Re: [PATCH 1/2] drm/amdgpu: protect waitq access with userq_mutex in wait IOCTL

[Christian König <christian.koenig@amd.com>]

On 3/9/26 03:22, Chenyuan Mi wrote:
> amdgpu_userq_wait_ioctl() accesses the wait queue object obtained
> from xa_load() without holding userq_mutex or taking a reference on
> the queue. A concurrent AMDGPU_USERQ_OP_FREE call can destroy and
> free the queue between the xa_load() and the subsequent
> xa_alloc(&waitq->fence_drv_xa, ...), resulting in a use-after-free.
> 
> This is a regression introduced by commit 4b27406380b0
> ("drm/amdgpu: Add queue id support to the user queue wait IOCTL"),
> which removed the indirect fence_drv_xa_ptr model and its NULL
> check safety net from commit ed5fdc1fc282 ("drm/amdgpu: Fix the
> use-after-free issue in wait IOCTL") and replaced it with a direct
> waitq->fence_drv_xa access, but did not add any lifetime protection
> around the new waitq pointer.
> 
> Fix this by holding userq_mutex across the xa_load() and the
> subsequent fence_drv_xa operations, matching the locking used by
> the destroy path.
> 
> Fixes: 4b27406380b0 ("drm/amdgpu: Add queue id support to the user queue wait IOCTL")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chenyuan Mi <chenyuan_mi@163.com>

Well this trivially causes a deadlock.

The correct fix has already been published by Sunil quite a while ago.

Regards,
Christian.

> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
> index 8013260e29dc..1785ea7c18fe 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
> @@ -912,8 +912,10 @@ int amdgpu_userq_wait_ioctl(struct drm_device *dev, void *data,
>                  */
>                 num_fences = dma_fence_dedup_array(fences, num_fences);
> 
> +               mutex_lock(&userq_mgr->userq_mutex);
>                 waitq = xa_load(&userq_mgr->userq_xa, wait_info->waitq_id);
>                 if (!waitq) {
> +                       mutex_unlock(&userq_mgr->userq_mutex);
>                         r = -EINVAL;
>                         goto free_fences;
>                 }
> @@ -932,6 +934,7 @@ int amdgpu_userq_wait_ioctl(struct drm_device *dev, void *data,
>                                 r = dma_fence_wait(fences[i], true);
>                                 if (r) {
>                                         dma_fence_put(fences[i]);
> +                                       mutex_unlock(&userq_mgr->userq_mutex);
>                                         goto free_fences;
>                                 }
> 
> @@ -948,8 +951,10 @@ int amdgpu_userq_wait_ioctl(struct drm_device *dev, void *data,
>                          */
>                         r = xa_alloc(&waitq->fence_drv_xa, &index, fence_drv,
>                                      xa_limit_32b, GFP_KERNEL);
> -                       if (r)
> +                       if (r) {
> +                               mutex_unlock(&userq_mgr->userq_mutex);
>                                 goto free_fences;
> +                       }
> 
>                         amdgpu_userq_fence_driver_get(fence_drv);
> 
> @@ -961,6 +966,7 @@ int amdgpu_userq_wait_ioctl(struct drm_device *dev, void *data,
>                         /* Increment the actual userq fence count */
>                         cnt++;
>                 }
> +               mutex_unlock(&userq_mgr->userq_mutex);
> 
>                 wait_info->num_fences = cnt;
>                 /* Copy userq fence info to user space */
> --
> 2.53.0
>