Re: [PATCH v2] btrfs: prevent deleting the wrong subvolume by ID

[Qu Wenruo <wqu@suse.com>]

在 2026/10/19 02:08, guolingxing 写道:
> When deleting a subvolume by ID, btrfs_ioctl_snap_destroy() first
> resolves the requested ID to a name, then looks up that name again
> under the parent directory lock.
> 
> A concurrent rename can change which subvolume is associated with
> the name between these operations. Since the deletion path does not
> verify the ID of the resolved subvolume, it can delete a different
> subvolume while leaving the requested one intact.
> 
> This was reproduced by repeatedly exchanging the names of two
> subvolumes using renameat2(RENAME_EXCHANGE) while deleting one of
> them by ID. The wrong subvolume was deleted in 9 out of 14 independent
> tests.
> 
> Fix this by checking that the resolved subvolume ID matches the
> requested ID while holding the parent directory lock. Return -EAGAIN
> on a mismatch so that userspace can retry the operation instead of
> being told that the requested subvolume does not exist.
> 
> Fixes: 949964c92843 ("btrfs: add new BTRFS_IOC_SNAP_DESTROY_V2 ioctl")
> Cc: stable@vger.kernel.org
> Signed-off-by: guolingxing <guolingxing@tju.edu.cn>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
> Changes since v1:
> - Return -EAGAIN instead of -ENOENT on a subvolume ID mismatch,
>    as suggested by Qu Wenruo.
> 
> 
> Testing:
> - Unpatched kernel: 9 wrong deletions in 14 independent runs.
> - v2 patched kernel: 0 wrong deletions in 30 independent runs.
> - EAGAIN was observed in 14 runs; all 30 runs successfully deleted
>    the requested subvolume while preserving the other one.
> 
>   fs/btrfs/ioctl.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 576cd76ca0bf..d82a7f3f3d14 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -2380,6 +2380,17 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
>   
>   	inode = d_inode(dentry);
>   	dest = BTRFS_I(inode)->root;
> +
> +	/*
> +	 * A concurrent rename may have changed which subvolume the name
> +	 * refers to. Verify that the resolved ID still matches the requested ID.
> +	 */
> +	if (destroy_v2 && (vol_args2->flags & BTRFS_SUBVOL_SPEC_BY_ID) &&
> +	    btrfs_root_id(dest) != vol_args2->subvolid) {
> +		ret = -EAGAIN;
> +		goto out_end_removing;
> +	}
> +
>   	if (!capable(CAP_SYS_ADMIN)) {
>   		/*
>   		 * Regular user.  Only allow this with a special mount