linux-kernel - [PATCH 1/2] uprobes: fix the memcg accounting

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Date:	Wed, 17 Aug 2016 17:36:29 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Brenden Blanco <bblanco@...mgrid.com>,
	Ingo Molnar <mingo@...hat.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: [PATCH 1/2] uprobes: fix the memcg accounting

__replace_page() wronlgy calls mem_cgroup_cancel_charge() in "success" path,
it should only do this if page_check_address() fails.

This means that every enable/disable leads to unbalanced mem_cgroup_uncharge()
from put_page(old_page), it is trivial to underflow the page_counter->count
and trigger OOM.

Reported-and-tested-by: Brenden Blanco <bblanco@...mgrid.com>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Acked-by: Michal Hocko <mhocko@...nel.org>
Reviewed-by: Johannes Weiner <hannes@...xchg.org>
Fixes: 00501b531c47 ("mm: memcontrol: rewrite charge API")
Cc: stable@...r.kernel.org # 3.17+
---
 kernel/events/uprobes.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index b7a525a..8c50276 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -172,8 +172,10 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
 	err = -EAGAIN;
 	ptep = page_check_address(page, mm, addr, &ptl, 0);
-	if (!ptep)
+	if (!ptep) {
+		mem_cgroup_cancel_charge(kpage, memcg, false);
 		goto unlock;
+	}
 
 	get_page(kpage);
 	page_add_new_anon_rmap(kpage, vma, addr, false);
@@ -200,7 +202,6 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 
 	err = 0;
  unlock:
-	mem_cgroup_cancel_charge(kpage, memcg, false);
 	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
 	unlock_page(page);
 	return err;
-- 
2.5.0