linux-kernel - [PATCH 3.12 094/175] mm/nommu.c: fix arithmetic overflow in __vm_enough

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Message-Id: <56ade69589c4c04442f103b1f0d5305851c3071b.1426581621.git.jslaby@suse.cz>
Date:	Tue, 17 Mar 2015 09:41:12 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	Roman Gushchin <klamm@...dex-team.ru>,
	Andrew Shewmaker <agshew@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 094/175] mm/nommu.c: fix arithmetic overflow in __vm_enough_memory()

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

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

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>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 mm/nommu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/nommu.c b/mm/nommu.c
index 1221d2b66e97..97d19be38233 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1904,7 +1904,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
  */
 int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 {
-	unsigned long free, allowed, reserve;
+	long free, allowed, reserve;
 
 	vm_acct_memory(pages);
 
@@ -1969,7 +1969,7 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 	 */
 	if (mm) {
 		reserve = sysctl_user_reserve_kbytes >> (PAGE_SHIFT - 10);
-		allowed -= min(mm->total_vm / 32, reserve);
+		allowed -= min_t(long, mm->total_vm / 32, reserve);
 	}
 
 	if (percpu_counter_read_positive(&vm_committed_as) < allowed)
-- 
2.3.0

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