[PATCH 5.15 090/110] virtio: acknowledge all features before access

Greg Kroah-Hartman
2022-03-14T12:10:46Z

From: Michael S. Tsirkin <mst@redhat.com>

commit 4fa59ede95195f267101a1b8916992cf3f245cdb upstream.

The feature negotiation was designed in a way that
makes it possible for devices to know which config
fields will be accessed by drivers.

This is broken since commit 404123c2db79 ("virtio: allow drivers to
validate features") with fallout in at least block and net. We have a
partial work-around in commit 2f9a174f918e ("virtio: write back
F_VERSION_1 before validate") which at least lets devices find out which
format should config space have, but this is a partial fix: guests
should not access config space without acknowledging features since
otherwise we'll never be able to change the config space format.

To fix, split finalize_features from virtio_finalize_features and
call finalize_features with all feature bits before validation,
and then - if validation changed any bits - once again after.

Since virtio_finalize_features no longer writes out features
rename it to virtio_features_ok - since that is what it does:
checks that features are ok with the device.

As a side effect, this also reduces the amount of hypervisor accesses -
we now only acknowledge features once unless we are clearing any
features when validating (which is uncommon).

IRC I think that this was more or less always the intent in the spec but
unfortunately the way the spec is worded does not say this explicitly, I
plan to address this at the spec level, too.

Acked-by: Jason Wang <jasowang@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 404123c2db79 ("virtio: allow drivers to validate features")
Fixes: 2f9a174f918e ("virtio: write back F_VERSION_1 before validate")
Cc: "Halil Pasic" <pasic@linux.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/virtio/virtio.c | 39 ++++++++++++++++++++++-----------------
include/linux/virtio_config.h | 3 ++-
2 files changed, 24 insertions(+), 18 deletions(-)

--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -166,14 +166,13 @@ void virtio_add_status(struct virtio_dev
}
EXPORT_SYMBOL_GPL(virtio_add_status);

-static int virtio_finalize_features(struct virtio_device *dev)
+/* Do some validation, then set FEATURES_OK */
+static int virtio_features_ok(struct virtio_device *dev)
{
- int ret = dev->config->finalize_features(dev);
unsigned status;
+ int ret;

might_sleep();
- if (ret)
- return ret;

ret = arch_has_restricted_virtio_memory_access();
if (ret) {
@@ -238,17 +237,6 @@ static int virtio_dev_probe(struct devic
driver_features_legacy = driver_features;
}

- /*
- * Some devices detect legacy solely via F_VERSION_1. Write
- * F_VERSION_1 to force LE config space accesses before FEATURES_OK for
- * these when needed.
- */
- if (drv->validate && !virtio_legacy_is_little_endian()
- && device_features & BIT_ULL(VIRTIO_F_VERSION_1)) {
- dev->features = BIT_ULL(VIRTIO_F_VERSION_1);
- dev->config->finalize_features(dev);
- }
-
if (device_features & (1ULL << VIRTIO_F_VERSION_1))
dev->features = driver_features & device_features;
else
@@ -259,13 +247,26 @@ static int virtio_dev_probe(struct devic
if (device_features & (1ULL << i))
__virtio_set_bit(dev, i);

+ err = dev->config->finalize_features(dev);
+ if (err)
+ goto err;
+
if (drv->validate) {
+ u64 features = dev->features;
+
err = drv->validate(dev);
if (err)
goto err;
+
+ /* Did validation change any features? Then write them again. */
+ if (features != dev->features) {
+ err = dev->config->finalize_features(dev);
+ if (err)
+ goto err;
+ }
}

- err = virtio_finalize_features(dev);
+ err = virtio_features_ok(dev);
if (err)
goto err;

@@ -489,7 +490,11 @@ int virtio_device_restore(struct virtio_
/* We have a driver! */
virtio_add_status(dev, VIRTIO_CONFIG_S_DRIVER);

- ret = virtio_finalize_features(dev);
+ ret = dev->config->finalize_features(dev);
+ if (ret)
+ goto err;
+
+ ret = virtio_features_ok(dev);
if (ret)
goto err;

--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -62,8 +62,9 @@ struct virtio_shm_region {
* Returns the first 64 feature bits (all we currently need).
* @finalize_features: confirm what device features we'll be using.
* vdev: the virtio_device
- * This gives the final feature bits for the device: it can change
+ * This sends the driver feature bits to the device: it can change
* the dev->feature bits if it wants.
+ * Note: despite the name this can be called any number of times.
* Returns 0 on success or error status
* @bus_name: return the bus name associated with the device (optional)
* vdev: the virtio_device