Re: [PATCH] fhandle: reject detached mounts in capable_wrt_mount()

On Wed, 2026-07-01 at 11:44 +0000, David Lee wrote:
> The recent fhandle RCU fix moved the mount namespace capability check
> into capable_wrt_mount(), so a non-NULL mnt_namespace survives the
> ns_capable() dereference. The helper still assumes the later
> READ_ONCE(mount->mnt_ns) must be non-NULL because may_decode_fh()
> checked is_mounted() first.
> 
> That assumption is not stable. A detached mount from
> open_tree(..., OPEN_TREE_CLONE) can be dissolved on fput while
> open_by_handle_at() is between those checks, and umount_tree() can
> clear mount->mnt_ns. If the helper observes NULL, it dereferences
> mnt_ns->user_ns and panics.
> 
> Return false when the RCU read observes a detached mount. This keeps
> the relaxed permission path conservative: a mount no longer attached
> to a namespace cannot authorize open_by_handle_at() access.
> 
> Fixes: 620c266f3949 ("fhandle: relax open_by_handle_at() permission checks")
> Cc: [email protected]
> Signed-off-by: David Lee <[email protected]>
> Assisted-by: Codex:gpt-5
> ---
> Bug found and triaged by David Lee from Trail of Bits.
> 
> Trail of Bits has a minimal PoC that triggers this crash on a custom
> kernel build, which can be shared further if needed.
> 
>  fs/fhandle.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/fhandle.c b/fs/fhandle.c
> index 1ca7eb3a6cb5..f8829231e3d7 100644
> --- a/fs/fhandle.c
> +++ b/fs/fhandle.c
> @@ -295,7 +295,7 @@ static bool capable_wrt_mount(struct mount *mount)
>  	 */
>  	guard(rcu)();
>  	mnt_ns = READ_ONCE(mount->mnt_ns);
> -	return ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN);
> +	return mnt_ns && ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN);
>  }
>  
>  static inline int may_decode_fh(struct handle_to_path_ctx *ctx,

Seems sane.

Reviewed-by: Jeff Layton <[email protected]>