[PATCH] drm/virtio: fix NULL pointer dereference on fence allocation failure

[PATCH] drm/virtio: fix NULL pointer dereference on fence allocation failure

[Peiyang He]

virtio_gpu_fence_alloc() can fail due to memory pressure and return NULL,
but its caller like virtio_gpu_init_submit() never checks it. Later,
virtio_gpu_init_submit() passes the NULL fence to
virtio_gpu_fence_event_create(), which unconditionally dereferences it.

Found when fuzzing the virtio driver with Syzkaller:

	Oops: general protection fault, probably for non-canonical address 0xdffffc0000000012: 0000 [#1] SMP KASAN NOPTI
	KASAN: null-ptr-deref in range [0x0000000000000090-0x0000000000000097]
	CPU: 1 UID: 0 PID: 9991 Comm: syz.0.121 Not tainted 7.2.0 #4 PREEMPT(full) 
	Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
	RIP: 0010:virtio_gpu_fence_event_create drivers/gpu/drm/virtio/virtgpu_submit.c:295 [inline]
	RIP: 0010:virtio_gpu_init_submit drivers/gpu/drm/virtio/virtgpu_submit.c:398 [inline]
	RIP: 0010:virtio_gpu_execbuffer_ioctl+0xc78/0x1aa0 drivers/gpu/drm/virtio/virtgpu_submit.c:505
	Code: 85 ed 0f 85 21 09 00 00 e8 05 5a c9 fb 48 8b 44 24 10 48 8d b8 90 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 9a 0d 00 00 48 8b 44 24 10 4c 89 b0 90 00 00 00
	RSP: 0018:ffffc900039dfad0 EFLAGS: 00010216
	RAX: dffffc0000000000 RBX: ffffc900039dfdd8 RCX: ffffffff85f6fd3d
	RDX: 0000000000000012 RSI: ffffffff85f6fd4b RDI: 0000000000000090
	RBP: 0000000000000000 R0virtio_gpu_virgl_process_cmd: ctrl 0x102, error 0x1203
	R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880132c4000
	R13: 0000000000000000 R14: ffff888073b6c700 R15: 000000000000003b
	FS:  00007fab480b96c0(0000) GS:ffff8880eb6e9000(0000) knlGS:0000000000000000
	CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
	CR2: 00007effbf5e55a8 CR3: 0000000048d19000 CR4: 0000000000350ef0
	Call Trace:
	<TASK>
	drm_ioctl_kernel+0x1f4/0x3e0 drivers/gpu/drm/drm_ioctl.c:817
	drm_ioctl+0x5f4/0xc70 drivers/gpu/drm/drm_ioctl.c:914
	vfs_ioctl fs/ioctl.c:51 [inline]
	__do_sys_ioctl fs/ioctl.c:597 [inline]
	__se_sys_ioctl fs/ioctl.c:583 [inline]
	__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
	do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
	do_syscall_64+0x116/0x800 arch/x86/entry/syscall_64.c:94
	entry_SYSCALL_64_after_hwframe+0x77/0x7f
	RIP: 0033:0x7fab471a82bd
	Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
	RSP: 002b:00007fab480b9018 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
	RAX: ffffffffffffffda RBX: 00007fab47435fa0 RCX: 00007fab471a82bd
	RDX: 00002000000000c0 RSI: 00000000c0406442 RDI: 0000000000000003
	RBP: 00007fab480b9080 R08: 0000000000000000 R09: 0000000000000000
	R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
	R13: 00007fab47436038 R14: 00007fab47435fa0 R15: 00007ffe85ab0740
	</TASK>
	Modules linked in:
	---[ end trace 0000000000000000 ]---
	RIP: 0010:virtio_gpu_fence_event_create drivers/gpu/drm/virtio/virtgpu_submit.c:295 [inline]
	RIP: 0010:virtio_gpu_init_submit drivers/gpu/drm/virtio/virtgpu_submit.c:398 [inline]
	RIP: 0010:virtio_gpu_execbuffer_ioctl+0xc78/0x1aa0 drivers/gpu/drm/virtio/virtgpu_submit.c:505
	Code: 85 ed 0f 85 21 09 00 00 e8 05 5a c9 fb 48 8b 44 24 10 48 8d b8 90 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 9a 0d 00 00 48 8b 44 24 10 4c 89 b0 90 00 00 00
	RSP: 0018:ffffc900039dfad0 EFLAGS: 00010216
	RAX: dffffc0000000000 RBX: ffffc900039dfdd8 RCX: ffffffff85f6fd3d
	RDX: 0000000000000012 RSI: ffffffff85f6fd4b RDI: 0000000000000090
	RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000000
	R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880132c4000
	R13: 0000000000000000 R14: ffff888073b6c700 R15: 000000000000003b
	FS:  00007fab480b96c0(0000) GS:ffff888098ae9000(0000) knlGS:0000000000000000
	CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
	CR2: 00007f24c3759000 CR3: 0000000048d19000 CR4: 0000000000350ef0
	----------------
	Code disassembly (best guess):
	0:	85 ed                	test   %ebp,%ebp
	2:	0f 85 21 09 00 00    	jne    0x929
	8:	e8 05 5a c9 fb       	call   0xfbc95a12
	d:	48 8b 44 24 10       	mov    0x10(%rsp),%rax
	12:	48 8d b8 90 00 00 00 	lea    0x90(%rax),%rdi
	19:	48 b8 00 00 00 00 00 	movabs $0xdffffc0000000000,%rax
	20:	fc ff df
	23:	48 89 fa             	mov    %rdi,%rdx
	26:	48 c1 ea 03          	shr    $0x3,%rdx
	* 2a:	80 3c 02 00          	cmpb   $0x0,(%rdx,%rax,1) <-- trapping instruction
	2e:	0f 85 9a 0d 00 00    	jne    0xdce
	34:	48 8b 44 24 10       	mov    0x10(%rsp),%rax
	39:	4c 89 b0 90 00 00 00 	mov    %r14,0x90(%rax)

Fix by checking virtio_gpu_fence_alloc() in virtio_gpu_init_submit() and
returning -ENOMEM before any later code can dereference the NULL fence.

Fixes: 70d1ace56db6 ("drm/virtio: Conditionally allocate virtio_gpu_fence")
Cc: stable@vger.kernel.org
Signed-off-by: Peiyang He <peiyang_he@smail.nju.edu.cn>
Assisted-by: Codex:gpt-5.5
---
 drivers/gpu/drm/virtio/virtgpu_submit.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_submit.c b/drivers/gpu/drm/virtio/virtgpu_submit.c
index 32cb1e4aa425..857ed5c5f2a8 100644
--- a/drivers/gpu/drm/virtio/virtgpu_submit.c
+++ b/drivers/gpu/drm/virtio/virtgpu_submit.c
@@ -389,10 +389,13 @@ static int virtio_gpu_init_submit(struct virtio_gpu_submit *submit,
 	if ((exbuf->flags & VIRTGPU_EXECBUF_FENCE_FD_OUT) ||
 	    exbuf->num_out_syncobjs ||
 	    exbuf->num_bo_handles ||
-	    drm_fence_event)
+	    drm_fence_event) {
 		out_fence = virtio_gpu_fence_alloc(vgdev, fence_ctx, ring_idx);
-	else
+		if (!out_fence)
+			return -ENOMEM;
+	} else {
 		out_fence = NULL;
+	}
 
 	if (drm_fence_event) {
 		err = virtio_gpu_fence_event_create(dev, file, out_fence, ring_idx);
-- 
2.43.0

Re: [PATCH] drm/virtio: fix NULL pointer dereference on fence allocation failure

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:

Pre-existing issues:
- [High] Erroneous release of file descriptor 0 on early error returns due to 0-initialized out_fence_fd.
- [High] Leak of DRM event memory and event space quota on early submission failure.
- [High] Premature reset of in_syncobjs on error paths breaks ioctl restart and destroys dependencies.
--

commit 1f57c5e182bb2310a6b5538ff7f993fd7e45dee5
Author: Peiyang He <peiyang_he@smail.nju.edu.cn>
drm/virtio: fix NULL pointer dereference on fence allocation failure

This commit adds a missing NULL check after allocating a fence in
virtio_gpu_init_submit() to prevent a NULL pointer dereference under
memory pressure.

> diff --git a/drivers/gpu/drm/virtio/virtgpu_submit.c b/drivers/gpu/drm/virtio/virtgpu_submit.c
> --- a/drivers/gpu/drm/virtio/virtgpu_submit.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_submit.c
> @@ -389,10 +389,13 @@ static int virtio_gpu_init_submit(struct virtio_gpu_submit *submit,
>  	if ((exbuf->flags & VIRTGPU_EXECBUF_FENCE_FD_OUT) ||
>  	    exbuf->num_out_syncobjs ||
>  	    exbuf->num_bo_handles ||
> -	    drm_fence_event)
> +	    drm_fence_event) {
>  		out_fence = virtio_gpu_fence_alloc(vgdev, fence_ctx, ring_idx);
> -	else
> +		if (!out_fence)
> +			return -ENOMEM;

[Severity: High]
This isn't a bug introduced by this patch, but returning early here
exposes a pre-existing issue with file descriptor handling.

Because virtio_gpu_init_submit() initializes the submit struct using
memset(submit, 0, sizeof(*submit)), submit->out_fence_fd is left as 0.
If we return -ENOMEM here, the caller eventually invokes
virtio_gpu_cleanup_submit(), which checks:

    if (submit->out_fence_fd >= 0)
        put_unused_fd(submit->out_fence_fd);

Since 0 is >= 0, this will erroneously release file descriptor 0
(typically stdin) back to the kernel, potentially allowing subsequent
open() calls to hijack the file descriptor. Could out_fence_fd be
initialized to -1 before taking this early return?

[Severity: High]
This is a pre-existing issue, but does taking an error path (such as
this new -ENOMEM return) break system call restarts?

When the submission ioctl fails, the error path calls
virtio_gpu_cleanup_submit(). This function unconditionally calls
virtio_gpu_reset_syncobjs(), replacing all input syncobj fences with
NULL. When the kernel automatically restarts the ioctl (e.g. after
-ERESTARTSYS is returned during a signal interruption), the dependencies
are already destroyed, causing drm_syncobj_find_fence() to return
-EINVAL and breaking userspace synchronization state.

> +	} else {
>  		out_fence = NULL;
> +	}
>  
>  	if (drm_fence_event) {
>  		err = virtio_gpu_fence_event_create(dev, file, out_fence, ring_idx);

[Severity: High]
This is a pre-existing issue, but how does the code handle event cleanup
if an error occurs later in the submission process?

If virtio_gpu_fence_event_create() succeeds, it allocates the event and
deducts from file_priv->event_space. If a subsequent initialization step
fails, virtio_gpu_cleanup_submit() drops the fence reference via
dma_fence_put(&submit->out_fence->f).

However, because virtio_gpu_fence_ops lacks a .release callback,
dma_fence_release() simply frees the fence without ever calling
drm_event_cancel_free(), leaking both the event memory and the client's
event space quota.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/00EFE4BA92889B14+20260909091114.2622550-1-peiyang_he@smail.nju.edu.cn?part=1