synthvid header underflow, type-specific payload underflow, feature
change underflow). The driver already uses drm_err_ratelimited() in
hyperv_sendpacket() for the corresponding send path.

> Additional logic is needed here. Each of the three message types
> in the "if" statement has data beyond just the header. Before doing
> the memcpy() and complete(), the code should validate that the msg
> is big enough to contain that expected data.

Fixed in v3. For the three completion types I now compute the required
payload size with a switch on msg->vid_hdr.type and reject the packet
before memcpy/complete:

  SYNTHVID_VERSION_RESPONSE    -> sizeof(struct synthvid_version_resp)
  SYNTHVID_RESOLUTION_RESPONSE -> sizeof(struct synthvid_supported_resolution_resp)
  SYNTHVID_VRAM_LOCATION_ACK   -> sizeof(struct synthvid_vram_location_ack)

The memcpy then uses bytes_recvd, so wait_for_completion_timeout()
consumers never see truncated data and never read past what the host
wrote.

Series: <cover.1779221339.git.me@berkoc.com>

The v3 patches carry an `Assisted-by: Claude:claude-opus-4-7
berkoc-pipeline` trailer per the kernel coding-assistants policy.
Code, analysis and review responses are mine; the model is used as
a structured reviewer under human verification.

Thanks,
Berkant

Re: [PATCH v3 1/2] drm/hyperv: validate resolution_count and fix WIN8 fallback

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [Critical] Unsynchronized access to the shared `hv->init_buf` introduces Double Fetch/TOCTOU vulnerabilities, allowing a malicious host to bypass bounds checks and leak kernel memory.
- [High] The driver reuses the `hv->wait` completion for synchronous messages without re-initializing it, which can cause spurious wakeups and synchronization bypass.
--

commit dde0fd11b79967ca7df11b1049abebec363d25a5
Author: Berkant Koc <me@berkoc.com>

drm/hyperv: validate resolution_count and fix WIN8 fallback

Adds bounds checking for resolution_count to prevent walking past the
supported_resolution array. It also adds fallback handling to ensure WIN8
defaults are used when the resolution probe fails.

> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> index 051ecc5268321..c3d0ff229e3d0 100644
> --- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> @@ -387,12 +387,15 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  	hyperv_sendpacket(hdev, msg);
>  
>  	t = wait_for_completion_timeout(&hv->wait, VMBUS_VSP_TIMEOUT);

[Severity: High]
This isn't a bug introduced by this patch, but does the driver need to call
reinit_completion(&hv->wait) before sending requests?

If a delayed or unsolicited response arrives, hyperv_receive_sub() calls
complete(&hv->wait), incrementing its internal counter.  When this function
is called later, wait_for_completion_timeout() will return immediately
without waiting for the actual response.

Could this cause the driver to erroneously parse its own outgoing request
buffer as a response?

>  	if (!t) {
>  		drm_err(dev, "Time out on waiting resolution response\n");
>  		return -ETIMEDOUT;
>  	}
>  
> -	if (msg->resolution_resp.resolution_count == 0) {
> -		drm_err(dev, "No supported resolutions\n");
> +	if (msg->resolution_resp.resolution_count == 0 ||
> +	    msg->resolution_resp.resolution_count >
> +	    SYNTHVID_MAX_RESOLUTION_COUNT) {

[Severity: Critical]
This is a pre-existing issue, but does reading directly from the shared
hv->init_buf introduce a TOCTOU or Double Fetch problem here?

Since hv->init_buf is also used by the asynchronous hyperv_receive_sub()
callback, could a malicious host send a valid count to pass this check,
and then immediately overwrite hv->init_buf with a large value before the
subsequent for-loop evaluates i < msg->resolution_resp.resolution_count?

Additionally, when the driver prepares requests earlier in this function
(e.g., setting msg->vid_hdr.size), an unsolicited response could overwrite
hv->init_buf. Could this trick vmbus_sendpacket() into using a
host-controlled size and leaking kernel memory?

> +		drm_err(dev, "Invalid resolution count: %d\n",
> +			msg->resolution_resp.resolution_count);
>  		return -ENODEV;
>  	}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1779221339.git.me@berkoc.com?part=1

Re: [PATCH v3 2/2] drm/hyperv: validate VMBus packet size in receive callback

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 4 potential issue(s) to consider:

New issues:
- [Critical] Using `bytes_recvd` for `memcpy()` without checking `vmbus_recvpacket()` return value leads to a massive heap buffer overflow.
- [High] Strict sizeof() validation incorrectly rejects dynamically-sized SYNTHVID_RESOLUTION_RESPONSE packets.

Pre-existing issues:
- [High] Concurrent lockless write to `hv->init_buf` from VMBus callback allows a malicious host to overwrite data while the guest is validating it.
- [High] Missing `reinit_completion()` before reusing the shared `hv->wait` completion object.
--

commit 8035423b95c9b24007478019421c5f94fe1179ce
Author: Berkant Koc <me@berkoc.com>
Subject: drm/hyperv: validate VMBus packet size in receive callback

This patch passes bytes_recvd into hyperv_receive_sub() to validate that
packets cover the pipe and synthvid headers as well as the type-specific
payload. The memcpy operation is also modified to use bytes_recvd.

Cc: stable@vger.kernel.org # 5.14+

> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> index c3d0ff229e3d0..12d3feb4f88ac 100644
> --- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
[ ... ]
> @@ -420,26 +420,62 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  	return 0;
>  }
>  
> -static void hyperv_receive_sub(struct hv_device *hdev)
> +static void hyperv_receive_sub(struct hv_device *hdev, u32 bytes_recvd)
>  {
[ ... ]
>  	if (msg->vid_hdr.type == SYNTHVID_VERSION_RESPONSE ||
>  	    msg->vid_hdr.type == SYNTHVID_RESOLUTION_RESPONSE ||
>  	    msg->vid_hdr.type == SYNTHVID_VRAM_LOCATION_ACK) {
> -		memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE);
> +		size_t need = hdr_size;
> +
> +		switch (msg->vid_hdr.type) {
> +		case SYNTHVID_VERSION_RESPONSE:
> +			need += sizeof(struct synthvid_version_resp);
> +			break;
> +		case SYNTHVID_RESOLUTION_RESPONSE:
> +			need += sizeof(struct synthvid_supported_resolution_resp);

[Severity: High]
Does this strict sizeof() check correctly handle dynamically-sized packets?

If the host sends a response sized only for the active elements (up to
resolution_count) rather than the maximum 64 elements, bytes_recvd will
be less than need. Will this cause valid packets to be falsely rejected
and break the driver initialization sequence?

> +			break;
> +		case SYNTHVID_VRAM_LOCATION_ACK:
> +			need += sizeof(struct synthvid_vram_location_ack);
> +			break;
> +		}
> +		if (bytes_recvd < need) {
> +			drm_err_ratelimited(&hv->dev,
> +					    "synthvid packet too small for type %u: %u < %zu\n",
> +					    msg->vid_hdr.type, bytes_recvd, need);
> +			return;
> +		}
> +		memcpy(hv->init_buf, msg, bytes_recvd);

[Severity: High]
This is a pre-existing issue, but does this pattern allow a malicious host
to overwrite data while the guest is validating it?

The callback unconditionally copies incoming response packets into
hv->init_buf via memcpy() and calls complete().

Since the waiting thread processes hv->init_buf in process context without
copying it to isolated local memory, can an untrusted host send a second
packet immediately after validation to trigger a time-of-check to
time-of-use (TOCTOU) data race?

>  		complete(&hv->wait);

[Severity: High]
This isn't a bug introduced by this patch, but does the driver need to call
reinit_completion(&hv->wait) before sending a new request?

If a prior request times out and its response arrives late, the completion
counter is incremented. Will the next wait immediately consume the
completion counter and process stale data left in hv->init_buf?

>  		return;
>  	}
[ ... ]
> @@ -466,7 +502,7 @@ static void hyperv_receive(void *ctx)
>  				       &bytes_recvd, &req_id);
>  		if (bytes_recvd > 0 &&
>  		    recv_buf->pipe_hdr.type == PIPE_MSG_DATA)
> -			hyperv_receive_sub(hdev);
> +			hyperv_receive_sub(hdev, bytes_recvd);

[Severity: Critical]
If vmbus_recvpacket() encounters a packet larger than VMBUS_MAX_PACKET_SIZE
(16KB), it returns -ENOBUFS and sets bytes_recvd to the required packet
length.

Since ret == 0 is only checked in the while condition, does this pass an
unbounded bytes_recvd into the callback? If recv_buf still contains the
header from a previous successful iteration, it looks like
memcpy(hv->init_buf, msg, bytes_recvd) will copy more than 16KB into
hv->init_buf, causing a heap buffer overflow.

>  	} while (bytes_recvd > 0 && ret == 0);
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1779221339.git.me@berkoc.com?part=2

Re: [PATCH v3 2/2] drm/hyperv: validate VMBus packet size in receive callback

[Berkant Koc]

sashiko-bot@kernel.org wrote:
> - [Critical] Using `bytes_recvd` for `memcpy()` without checking
>   `vmbus_recvpacket()` return value leads to a massive heap buffer
>   overflow.

This one is bounded on this channel. hyperv_connect_vsp() calls
vmbus_open() without setting max_pkt_size, so the inbound ring uses
VMBUS_DEFAULT_MAX_PKT_SIZE (4096) and hv_pkt_iter_first() clamps the
packet length to pkt_buffer_size. bytes_recvd therefore cannot exceed
4096, well under the 16 KiB recv_buf and init_buf, and
vmbus_recvpacket() does not return -ENOBUFS here, so the memcpy length
stays bounded.

I will still gate the dispatch on a successful vmbus_recvpacket()
return in the next revision, as defense in depth, so the bound is
local instead of relying on the ring clamp.

> - [High] Strict sizeof() validation incorrectly rejects
>   dynamically-sized SYNTHVID_RESOLUTION_RESPONSE packets.

Agreed. The response carries resolution_count entries, not the full
SYNTHVID_MAX_RESOLUTION_COUNT array, so checking against
sizeof(struct synthvid_supported_resolution_resp) is too strict. The
next revision validates the fixed prefix, reads and bounds
resolution_count, then requires only the count-sized array.

> - [High] Concurrent lockless write to `hv->init_buf` from VMBus
>   callback allows a malicious host to overwrite data while the guest
>   is validating it.
> - [High] Missing `reinit_completion()` before reusing the shared
>   `hv->wait` completion object.

Both pre-existing. On v2 Michael Kelley suggested splitting the
completion reinit into a separate patch on the resume path. The
init_buf reuse sits in the same area, so I plan to send the reinit and
the related response-type handling as a separate follow-up rather than
fold them into this size-validation change.

Thanks for the review.

Berkant

RE: [PATCH v3 2/2] drm/hyperv: validate VMBus packet size in receive callback

[Michael Kelley]

From: Berkant Koc <me@berkoc.com> Sent: Wednesday, May 20, 2026 6:24 AM
> 
> sashiko-bot@kernel.org wrote:
> > - [Critical] Using `bytes_recvd` for `memcpy()` without checking
> >   `vmbus_recvpacket()` return value leads to a massive heap buffer
> >   overflow.
> 
> This one is bounded on this channel. hyperv_connect_vsp() calls
> vmbus_open() without setting max_pkt_size, so the inbound ring uses
> VMBUS_DEFAULT_MAX_PKT_SIZE (4096) and hv_pkt_iter_first() clamps the
> packet length to pkt_buffer_size. bytes_recvd therefore cannot exceed
> 4096, well under the 16 KiB recv_buf and init_buf, and
> vmbus_recvpacket() does not return -ENOBUFS here, so the memcpy length
> stays bounded.

Actually, the behavior of vmbus_recvpacket() is more subtle,
and the problem pointed out by Sashiko AI is real. I had forgotten
about this subtle behavior in my first reply to you on this topic. :-(

The incoming message from Hyper-V has a length encoded in it.
hv_ringbuffer_read() gets that message length, and if it is larger
than the buflen_parameter, -ENOBUFS is returned. But the
returned buffer_actual_len is also set to the incoming message
length provided by Hyper-V. This allows the caller to realize
it didn't provide enough buffer space, and also tells it how much
buffer space is needed. hv_pci_onchannelcallback() uses
this functionality to retry the receive operation with a larger
buffer. I think all the other callers just treat -ENOBUFS as a
fatal error, which is also fine.

> 
> I will still gate the dispatch on a successful vmbus_recvpacket()
> return in the next revision, as defense in depth, so the bound is
> local instead of relying on the ring clamp.

Indeed, the error check is needed, and it's not just defense in depth.

> 
> > - [High] Strict sizeof() validation incorrectly rejects
> >   dynamically-sized SYNTHVID_RESOLUTION_RESPONSE packets.
> 
> Agreed. The response carries resolution_count entries, not the full
> SYNTHVID_MAX_RESOLUTION_COUNT array, so checking against
> sizeof(struct synthvid_supported_resolution_resp) is too strict. The
> next revision validates the fixed prefix, reads and bounds
> resolution_count, then requires only the count-sized array.

OK, good.

> 
> > - [High] Concurrent lockless write to `hv->init_buf` from VMBus
> >   callback allows a malicious host to overwrite data while the guest
> >   is validating it.

I don't think this actually happens. In hv_pkt_iter_first(), the message
is copied out of the ring buffer into a temporary buffer that is not
explicitly shared with the host. This temporary copy prevents a
malicious host from modifying the data while the guest is validating it.

> > - [High] Missing `reinit_completion()` before reusing the shared
> >   `hv->wait` completion object.
> 
> Both pre-existing. On v2 Michael Kelley suggested splitting the
> completion reinit into a separate patch on the resume path. The
> init_buf reuse sits in the same area, so I plan to send the reinit and
> the related response-type handling as a separate follow-up rather than
> fold them into this size-validation change.

Handling a timeout, and then the host providing a belated response
is a really messy problem. There's some mechanism using request IDs
in hv_ringbuffer_write() and vmbus_sendpacket_getid() to help match
up requests and responses, but my recollection is that even this
extra machinery is not 100% foolproof. You may or may not want
to go down the path of trying to fix it. :-)
	
A process comment:  The emails for v2 and v3 of your patch set
are being threaded in an unexpected way. They are showing up
as replies under the original v1. See https://lore.kernel.org/linux-hyperv/.
The preferred approach is for each version to start a new email
thread. The cover letter should start the new thread, and the
patches should show up as threaded under the cover letter.

Also, don't post a new version more frequently than every
24 hours at a minimum, and there's no problem with waiting
2 to 3 days. The idea is to give people a chance to review and
provide comments so that you can batch any changes in
response to the feedback.

Michael