Commit 2fc66e9f authored by Roman Gushchin's avatar Roman Gushchin Committed by Luis Henriques

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

commit 5703b087 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 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.

[akpm@linux-foundation.org: use min_t]
Signed-off-by: default avatarRoman 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: default avatarMichal Hocko <mhocko@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarLuis Henriques <luis.henriques@canonical.com>
parent 6333e244
...@@ -132,7 +132,7 @@ EXPORT_SYMBOL_GPL(vm_memory_committed); ...@@ -132,7 +132,7 @@ EXPORT_SYMBOL_GPL(vm_memory_committed);
*/ */
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, reserve; long free, allowed, reserve;
vm_acct_memory(pages); vm_acct_memory(pages);
...@@ -196,7 +196,7 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin) ...@@ -196,7 +196,7 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
*/ */
if (mm) { if (mm) {
reserve = sysctl_user_reserve_kbytes >> (PAGE_SHIFT - 10); 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) if (percpu_counter_read_positive(&vm_committed_as) < allowed)
......
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