[PATCH v2] ACPI: video: Fix random crashes due to bad kfree

[Chris Bainbridge <chris.bainbridge@gmail.com>]

Commit c6a837088bed ("drm/amd/display: Fetch the EDID from _DDC if
available for eDP") added function dm_helpers_probe_acpi_edid, which
fetches the EDID from the BIOS by calling acpi_video_get_edid.
acpi_video_get_edid returns a pointer to the EDID, but this pointer does
not originate from kmalloc - it is actually the internal "pointer" field
from an acpi_buffer struct (which did come from kmalloc).
dm_helpers_probe_acpi_edid then attempts to kfree the EDID pointer,
resulting in memory corruption which leads to random, intermittent
crashes (e.g. 4% of boots will fail with some Oops).

Fix this by allocating a new array (which can be safely freed) for the
EDID data, and correctly freeing the acpi_buffer pointer.

The only other caller of acpi_video_get_edid is nouveau_acpi_edid:
remove the extraneous kmemdup here as the EDID data is now copied in
acpi_video_device_EDID.

Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Fixes: c6a837088bed ("drm/amd/display: Fetch the EDID from _DDC if available for eDP")
---
Changes in v2:
	- check kmemdup() return value
	- move buffer management into acpi_video_device_EDID()
	- return actual length value of buffer
---
 drivers/acpi/acpi_video.c              | 50 ++++++++++++++------------
 drivers/gpu/drm/nouveau/nouveau_acpi.c |  2 +-
 2 files changed, 29 insertions(+), 23 deletions(-)

diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
index 8274a17872ed..3c627bdf2d1b 100644
--- a/drivers/acpi/acpi_video.c
+++ b/drivers/acpi/acpi_video.c
@@ -610,16 +610,29 @@ acpi_video_device_lcd_get_level_current(struct acpi_video_device *device,
 	return 0;
 }
 
+/*
+ *  Arg:
+ *	device	: video output device (LCD, CRT, ..)
+ *	edid    : address for returned EDID pointer
+ *	length  : _DDC length to request (must be a multiple of 128)
+ *
+ *  Return Value:
+ *	Length of EDID (positive value) or error (negative value)
+ *
+ *  Get EDID from ACPI _DDC. On success, a pointer to the EDID data is written
+ *  to the edid address, and the length of the EDID is returned. The caller is
+ *  responsible for freeing the edid pointer.
+ */
+
 static int
-acpi_video_device_EDID(struct acpi_video_device *device,
-		       union acpi_object **edid, int length)
+acpi_video_device_EDID(struct acpi_video_device *device, void **edid, int length)
 {
-	int status;
+	acpi_status status;
 	struct acpi_buffer buffer = { ACPI_ALLOCATE_BUFFER, NULL };
 	union acpi_object *obj;
 	union acpi_object arg0 = { ACPI_TYPE_INTEGER };
 	struct acpi_object_list args = { 1, &arg0 };
-
+	int ret;
 
 	*edid = NULL;
 
@@ -636,16 +649,17 @@ acpi_video_device_EDID(struct acpi_video_device *device,
 
 	obj = buffer.pointer;
 
-	if (obj && obj->type == ACPI_TYPE_BUFFER)
-		*edid = obj;
-	else {
+	if (obj && obj->type == ACPI_TYPE_BUFFER) {
+		*edid = kmemdup(obj->buffer.pointer, obj->buffer.length, GFP_KERNEL);
+		ret = *edid ? obj->buffer.length : -ENOMEM;
+	} else {
 		acpi_handle_debug(device->dev->handle,
 				 "Invalid _DDC data for length %d\n", length);
-		status = -EFAULT;
-		kfree(obj);
+		ret = -EFAULT;
 	}
 
-	return status;
+	kfree(obj);
+	return ret;
 }
 
 /* bus */
@@ -1435,9 +1449,7 @@ int acpi_video_get_edid(struct acpi_device *device, int type, int device_id,
 {
 	struct acpi_video_bus *video;
 	struct acpi_video_device *video_device;
-	union acpi_object *buffer = NULL;
-	acpi_status status;
-	int i, length;
+	int i, length, ret;
 
 	if (!device || !acpi_driver_data(device))
 		return -EINVAL;
@@ -1477,16 +1489,10 @@ int acpi_video_get_edid(struct acpi_device *device, int type, int device_id,
 		}
 
 		for (length = 512; length > 0; length -= 128) {
-			status = acpi_video_device_EDID(video_device, &buffer,
-							length);
-			if (ACPI_SUCCESS(status))
-				break;
+			ret = acpi_video_device_EDID(video_device, edid, length);
+			if (ret > 0)
+				return ret;
 		}
-		if (!length)
-			continue;
-
-		*edid = buffer->buffer.pointer;
-		return length;
 	}
 
 	return -ENODEV;
diff --git a/drivers/gpu/drm/nouveau/nouveau_acpi.c b/drivers/gpu/drm/nouveau/nouveau_acpi.c
index 8f0c69aad248..21b56cc7605c 100644
--- a/drivers/gpu/drm/nouveau/nouveau_acpi.c
+++ b/drivers/gpu/drm/nouveau/nouveau_acpi.c
@@ -384,7 +384,7 @@ nouveau_acpi_edid(struct drm_device *dev, struct drm_connector *connector)
 	if (ret < 0)
 		return NULL;
 
-	return kmemdup(edid, EDID_LENGTH, GFP_KERNEL);
+	return edid;
 }
 
 bool nouveau_acpi_video_backlight_use_native(void)
-- 
2.39.5