[Intel-gfx][PATCH v2] drm/i915/gvt: Prevent NULL pointer dereference in intel_vgpu_reg_rw_edid()

[Alejandro Sior]

In the function intel_vgpu_reg_rw_edid of kvmgt.c, pos can be equal
to NULL for GPUs that do not properly support EDID. In those cases, when
pos gets passed to the handle_edid functions, it gets added a short offset
then it's dereferenced in memcpy's, leading to NULL pointer
dereference kernel oops.

More concretely, that kernel oops renders some Broadwell GPUs users
unable to set up virtual machines with virtual GPU passthrough (virtual
machines hang indefinitely when trying to make use of the virtual GPU),
and make them unable to remove the virtual GPUs once the kernel oops has
happened (it hangs indefinitely, and notably too when the kernel tries to
shutdown). The issues that this causes and steps to reproduce are
discussed in more details in this github issue post:
https://github.com/intel/gvt-linux/issues/170#issuecomment-685806160

Check if pos is equal to NULL, and if it is, set ret to a negative
value, making the module simply indicate that the access to EDID region
has failed, without any fatal repercussion.

Signed-off-by: Alejandro Sior <aho at sior.be>

---
Changes in v2:
- removed middle name of author to comply with git name
- rephrased the patch description with imperative phrasing
- removed useless paragraph
- made a paragraph more concise
- fixed typos
- made individual lines shorter than 75 chars

 drivers/gpu/drm/i915/gvt/kvmgt.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
index ad8a9df49f29..49163363ba4a 100644
--- a/drivers/gpu/drm/i915/gvt/kvmgt.c
+++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
@@ -557,7 +557,9 @@ static size_t intel_vgpu_reg_rw_edid(struct intel_vgpu *vgpu, char *buf,
 		(struct vfio_edid_region *)kvmgt_vdev(vgpu)->region[i].data;
 	loff_t pos = *ppos & VFIO_PCI_OFFSET_MASK;
 
-	if (pos < region->vfio_edid_regs.edid_offset) {
+	if (pos == NULL) {
+		ret = -EINVAL;
+	} else if (pos < region->vfio_edid_regs.edid_offset) {
 		ret = handle_edid_regs(vgpu, region, buf, count, pos, iswrite);
 	} else {
 		pos -= EDID_BLOB_OFFSET;
-- 
2.28.0