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

[Zhenyu Wang]

On 2020.09.08 20:11:21 +0200, Alejandro Sior wrote:
> 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;

Thanks for reporting this! Sorry that we failed to do sanity validation on older
platform when enabling vfio edid function for all platform.

Could you try below one to see if it fixes your problem? Instead of refusing
possible read of edid, this fixes port number for BDW.

From d5d9304b6bfdc31356fd2feb1ddbbf28073fe3d4 Mon Sep 17 00:00:00 2001
From: Zhenyu Wang <zhenyuw at linux.intel.com>
Date: Fri, 11 Sep 2020 13:50:20 +0800
Subject: [PATCH] drm/i915/gvt: Fix port number for BDW on EDID region setup

Current BDW virtual display port is initialized as PORT_B, so need
to use same port for VFIO EDID region, otherwise invalid EDID blob
pointer is assigned.

Fixes: 0178f4ce3c3b ("drm/i915/gvt: Enable vfio edid for all GVT supported platform")
Signed-off-by: Zhenyu Wang <zhenyuw at linux.intel.com>
---
 drivers/gpu/drm/i915/gvt/vgpu.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gvt/vgpu.c b/drivers/gpu/drm/i915/gvt/vgpu.c
index 8fa9b31a2484..f6d7e33c7099 100644
--- a/drivers/gpu/drm/i915/gvt/vgpu.c
+++ b/drivers/gpu/drm/i915/gvt/vgpu.c
@@ -368,6 +368,7 @@ void intel_gvt_destroy_idle_vgpu(struct intel_vgpu *vgpu)
 static struct intel_vgpu *__intel_gvt_create_vgpu(struct intel_gvt *gvt,
 		struct intel_vgpu_creation_params *param)
 {
+	struct drm_i915_private *dev_priv = gvt->gt->i915;
 	struct intel_vgpu *vgpu;
 	int ret;
 
@@ -436,7 +437,10 @@ static struct intel_vgpu *__intel_gvt_create_vgpu(struct intel_gvt *gvt,
 	if (ret)
 		goto out_clean_sched_policy;
 
-	ret = intel_gvt_hypervisor_set_edid(vgpu, PORT_D);
+	if (IS_BROADWELL(dev_priv))
+		ret = intel_gvt_hypervisor_set_edid(vgpu, PORT_B);
+	else
+		ret = intel_gvt_hypervisor_set_edid(vgpu, PORT_D);
 	if (ret)
 		goto out_clean_sched_policy;
 
-- 
2.28.0



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