+ mm-memcg-use-read_once-write_once-to-access-stock-cached.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm: memcg: use READ_ONCE()/WRITE_ONCE() to access stock->cached
has been added to the -mm mm-unstable branch.  Its filename is
     mm-memcg-use-read_once-write_once-to-access-stock-cached.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memcg-use-read_once-write_once-to-access-stock-cached.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Roman Gushchin <roman.gushchin@linux.dev>
Subject: mm: memcg: use READ_ONCE()/WRITE_ONCE() to access stock->cached
Date: Tue, 2 May 2023 09:08:39 -0700

A memcg pointer in the percpu stock can be accessed by drain_all_stock()
from another cpu in a lockless way.  In theory it might lead to an issue,
similar to the one which has been discovered with stock->cached_objcg,
where the pointer was zeroed between the check for being NULL and
dereferencing.  In this case the issue is unlikely a real problem, but to
make it bulletproof and similar to stock->cached_objcg, let's annotate all
accesses to stock->cached with READ_ONCE()/WTRITE_ONCE().

Link: https://lkml.kernel.org/r/20230502160839.361544-2-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memcontrol.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/mm/memcontrol.c~mm-memcg-use-read_once-write_once-to-access-stock-cached
+++ a/mm/memcontrol.c
@@ -2275,7 +2275,7 @@ static bool consume_stock(struct mem_cgr
 	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
-	if (memcg == stock->cached && stock->nr_pages >= nr_pages) {
+	if (memcg == READ_ONCE(stock->cached) && stock->nr_pages >= nr_pages) {
 		stock->nr_pages -= nr_pages;
 		ret = true;
 	}
@@ -2290,7 +2290,7 @@ static bool consume_stock(struct mem_cgr
  */
 static void drain_stock(struct memcg_stock_pcp *stock)
 {
-	struct mem_cgroup *old = stock->cached;
+	struct mem_cgroup *old = READ_ONCE(stock->cached);
 
 	if (!old)
 		return;
@@ -2303,7 +2303,7 @@ static void drain_stock(struct memcg_sto
 	}
 
 	css_put(&old->css);
-	stock->cached = NULL;
+	WRITE_ONCE(stock->cached, NULL);
 }
 
 static void drain_local_stock(struct work_struct *dummy)
@@ -2338,10 +2338,10 @@ static void __refill_stock(struct mem_cg
 	struct memcg_stock_pcp *stock;
 
 	stock = this_cpu_ptr(&memcg_stock);
-	if (stock->cached != memcg) { /* reset if necessary */
+	if (READ_ONCE(stock->cached) != memcg) { /* reset if necessary */
 		drain_stock(stock);
 		css_get(&memcg->css);
-		stock->cached = memcg;
+		WRITE_ONCE(stock->cached, memcg);
 	}
 	stock->nr_pages += nr_pages;
 
@@ -2383,7 +2383,7 @@ static void drain_all_stock(struct mem_c
 		bool flush = false;
 
 		rcu_read_lock();
-		memcg = stock->cached;
+		memcg = READ_ONCE(stock->cached);
 		if (memcg && stock->nr_pages &&
 		    mem_cgroup_is_descendant(memcg, root_memcg))
 			flush = true;
_

Patches currently in -mm which might be from roman.gushchin@linux.dev are

mm-kmem-fix-a-null-pointer-dereference-in-obj_stock_flush_required.patch
mm-memcg-use-read_once-write_once-to-access-stock-cached.patch