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From: Roman Gushchin <klamm@yandex-team.ru>
To: linux-mm@kvack.kernel.org
Cc: linux-kernel@vger.kernel.org,
	Roman Gushchin <klamm@yandex-team.ru>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrew Shewmaker <agshew@gmail.com>,
	Rik van Riel <riel@redhat.com>,
	Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
	stable@vger.kernel.org
Subject: [PATCH] mm/nommu.c: fix arithmetic overflow in __vm_enough_memory()
Date: Fri, 30 Jan 2015 16:09:45 +0300	[thread overview]
Message-ID: <1422623385-13614-1-git-send-email-klamm@yandex-team.ru> (raw)
In-Reply-To: <20150129195735.GA9331@scruffy>

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 Morton <akpm@linux-foundation.org>
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
---
 mm/nommu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/nommu.c b/mm/nommu.c
index b51eadf..e1fd67b 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1894,7 +1894,7 @@ 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);
 
@@ -1958,7 +1958,7 @@ 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)
-- 
2.1.0


  reply	other threads:[~2015-01-30 13:21 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-29 13:06 [PATCH] mm: " Roman Gushchin
2015-01-29 19:57 ` Andrew Shewmaker
2015-01-30 13:09   ` Roman Gushchin [this message]
2015-01-30 13:14   ` Roman Gushchin
2015-02-03 13:29 ` Michal Hocko

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