[PATCH 3.13.y-ckt 091/143] mm/nommu.c: fix arithmetic overflow in __vm_enough_memory()

[Kamal Mostafa]

3.13.11-ckt18 -stable review patch.  If anyone has any objections, please let me know.

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

From: Roman Gushchin <klamm at yandex-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 at yandex-team.ru>
Cc: Andrew Shewmaker <agshew at gmail.com>
Cc: Rik van Riel <riel at redhat.com>
Cc: Konstantin Khlebnikov <khlebnikov at yandex-team.ru>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 mm/nommu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/nommu.c b/mm/nommu.c
index fec093a..0f28834 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1896,7 +1896,7 @@ 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);
 
@@ -1960,7 +1960,7 @@ 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)
-- 
1.9.1