NFSv4: Check for buffer length in __nfs4_get_acl_uncached

Commit 1f1ea6c2 "NFSv4: Fix buffer overflow checking in
__nfs4_get_acl_uncached" accidently dropped the checking for too small
result buffer length.

If someone uses getxattr on "system.nfs4_acl" on an NFSv4 mount
supporting ACLs, the ACL has not been cached and the buffer suplied is
too short, we still copy the complete ACL, resulting in kernel and user
space memory corruption.
Signed-off-by: NSven Wegener <sven.wegener@stealer.net>
Cc: stable@kernel.org
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4proc.c

@@ -3937,8 +3937,13 @@ static ssize_t __nfs4_get_acl_uncached(struct inode *inode, void *buf, size_t bu
 		goto out_free;
 	}
 	nfs4_write_cached_acl(inode, pages, res.acl_data_offset, res.acl_len);
-	if (buf)
+	if (buf) {
+		if (res.acl_len > buflen) {
+			ret = -ERANGE;
+			goto out_free;
+		}
 		_copy_from_pages(buf, pages, res.acl_data_offset, res.acl_len);
+	}
 out_ok:
 	ret = res.acl_len;
 out_free: