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mm/mmap.c: fix arithmetic overflow in __vm_enough_memory()

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commit 5703b087dc8eaf47bfb399d6cf512d471beff405 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 c9b1d0981f
("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.

[akpm@linux-foundation.org: use min_t]
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>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[lizf: Backported to 3.4:
 - adjust context
 - there's no variable reserve]
Signed-off-by: Zefan Li <lizefan@huawei.com>
This commit is contained in:
Roman Gushchin 2015-02-11 15:28:39 -08:00 committed by Zefan Li
parent 2a4edc62b3
commit dcdcb2bd6b
1 changed files with 1 additions and 1 deletions
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2
mm/mmap.c
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@ -112,7 +112,7 @@ struct percpu_counter vm_committed_as ____cacheline_aligned_in_smp;
*/ */
int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin) 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); vm_acct_memory(pages);