[PATCH 0/2] drm/amdgpu: fix use-after-free in userq signal/wait IOCTLs

[PATCH 0/2] drm/amdgpu: fix use-after-free in userq signal/wait IOCTLs

[Chenyuan Mi]

Both amdgpu_userq_wait_ioctl() and amdgpu_userq_signal_ioctl()
access user queue objects obtained from xa_load() without holding
userq_mutex. A concurrent AMDGPU_USERQ_OP_FREE can destroy and
kfree the queue in this window, leading to use-after-free.

The two bugs have different origins:

- Patch 1 fixes a wait-path 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").

- Patch 2 fixes a similar pre-existing lifetime bug in the signal
  path, present since commit a292fdecd728 ("drm/amdgpu: Implement
  userqueue signal/wait IOCTL").

Patch 1 adds explicit userq_mutex coverage around the xa_load and
subsequent fence_drv_xa operations in the wait path.

Patch 2 moves the ensure_ev_fence call (which acquires
userq_mutex) before xa_load in the signal path, so that the queue
lookup and all subsequent accesses are covered by the same lock.

Chenyuan Mi (2):
  drm/amdgpu: protect waitq access with userq_mutex in wait IOCTL
  drm/amdgpu: protect queue access in signal IOCTL

 .../gpu/drm/amd/amdgpu/amdgpu_userq_fence.c   | 25 +++++++++++++------
 1 file changed, 18 insertions(+), 7 deletions(-)

--
2.53.0

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

[Chenyuan Mi]

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>
---
 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

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

[Christian König]

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
> 

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

[Claude Code Review Bot]

Patch Review

**Already fixed upstream via refcounting.** The current tree uses:
```c
waitq = amdgpu_userq_get(userq_mgr, wait_info->waitq_id);
```
(line 866) instead of the raw `xa_load()` this patch targets. The `amdgpu_userq_get()` function takes a kref under xa_lock, and `amdgpu_userq_put()` is called at the end (line 951), providing proper lifetime protection without holding the mutex across the entire loop.

**Even evaluated against the older base, there is a concern:**

- **Sleeping under mutex**: The patch holds `userq_mutex` across `dma_fence_wait(fences[i], true)` (line 909 in the diff context). `dma_fence_wait` with `intr=true` can sleep for an extended/unbounded period waiting for GPU work to complete. Holding `userq_mutex` during this time blocks all other userqueue operations (create, destroy, signal) for this file descriptor. This is a significant liveness concern — a slow or stalled fence would effectively deadlock the entire userqueue subsystem for the process. The refcounting approach in drm-next avoids this problem entirely.

---
Generated by Claude Code Patch Reviewer

[PATCH 2/2] drm/amdgpu: protect queue access in signal IOCTL

[Chenyuan Mi]

amdgpu_userq_signal_ioctl() retrieves the user queue via xa_load()
and then dereferences it in amdgpu_userq_fence_read_wptr(),
amdgpu_userq_fence_create(), and direct queue->last_fence accesses,
all before userq_mutex is acquired by amdgpu_userq_ensure_ev_fence().

A concurrent AMDGPU_USERQ_OP_FREE can destroy and free the queue
in this window, leading to a use-after-free.

This bug predates the queue-id wait ioctl changes and has been
present since the original signal/wait ioctl implementation.

Fix this by moving amdgpu_userq_ensure_ev_fence() before xa_load()
so that the queue lookup and all subsequent accesses are performed
under the userq_mutex that ensure_ev_fence acquires. Add the
necessary mutex_unlock() calls to the error paths between the moved
ensure_ev_fence and the existing unlock.

Fixes: a292fdecd728 ("drm/amdgpu: Implement userqueue signal/wait IOCTL")
Cc: stable@vger.kernel.org
Signed-off-by: Chenyuan Mi <chenyuan_mi@163.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
index 1785ea7c18fe..7866f583eea4 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
@@ -545,23 +545,28 @@ int amdgpu_userq_signal_ioctl(struct drm_device *dev, void *data,
 		}
 	}
 
-	/* Retrieve the user queue */
+	/* We are here means UQ is active, make sure the eviction fence is valid */
+	amdgpu_userq_ensure_ev_fence(&fpriv->userq_mgr, &fpriv->evf_mgr);
+
+	/* Retrieve the user queue under userq_mutex (held by ensure_ev_fence) */
 	queue = xa_load(&userq_mgr->userq_xa, args->queue_id);
 	if (!queue) {
+		mutex_unlock(&userq_mgr->userq_mutex);
 		r = -ENOENT;
 		goto put_gobj_write;
 	}
 
 	r = amdgpu_userq_fence_read_wptr(adev, queue, &wptr);
-	if (r)
+	if (r) {
+		mutex_unlock(&userq_mgr->userq_mutex);
 		goto put_gobj_write;
+	}
 
 	r = amdgpu_userq_fence_alloc(&userq_fence);
-	if (r)
+	if (r) {
+		mutex_unlock(&userq_mgr->userq_mutex);
 		goto put_gobj_write;
-
-	/* We are here means UQ is active, make sure the eviction fence is valid */
-	amdgpu_userq_ensure_ev_fence(&fpriv->userq_mgr, &fpriv->evf_mgr);
+	}
 
 	/* Create a new fence */
 	r = amdgpu_userq_fence_create(queue, userq_fence, wptr, &fence);
-- 
2.53.0

Re: [PATCH 2/2] drm/amdgpu: protect queue access in signal IOCTL

[Christian König]

On 3/9/26 03:22, Chenyuan Mi wrote:
> [Some people who received this message don't often get email from chenyuan_mi@163.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> 
> amdgpu_userq_signal_ioctl() retrieves the user queue via xa_load()
> and then dereferences it in amdgpu_userq_fence_read_wptr(),
> amdgpu_userq_fence_create(), and direct queue->last_fence accesses,
> all before userq_mutex is acquired by amdgpu_userq_ensure_ev_fence().
> 
> A concurrent AMDGPU_USERQ_OP_FREE can destroy and free the queue
> in this window, leading to a use-after-free.
> 
> This bug predates the queue-id wait ioctl changes and has been
> present since the original signal/wait ioctl implementation.
> 
> Fix this by moving amdgpu_userq_ensure_ev_fence() before xa_load()

Again that trivially causes a deadlock. So the patch is just not working at all.

Regards,
Christian.

> so that the queue lookup and all subsequent accesses are performed
> under the userq_mutex that ensure_ev_fence acquires. Add the
> necessary mutex_unlock() calls to the error paths between the moved
> ensure_ev_fence and the existing unlock.
> 
> Fixes: a292fdecd728 ("drm/amdgpu: Implement userqueue signal/wait IOCTL")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chenyuan Mi <chenyuan_mi@163.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
> index 1785ea7c18fe..7866f583eea4 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
> @@ -545,23 +545,28 @@ int amdgpu_userq_signal_ioctl(struct drm_device *dev, void *data,
>                 }
>         }
> 
> -       /* Retrieve the user queue */
> +       /* We are here means UQ is active, make sure the eviction fence is valid */
> +       amdgpu_userq_ensure_ev_fence(&fpriv->userq_mgr, &fpriv->evf_mgr);
> +
> +       /* Retrieve the user queue under userq_mutex (held by ensure_ev_fence) */
>         queue = xa_load(&userq_mgr->userq_xa, args->queue_id);
>         if (!queue) {
> +               mutex_unlock(&userq_mgr->userq_mutex);
>                 r = -ENOENT;
>                 goto put_gobj_write;
>         }
> 
>         r = amdgpu_userq_fence_read_wptr(adev, queue, &wptr);
> -       if (r)
> +       if (r) {
> +               mutex_unlock(&userq_mgr->userq_mutex);
>                 goto put_gobj_write;
> +       }
> 
>         r = amdgpu_userq_fence_alloc(&userq_fence);
> -       if (r)
> +       if (r) {
> +               mutex_unlock(&userq_mgr->userq_mutex);
>                 goto put_gobj_write;
> -
> -       /* We are here means UQ is active, make sure the eviction fence is valid */
> -       amdgpu_userq_ensure_ev_fence(&fpriv->userq_mgr, &fpriv->evf_mgr);
> +       }
> 
>         /* Create a new fence */
>         r = amdgpu_userq_fence_create(queue, userq_fence, wptr, &fence);
> --
> 2.53.0
> 

Claude review: Re: [PATCH 2/2] drm/amdgpu: protect queue access in signal IOCTL

[Claude Code Review Bot]

Patch Review

**Already fixed upstream via refcounting.** The current tree uses:
```c
queue = amdgpu_userq_get(userq_mgr, args->queue_id);
```
(line 522) instead of raw `xa_load()`. Combined with `amdgpu_userq_put(queue)` at the cleanup (line 614), this provides proper lifetime protection.

**Additional concerns with this patch's approach:**

- **Reliance on implicit locking side-effect**: The patch relies on the fact that `amdgpu_userq_ensure_ev_fence()` acquires `userq_mutex` as a side-effect, and then adds `mutex_unlock()` calls in error paths between it and the existing unlock. This coupling is fragile — the comment `/* Retrieve the user queue under userq_mutex (held by ensure_ev_fence) */` documents a non-obvious invariant. If `ensure_ev_fence`'s locking behavior ever changes, this code silently breaks. It would be cleaner to acquire the mutex explicitly and separately.

- **`amdgpu_userq_fence_alloc` under mutex**: Moving `ensure_ev_fence` earlier means `amdgpu_userq_fence_alloc()` (a `GFP_KERNEL` allocation from a slab cache) is now called under `userq_mutex`. While this works, it needlessly extends the critical section. The allocation doesn't access queue state and doesn't need mutex protection.

- **Fixes tag**: The `Fixes:` tag references `a292fdecd728`, the original signal/wait IOCTL commit. The `Cc: stable` tag is appropriate for the severity, but given the patch doesn't apply to current trees, backporting would need to be done carefully against the actual stable tree base.

---
Generated by Claude Code Patch Reviewer

Claude review: drm/amdgpu: fix use-after-free in userq signal/wait IOCTLs

[Claude Code Review Bot]

Overall Series Review

Subject: drm/amdgpu: fix use-after-free in userq signal/wait IOCTLs
Author: Chenyuan Mi <chenyuan_mi@163.com>
Patches: 5
Reviewed: 2026-03-10T12:44:42.922557

---

This 2-patch series aims to fix use-after-free bugs in the amdgpu userqueue signal and wait IOCTLs by adding mutex protection around queue lookups. The identified race conditions are real: without lifetime protection, a concurrent `AMDGPU_USERQ_OP_FREE` can destroy a queue between `xa_load()` and subsequent dereferences.

**However, these patches are based on an older version of the code and are already superseded by changes in the current drm-next tree.** The current tree (as seen at `amdgpu_userq_fence.c:522` and `:866`) has replaced the raw `xa_load()` calls with `amdgpu_userq_get()`/`amdgpu_userq_put()`, which use `kref` refcounting to protect the queue lifetime. This refcounting approach is a better fix — it takes a reference under the xa_lock, preventing the queue from being freed while in use, without needing to hold `userq_mutex` across the entire operation.

**Recommendation: NAK — these patches are not needed against current drm-next.**

---
Generated by Claude Code Patch Reviewer