linux-kernel - [PATCH] mm: slab: fix potential double free in ___cache

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Message-Id: <20200926141341.785180-1-shakeelb@google.com>
Date:   Sat, 26 Sep 2020 07:13:41 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <guro@...com>, Ming Lei <ming.lei@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>, Jens Axboe <axboe@...nel.dk>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Shakeel Butt <shakeelb@...gle.com>,
        kernel test robot <rong.a.chen@...el.com>
Subject: [PATCH] mm: slab: fix potential double free in ___cache_free

With the commit 10befea91b61 ("mm: memcg/slab: use a single set of
kmem_caches for all allocations"), it becomes possible to call kfree()
from the slabs_destroy(). The functions cache_flusharray() and
do_drain() calls slabs_destroy() on array_cache of the local CPU without
updating the size of the array_cache. This enables the kfree() call from
the slabs_destroy() to recursively call cache_flusharray() which can
potentially call free_block() on the same elements of the array_cache of
the local CPU and causing double free and memory corruption.

To fix the issue, simply update the local CPU array_cache cache before
calling slabs_destroy().

Fixes: 10befea91b61 ("mm: memcg/slab: use a single set of kmem_caches for all allocations")
Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
Reviewed-by: Roman Gushchin <guro@...com>
Tested-by: Ming Lei <ming.lei@...hat.com>
Reported-by: kernel test robot <rong.a.chen@...el.com>
---
 mm/slab.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 3160dff6fd76..f658e86ec8ce 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1632,6 +1632,10 @@ static void slab_destroy(struct kmem_cache *cachep, struct page *page)
 		kmem_cache_free(cachep->freelist_cache, freelist);
 }
 
+/*
+ * Update the size of the caches before calling slabs_destroy as it may
+ * recursively call kfree.
+ */
 static void slabs_destroy(struct kmem_cache *cachep, struct list_head *list)
 {
 	struct page *page, *n;
@@ -2153,8 +2157,8 @@ static void do_drain(void *arg)
 	spin_lock(&n->list_lock);
 	free_block(cachep, ac->entry, ac->avail, node, &list);
 	spin_unlock(&n->list_lock);
-	slabs_destroy(cachep, &list);
 	ac->avail = 0;
+	slabs_destroy(cachep, &list);
 }
 
 static void drain_cpu_caches(struct kmem_cache *cachep)
@@ -3402,9 +3406,9 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
 	}
 #endif
 	spin_unlock(&n->list_lock);
-	slabs_destroy(cachep, &list);
 	ac->avail -= batchcount;
 	memmove(ac->entry, &(ac->entry[batchcount]), sizeof(void *)*ac->avail);
+	slabs_destroy(cachep, &list);
 }
 
 /*
-- 
2.28.0.681.g6f77f65b4e-goog