Commit 5703b087 authored by Roman Gushchin's avatar Roman Gushchin Committed by Linus Torvalds
Browse files

mm/mmap.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.

[ use min_t]
Signed-off-by: default avatarRoman Gushchin <>
Cc: Andrew Shewmaker <>
Cc: Rik van Riel <>
Cc: Konstantin Khlebnikov <>
Reviewed-by: default avatarMichal Hocko <>
Cc: <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 57c2e36b
......@@ -152,7 +152,7 @@ EXPORT_SYMBOL_GPL(vm_memory_committed);
int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
unsigned long free, allowed, reserve;
long free, allowed, reserve;
VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
-(s64)vm_committed_as_batch * num_online_cpus(),
......@@ -220,7 +220,7 @@ 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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