Re: [PATCH] umount: Allow superblock owners to force umount

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2025-03-18 at 12:29 -0400, trondmy@kernel.org wrote:
> From: Trond Myklebust <trond.myklebust@hammerspace.com>
> 
> Loosen the permission check on forced umount to allow users holding
> CAP_SYS_ADMIN privileges in namespaces that are privileged with respect
> to the userns that originally mounted the filesystem.
> 
> Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> ---
>  fs/namespace.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 8f1000f9f3df..d401486fe95d 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2026,6 +2026,7 @@ static void warn_mandlock(void)
>  static int can_umount(const struct path *path, int flags)
>  {
>  	struct mount *mnt = real_mount(path->mnt);
> +	struct super_block *sb = path->dentry->d_sb;
>  
>  	if (!may_mount())
>  		return -EPERM;
> @@ -2035,7 +2036,7 @@ static int can_umount(const struct path *path, int flags)
>  		return -EINVAL;
>  	if (mnt->mnt.mnt_flags & MNT_LOCKED) /* Check optimistically */
>  		return -EINVAL;
> -	if (flags & MNT_FORCE && !capable(CAP_SYS_ADMIN))
> +	if (flags & MNT_FORCE && !ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
>  		return -EPERM;
>  	return 0;
>  }

Reviewed-by: Jeff Layton <jlayton@kernel.org>