linux-kernel - [PATCH 3.2 038/221] mm/nommu.c: fix arithmetic overflow in __vm_enough

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Date:	Tue, 05 May 2015 02:16:39 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Andrew Shewmaker" <agshew@...il.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Konstantin Khlebnikov" <khlebnikov@...dex-team.ru>,
	"Rik van Riel" <riel@...hat.com>,
	"Roman Gushchin" <klamm@...dex-team.ru>
Subject: [PATCH 3.2 038/221] mm/nommu.c: fix arithmetic overflow in
 __vm_enough_memory()

3.2.69-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Roman Gushchin <klamm@...dex-team.ru>

commit 8138a67a5557ffea3a21dfd6f037842d4e748513 upstream.

I noticed that "allowed" can easily overflow by falling below 0, because
(total_vm / 32) can be larger than "allowed".  The problem occurs in
OVERCOMMIT_NONE mode.

In this case, a huge allocation can success and overcommit the system
(despite OVERCOMMIT_NONE mode).  All subsequent allocations will fall
(system-wide), so system become unusable.

The problem was masked out by commit c9b1d0981fcc
("mm: limit growth of 3% hardcoded other user reserve"),
but it's easy to reproduce it on older kernels:
1) set overcommit_memory sysctl to 2
2) mmap() large file multiple times (with VM_SHARED flag)
3) try to malloc() large amount of memory

It also can be reproduced on newer kernels, but miss-configured
sysctl_user_reserve_kbytes is required.

Fix this issue by switching to signed arithmetic here.

Signed-off-by: Roman Gushchin <klamm@...dex-team.ru>
Cc: Andrew Shewmaker <agshew@...il.com>
Cc: Rik van Riel <riel@...hat.com>
Cc: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2: there is no 'reserved' variable]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1885,7 +1885,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
  */
 int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 {
-	unsigned long free, allowed;
+	long free, allowed;
 
 	vm_acct_memory(pages);
 

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