Commit 8138a67a authored Feb 11, 2015 by Roman Gushchin Committed by Linus Torvalds Feb 11, 2015

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



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 c9b1d098
("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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 5703b087

mm/nommu.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -1928,7 +1928,7 @@ 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);

		@@ -1992,7 +1992,7 @@ 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)

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