[PATCH 2/2] slab: Add naive detection of double free

Kees Cook
2020-06-17T19:54:07Z

Similar to commit ce6fa91b9363 ("mm/slub.c: add a naive detection
of double free or corruption"), add a very cheap double-free check
for SLAB under CONFIG_SLAB_FREELIST_HARDENED. With this added, the
"SLAB_FREE_DOUBLE" LKDTM test passes under SLAB:

lkdtm: Performing direct entry SLAB_FREE_DOUBLE
lkdtm: Attempting double slab free ...
------------[ cut here ]------------
WARNING: CPU: 2 PID: 2193 at mm/slab.c:757 ___cache _free+0x325/0x390

Signed-off-by: Kees Cook <keescook@chromium.org>
---
mm/slab.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 9350062ffc1a..c4e3a194b271 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -749,6 +749,16 @@ static void drain_alien_cache(struct kmem_cache *cachep,
}
}

+/* &alien->lock must be held by alien callers. */
+static __always_inline void __free_one(struct array_cache *ac, void *objp)
+{
+ /* Avoid trivial double-free. */
+ if (IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
+ WARN_ON_ONCE(ac->avail > 0 && ac->entry[ac->avail - 1] == objp))
+ return;
+ ac->entry[ac->avail++] = objp;
+}
+
static int __cache_free_alien(struct kmem_cache *cachep, void *objp,
int node, int page_node)
{
@@ -767,7 +777,7 @@ static int __cache_free_alien(struct kmem_cache *cachep, void *objp,
STATS_INC_ACOVERFLOW(cachep);
__drain_alien_cache(cachep, ac, page_node, &list);
}
- ac->entry[ac->avail++] = objp;
+ __free_one(ac, objp);
spin_unlock(&alien->lock);
slabs_destroy(cachep, &list);
} else {
@@ -3466,7 +3476,7 @@ void ___cache_free(struct kmem_cache *cachep, void *objp,
}
}

- ac->entry[ac->avail++] = objp;
+ __free_one(ac, objp);
}

/**
--
2.25.1