[PATCH] virtiofs: fix NULL dereference in virtio_fs_add_queues_sysfs()

[PATCH] virtiofs: fix NULL dereference in virtio_fs_add_queues_sysfs()

[Qianchang Zhao]

virtio_fs_add_queues_sysfs() creates per-queue sysfs kobjects via
kobject_create_and_add(). The current code checks the wrong variable
after the allocation:

- kobject_create_and_add() may return NULL on failure.
- The code incorrectly checks fs->mqs_kobj (the parent kobject), which is
  expected to be non-NULL at this point.
- If kobject_create_and_add() fails, fsvq->kobj is NULL but the code can
  still call sysfs_create_group(fsvq->kobj, ...), leading to a NULL pointer
  dereference and kernel panic (DoS).

Fix by validating fsvq->kobj immediately after kobject_create_and_add()
and aborting on failure, so sysfs_create_group() is never called with a
NULL kobject.

Reported-by: Qianchang Zhao <pioooooooooip@gmail.com>
Reported-by: Zhitong Liu <liuzhitong1993@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Qianchang Zhao <pioooooooooip@gmail.com>
---
 fs/fuse/virtio_fs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 6bc7c97b0..b2f6486fe 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -373,7 +373,7 @@ static int virtio_fs_add_queues_sysfs(struct virtio_fs *fs)
 
 		sprintf(buff, "%d", i);
 		fsvq->kobj = kobject_create_and_add(buff, fs->mqs_kobj);
-		if (!fs->mqs_kobj) {
+		if (!fsvq->kobj) {
 			ret = -ENOMEM;
 			goto out_del;
 		}
-- 
2.34.1

Re: [PATCH] virtiofs: fix NULL dereference in virtio_fs_add_queues_sysfs()

[Stefan Hajnoczi]

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On Sat, Dec 13, 2025 at 10:28:29AM +0900, Qianchang Zhao wrote:
> virtio_fs_add_queues_sysfs() creates per-queue sysfs kobjects via
> kobject_create_and_add(). The current code checks the wrong variable
> after the allocation:
> 
> - kobject_create_and_add() may return NULL on failure.
> - The code incorrectly checks fs->mqs_kobj (the parent kobject), which is
>   expected to be non-NULL at this point.
> - If kobject_create_and_add() fails, fsvq->kobj is NULL but the code can
>   still call sysfs_create_group(fsvq->kobj, ...), leading to a NULL pointer
>   dereference and kernel panic (DoS).
> 
> Fix by validating fsvq->kobj immediately after kobject_create_and_add()
> and aborting on failure, so sysfs_create_group() is never called with a
> NULL kobject.
> 
> Reported-by: Qianchang Zhao <pioooooooooip@gmail.com>
> Reported-by: Zhitong Liu <liuzhitong1993@gmail.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Qianchang Zhao <pioooooooooip@gmail.com>
> ---
>  fs/fuse/virtio_fs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

This issue was fixed in October. It no longer exists in mainline Linux:

  commit c014021253d77cd89b2d8788ce522283d83fbd40
  Author: Alok Tiwari <alok.a.tiwari@oracle.com>
  Date:   Mon Oct 27 03:46:47 2025 -0700
  
      virtio-fs: fix incorrect check for fsvq->kobj

> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 6bc7c97b0..b2f6486fe 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -373,7 +373,7 @@ static int virtio_fs_add_queues_sysfs(struct virtio_fs *fs)
>  
>  		sprintf(buff, "%d", i);
>  		fsvq->kobj = kobject_create_and_add(buff, fs->mqs_kobj);
> -		if (!fs->mqs_kobj) {
> +		if (!fsvq->kobj) {
>  			ret = -ENOMEM;
>  			goto out_del;
>  		}
> -- 
> 2.34.1
> 

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