Re: [PATCH] cifs: fix missing null session check in mount

[Shyam Prasad N <nspmangalore@gmail.com>]

On Wed, Jul 14, 2021 at 6:19 AM Steve French <smfrench@gmail.com> wrote:
>
> Although it is unlikely to be have ended up with a null
> session pointer calling cifs_try_adding_channels in cifs_mount.
> Coverity correctly notes that we are already checking for
> it earlier (when we return from do_dfs_failover), so at
> a minimum to clarify the code we should make sure we also
> check for it when we exit the loop so we don't end up calling
> cifs_try_adding_channels or mount_setup_tlink with a null
> ses pointer.
>
> Addresses-Coverity: 1505608 ("Derefernce after null check")
> Reviewed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
> Signed-off-by: Steve French <stfrench@microsoft.com>
> ---
>  fs/cifs/connect.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
> index db6c607269f5..463cae116c12 100644
> --- a/fs/cifs/connect.c
> +++ b/fs/cifs/connect.c
> @@ -3577,7 +3577,7 @@ int cifs_mount(struct cifs_sb_info *cifs_sb,
> struct smb3_fs_context *ctx)
>   rc = -ELOOP;
>   } while (rc == -EREMOTE);
>
> - if (rc || !tcon)
> + if (rc || !tcon || !ses)
>   goto error;
>
>   kfree(ref_path);
>
> --
> Thanks,
>
> Steve

Hi Paulo,

Doesn't it make sense to check rc, tcon and ses values right after
mount_get_conns call?

        rc = mount_get_conns(ctx, cifs_sb, &xid, &server, &ses,
&tcon);          <<<<<<<<<<<<<<<<<<<
        /*
         * If called with 'nodfs' mount option, then skip DFS
resolving.  Otherwise unconditionally
         * try to get an DFS referral (even cached) to determine
whether it is an DFS mount.
         *
         * Skip prefix path to provide support for DFS referrals from
w2k8 servers which don't seem
         * to respond with PATH_NOT_COVERED to requests that include the prefix.
         */
        if ((cifs_sb->mnt_cifs_flags & CIFS_MOUNT_NO_DFS) ||
            dfs_cache_find(xid, ses, cifs_sb->local_nls,
cifs_remap(cifs_sb), ctx->UNC + 1, NULL,
                           NULL)) {
                if (rc)
                        goto error;
                /* Check if it is fully accessible and then mount it */
                rc = is_path_remote(cifs_sb, ctx, xid, server, tcon);
                if (!rc)
                        goto out;
                if (rc != -EREMOTE)
                        goto error;
        }

Why don't we check for all rc values that we don't expect, and call
dfs_cache_find only when it's an expected error?

-- 
Regards,
Shyam