[PATCH 3.4 030/172] mm/mmap.c: fix arithmetic overflow in __vm_enough_memory()

[lizf]

From: Roman Gushchin <klamm@xxxxxxxxxxxxxx>

3.4.108-rc1 review patch.  If anyone has any objections, please let me know.

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


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 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.

[akpm@xxxxxxxxxxxxxxxxxxxx: use min_t]
Signed-off-by: Roman Gushchin <klamm@xxxxxxxxxxxxxx>
Cc: Andrew Shewmaker <agshew@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
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/mmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 208e70f..cb6456d 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -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)
 {
-	unsigned long free, allowed;
+	long free, allowed;
 
 	vm_acct_memory(pages);
 
-- 
1.9.1

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