9p/net: put a lower bound on msize

If the requested msize is too small (either from command line argument
or from the server version reply), we won't get any work done.
If it's *really* too small, nothing will work, and this got caught by
syzbot recently (on a new kmem_cache_create_usercopy() call)

Just set a minimum msize to 4k in both code paths, until someone
complains they have a use-case for a smaller msize.

We need to check in both mount option and server reply individually
because the msize for the first version request would be unchecked
with just a global check on clnt->msize.

Link: http://lkml.kernel.org/r/1541407968-31350-1-git-send-email-asmadeus@codewreck.org
Reported-by: syzbot+0c1d61e4db7db94102ca@syzkaller.appspotmail.com
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: stable@vger.kernel.org

net/9p/client.c

@@ -181,6 +181,12 @@ static int parse_opts(char *opts, struct p9_client *clnt)
 				ret = r;
 				continue;
 			}
+			if (option < 4096) {
+				p9_debug(P9_DEBUG_ERROR,
+					 "msize should be at least 4k\n");
+				ret = -EINVAL;
+				continue;
+			}
 			clnt->msize = option;
 			break;
 		case Opt_trans:
@@ -983,10 +989,18 @@ static int p9_client_version(struct p9_client *c)
 	else if (!strncmp(version, "9P2000", 6))
 		c->proto_version = p9_proto_legacy;
 	else {
+		p9_debug(P9_DEBUG_ERROR,
+			 "server returned an unknown version: %s\n", version);
 		err = -EREMOTEIO;
 		goto error;
 	}
 
+	if (msize < 4096) {
+		p9_debug(P9_DEBUG_ERROR,
+			 "server returned a msize < 4096: %d\n", msize);
+		err = -EREMOTEIO;
+		goto error;
+	}
 	if (msize < c->msize)
 		c->msize = msize;
 
@@ -1043,6 +1057,13 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
 	if (clnt->msize > clnt->trans_mod->maxsize)
 		clnt->msize = clnt->trans_mod->maxsize;
 
+	if (clnt->msize < 4096) {
+		p9_debug(P9_DEBUG_ERROR,
+			 "Please specify a msize of at least 4k\n");
+		err = -EINVAL;
+		goto free_client;
+	}
+
 	err = p9_client_version(clnt);
 	if (err)
 		goto close_trans;