[PATCH 3.19 037/177] mm/nommu.c: fix arithmetic overflow in __vm_enough_memory()

Greg Kroah-Hartman
2015-03-16T15:58:00Z

From: Roman Gushchin <klamm@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@yandex-team.ru>
Cc: Andrew Shewmaker <agshew@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/nommu.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1895,7 +1895,7 @@ 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);

@@ -1959,7 +1959,7 @@ int __vm_enough_memory(struct mm_struct
*/
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)