cifs: Validate content of NFS reparse point buffer

Symlink target location stored in DataBuffer is encoded in UTF-16. So check
that symlink DataBuffer length is non-zero and even number. And check that
DataBuffer does not contain UTF-16 null codepoint because Linux cannot
process symlink with null byte.

DataBuffer for char and block devices is 8 bytes long as it contains two
32-bit numbers (major and minor). Add check for this.

DataBuffer buffer for sockets and fifos zero-length. Add checks for this.
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/reparse.c

@@ -330,6 +330,18 @@ static int parse_reparse_posix(struct reparse_posix_data *buf,
 
 	switch ((type = le64_to_cpu(buf->InodeType))) {
 	case NFS_SPECFILE_LNK:
+		if (len == 0 || (len % 2)) {
+			cifs_dbg(VFS, "srv returned malformed nfs symlink buffer\n");
+			return -EIO;
+		}
+		/*
+		 * Check that buffer does not contain UTF-16 null codepoint
+		 * because Linux cannot process symlink with null byte.
+		 */
+		if (UniStrnlen((wchar_t *)buf->DataBuffer, len/2) != len/2) {
+			cifs_dbg(VFS, "srv returned null byte in nfs symlink target location\n");
+			return -EIO;
+		}
 		data->symlink_target = cifs_strndup_from_utf16(buf->DataBuffer,
 							       len, true,
 							       cifs_sb->local_nls);
@@ -341,8 +353,19 @@ static int parse_reparse_posix(struct reparse_posix_data *buf,
 		break;
 	case NFS_SPECFILE_CHR:
 	case NFS_SPECFILE_BLK:
+		/* DataBuffer for block and char devices contains two 32-bit numbers */
+		if (len != 8) {
+			cifs_dbg(VFS, "srv returned malformed nfs buffer for type: 0x%llx\n", type);
+			return -EIO;
+		}
+		break;
 	case NFS_SPECFILE_FIFO:
 	case NFS_SPECFILE_SOCK:
+		/* DataBuffer for fifos and sockets is empty */
+		if (len != 0) {
+			cifs_dbg(VFS, "srv returned malformed nfs buffer for type: 0x%llx\n", type);
+			return -EIO;
+		}
 		break;
 	default:
 		cifs_dbg(VFS, "%s: unhandled inode type: 0x%llx\n",