--- /dev/null
+From 574d356b7a02c7e1b01a1d9cba8a26b3c2888f45 Mon Sep 17 00:00:00 2001
+From: Dominique Martinet <dominique.martinet@cea.fr>
+Date: Mon, 5 Nov 2018 09:52:48 +0100
+Subject: 9p/net: put a lower bound on msize
+
+From: Dominique Martinet <dominique.martinet@cea.fr>
+
+commit 574d356b7a02c7e1b01a1d9cba8a26b3c2888f45 upstream.
+
+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
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+
+---
+ net/9p/client.c | 21 +++++++++++++++++++++
+ 1 file changed, 21 insertions(+)
+
+--- a/net/9p/client.c
++++ b/net/9p/client.c
+@@ -155,6 +155,12 @@ static int parse_opts(char *opts, struct
+ 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:
+@@ -979,10 +985,18 @@ static int p9_client_version(struct p9_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;
+
+@@ -1047,6 +1061,13 @@ struct p9_client *p9_client_create(const
+ 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;