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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, Denis Kirjanov <kda@linux-powerpc.org>,
        "Lee Schermerhorn" <lee.schermerhorn@hp.com>,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        "Entropy Moe" <3ntr0py1337@gmail.com>,
        "Randy Dunlap" <rdunlap@infradead.org>,
        syzbot+b055b1a6b2b958707a21@syzkaller.appspotmail.com
Date:   Fri, 24 Apr 2020 00:07:33 +0100
Message-ID: <lsq.1587683028.540912170@decadent.org.uk>
Subject: [PATCH 3.16 226/245] mm: mempolicy: require at least one nodeid
 for MPOL_PREFERRED
In-Reply-To: <lsq.1587683027.831233700@decadent.org.uk>
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3.16.83-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Randy Dunlap <rdunlap@infradead.org>

commit aa9f7d5172fac9bf1f09e678c35e287a40a7b7dd upstream.

Using an empty (malformed) nodelist that is not caught during mount option
parsing leads to a stack-out-of-bounds access.

The option string that was used was: "mpol=prefer:,".  However,
MPOL_PREFERRED requires a single node number, which is not being provided
here.

Add a check that 'nodes' is not empty after parsing for MPOL_PREFERRED's
nodeid.

Fixes: 095f1fc4ebf3 ("mempolicy: rework shmem mpol parsing and display")
Reported-by: Entropy Moe <3ntr0py1337@gmail.com>
Reported-by: syzbot+b055b1a6b2b958707a21@syzkaller.appspotmail.com
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: syzbot+b055b1a6b2b958707a21@syzkaller.appspotmail.com
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Link: http://lkml.kernel.org/r/89526377-7eb6-b662-e1d8-4430928abde9@infradead.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 mm/mempolicy.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2711,7 +2711,9 @@ int mpol_parse_str(char *str, struct mem
 	switch (mode) {
 	case MPOL_PREFERRED:
 		/*
-		 * Insist on a nodelist of one node only
+		 * Insist on a nodelist of one node only, although later
+		 * we use first_node(nodes) to grab a single node, so here
+		 * nodelist (or nodes) cannot be empty.
 		 */
 		if (nodelist) {
 			char *rest = nodelist;
@@ -2719,6 +2721,8 @@ int mpol_parse_str(char *str, struct mem
 				rest++;
 			if (*rest)
 				goto out;
+			if (nodes_empty(nodes))
+				goto out;
 		}
 		break;
 	case MPOL_INTERLEAVE: