reviewed-by me
On Thu, 30 Mar 2023 at 06:23, David Disseldorp <ddiss@xxxxxxx> wrote:
>
> When compiled with CONFIG_CIFS_DFS_UPCALL disabled, cifs_dfs_d_automount
> NULL. cifs.ko logic for mapping CIFS_FATTR_DFS_REFERRAL attributes to
> S_AUTOMOUNT and corresponding dentry flags is retained regardless of
> CONFIG_CIFS_DFS_UPCALL, leading to a NULL pointer dereference in
> VFS follow_automount() when traversing a DFS referral link:
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> ...
> Call Trace:
> <TASK>
> __traverse_mounts+0xb5/0x220
> ? cifs_revalidate_mapping+0x65/0xc0 [cifs]
> step_into+0x195/0x610
> ? lookup_fast+0xe2/0xf0
> path_lookupat+0x64/0x140
> filename_lookup+0xc2/0x140
> ? __create_object+0x299/0x380
> ? kmem_cache_alloc+0x119/0x220
> ? user_path_at_empty+0x31/0x50
> user_path_at_empty+0x31/0x50
> __x64_sys_chdir+0x2a/0xd0
> ? exit_to_user_mode_prepare+0xca/0x100
> do_syscall_64+0x42/0x90
> entry_SYSCALL_64_after_hwframe+0x72/0xdc
>
> This fix adds an inline cifs_dfs_d_automount() {return -EREMOTE} handler
> when CONFIG_CIFS_DFS_UPCALL is disabled. An alternative would be to
> avoid flagging S_AUTOMOUNT, etc. without CONFIG_CIFS_DFS_UPCALL. This
> approach was chosen as it provides more control over the error path.
>
> Signed-off-by: David Disseldorp <ddiss@xxxxxxx>
> ---
> fs/cifs/cifsfs.h | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/fs/cifs/cifsfs.h b/fs/cifs/cifsfs.h
> index 71fe0a0a7992..415176b2cf32 100644
> --- a/fs/cifs/cifsfs.h
> +++ b/fs/cifs/cifsfs.h
> @@ -124,7 +124,10 @@ extern const struct dentry_operations cifs_ci_dentry_ops;
> #ifdef CONFIG_CIFS_DFS_UPCALL
> extern struct vfsmount *cifs_dfs_d_automount(struct path *path);
> #else
> -#define cifs_dfs_d_automount NULL
> +static inline struct vfsmount *cifs_dfs_d_automount(struct path *path)
> +{
> + return ERR_PTR(-EREMOTE);
> +}
> #endif
>
> /* Functions related to symlinks */
> --
> 2.35.3
>
