net: 9p: avoid freeing uninit memory in p9pdu_vreadf

If some of p9pdu_readf() calls inside case 'T' in p9pdu_vreadf() fails,
the error path is not handled properly. *wnames or members of *wnames
array may be left uninitialized and invalidly freed.

Initialize *wnames to NULL in beginning of case 'T'. Initialize the first
*wnames array element to NULL and nullify the failing *wnames element so
that the error path freeing loop stops on the first NULL element and
doesn't proceed further.

Found by Linux Verification Center (linuxtesting.org).

Fixes: ace51c4 ("9p: add new protocol support code")
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Message-ID: <20231206200913.16135-1-pchelkin@ispras.ru>
Cc: stable@vger.kernel.org
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

Author: pchelkin91

net/9p/protocol.c

@@ -394,6 +394,8 @@ p9pdu_vreadf(struct p9_fcall *pdu, int proto_version, const char *fmt,
 				uint16_t *nwname = va_arg(ap, uint16_t *);
 				char ***wnames = va_arg(ap, char ***);
 
+				*wnames = NULL;
+
 				errcode = p9pdu_readf(pdu, proto_version,
 								"w", nwname);
 				if (!errcode) {
@@ -403,6 +405,8 @@ p9pdu_vreadf(struct p9_fcall *pdu, int proto_version, const char *fmt,
 							  GFP_NOFS);
 					if (!*wnames)
 						errcode = -ENOMEM;
+					else
+						(*wnames)[0] = NULL;
 				}
 
 				if (!errcode) {
@@ -414,20 +418,25 @@ p9pdu_vreadf(struct p9_fcall *pdu, int proto_version, const char *fmt,
 								proto_version,
 								"s",
 								&(*wnames)[i]);
-						if (errcode)
+						if (errcode) {
+							(*wnames)[i] = NULL;
 							break;
+						}
 					}
 				}
 
 				if (errcode) {
 					if (*wnames) {
 						int i;
 
-						for (i = 0; i < *nwname; i++)
+						for (i = 0; i < *nwname; i++) {
+							if (!(*wnames)[i])
+								break;
 							kfree((*wnames)[i]);
+						}
+						kfree(*wnames);
+						*wnames = NULL;
 					}
-					kfree(*wnames);
-					*wnames = NULL;
 				}
 			}
 			break;