[PATCH v3] net: 9p: avoid freeing uninit memory in p9pdu_vreadf

[Fedor Pchelkin]

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.

In order not to complicate the code with array index processing, fix the
problem with initializing *wnames to NULL in beginning of case 'T' and
using kcalloc() to allocate and initialize the array. For assurance,
nullify the failing *wnames element (the callee handles that already -
e.g. see 's' case).

Found by Linux Verification Center (linuxtesting.org).

Fixes: ace51c4dd2f9 ("9p: add new protocol support code")
Signed-off-by: Fedor Pchelkin <pchelkin@xxxxxxxxx>
---
v2: I've missed that *wnames can also be left uninitialized. Please
ignore the patch v1. As an answer to Dominique's comment: my
organization marks this statement in all commits.
v3: Simplify the patch by using kcalloc() instead of array indices
manipulation per Christian Schoenebeck's remark. Update the commit
message accordingly.

 net/9p/protocol.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/net/9p/protocol.c b/net/9p/protocol.c
index 4e3a2a1ffcb3..7067fb49d713 100644
--- a/net/9p/protocol.c
+++ b/net/9p/protocol.c
@@ -394,13 +394,14 @@ 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) {
 					*wnames =
-					    kmalloc_array(*nwname,
-							  sizeof(char *),
-							  GFP_NOFS);
+					    kcalloc(*nwname, sizeof(char *),
+						    GFP_NOFS);
 					if (!*wnames)
 						errcode = -ENOMEM;
 				}
@@ -414,8 +415,10 @@ 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;
+						}
 					}
 				}
 
@@ -425,9 +428,9 @@ p9pdu_vreadf(struct p9_fcall *pdu, int proto_version, const char *fmt,
 
 						for (i = 0; i < *nwname; i++)
 							kfree((*wnames)[i]);
+						kfree(*wnames);
+						*wnames = NULL;
 					}
-					kfree(*wnames);
-					*wnames = NULL;
 				}
 			}
 			break;
-- 
2.43.0