[PATCH 3.4 031/172] mm/nommu.c: fix arithmetic overflow in __vm_enough_memory()

From: lizf
Date: Tue Jun 16 2015 - 04:42:11 EST

From: Roman Gushchin <klamm@xxxxxxxxxxxxxx>

3.4.108-rc1 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

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@xxxxxxxxxxxxxx>
Cc: Andrew Shewmaker <agshew@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[lizf: Backported to 3.4:
- adjust context
- there's no variable reserve]
Signed-off-by: Zefan Li <lizefan@xxxxxxxxxx>
mm/nommu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/nommu.c b/mm/nommu.c
index d3afb47..3f282f9 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1916,7 +1916,7 @@ 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;



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