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 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.

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>

diff --git a/mm/nommu.c b/mm/nommu.c
index 4d1b8a19986770ad56f5ad58dc9d0250745851da..1a19fb3b04635549f63d90c9e62408cf55ad162c 100644 (file)
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -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)