Re: [PATCH] NFSD: fix nfsd4_listxattr_validate_cookie

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2024-01-25 at 07:46 -0700, Jorge Mora wrote:
> If LISTXATTRS is sent with a correct cookie but a small maxcount,
> this could lead function nfsd4_listxattr_validate_cookie to
> return NFS4ERR_BAD_COOKIE. If maxcount = 20, then second check
> on function gives RHS = 3 thus any cookie larger than 3 returns
> NFS4ERR_BAD_COOKIE.
> 
> There is no need to validate the cookie on the return XDR buffer
> since attribute referenced by cookie will be the first in the
> return buffer.
> 
> Fixes: 23e50fe3a5e6 ("nfsd: implement the xattr functions and en/decode logic")
> Signed-off-by: Jorge Mora <mora@netapp.com>
> ---
>  fs/nfsd/nfs4xdr.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> index 913d566519bf..7f957061c4d5 100644
> --- a/fs/nfsd/nfs4xdr.c
> +++ b/fs/nfsd/nfs4xdr.c
> @@ -5116,16 +5116,11 @@ nfsd4_listxattr_validate_cookie(struct nfsd4_listxattrs *listxattrs,
>  
>  	/*
>  	 * If the cookie is larger than the maximum number we can fit
> -	 * in either the buffer we just got back from vfs_listxattr, or,
> -	 * XDR-encoded, in the return buffer, it's invalid.
> +	 * in the buffer we just got back from vfs_listxattr, it's invalid.
>  	 */
>  	if (cookie > (listxattrs->lsxa_len) / (XATTR_USER_PREFIX_LEN + 2))
>  		return nfserr_badcookie;
>  
> -	if (cookie > (listxattrs->lsxa_maxcount /
> -		      (XDR_QUADLEN(XATTR_USER_PREFIX_LEN + 2) + 4)))
> -		return nfserr_badcookie;
> -
>  	*offsetp = (u32)cookie;
>  	return 0;
>  }


The cookie is generated by the server, and the client generates the
maxcount value. They don't really have anything to do with one another,
so yeah, I agree that this check is nonsensical.

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