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.

[akpm@linux-foundation.org: use min_t]
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>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -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)