mm, mempool: poison elements backed by slab allocator

Mempools keep elements in a reserved pool for contexts in which allocation
may not be possible.  When an element is allocated from the reserved pool,
its memory contents is the same as when it was added to the reserved pool.

Because of this, elements lack any free poisoning to detect use-after-free
errors.

This patch adds free poisoning for elements backed by the slab allocator.
This is possible because the mempool layer knows the object size of each
element.

When an element is added to the reserved pool, it is poisoned with
POISON_FREE.  When it is removed from the reserved pool, the contents are
checked for POISON_FREE.  If there is a mismatch, a warning is emitted to
the kernel log.

This is only effective for configs with CONFIG_DEBUG_SLAB or
CONFIG_SLUB_DEBUG_ON.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Dave Kleikamp <shaggy@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sebastian Ott <sebott@linux.vnet.ibm.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/mempool.c b/mm/mempool.c
index b60fb85526ed62937e4545262b2cbb0b9894315f..13336e7ae332fb6ce0e8e9dfdc6c2b577afc9935 100644 (file)
--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -17,16 +17,77 @@
 #include <linux/writeback.h>
 #include "slab.h"
 
+#if defined(CONFIG_DEBUG_SLAB) || defined(CONFIG_SLUB_DEBUG_ON)
+static void poison_error(mempool_t *pool, void *element, size_t size,
+                        size_t byte)
+{
+       const int nr = pool->curr_nr;
+       const int start = max_t(int, byte - (BITS_PER_LONG / 8), 0);
+       const int end = min_t(int, byte + (BITS_PER_LONG / 8), size);
+       int i;
+
+       pr_err("BUG: mempool element poison mismatch\n");
+       pr_err("Mempool %p size %ld\n", pool, size);
+       pr_err(" nr=%d @ %p: %s0x", nr, element, start > 0 ? "... " : "");
+       for (i = start; i < end; i++)
+               pr_cont("%x ", *(u8 *)(element + i));
+       pr_cont("%s\n", end < size ? "..." : "");
+       dump_stack();
+}
+
+static void check_slab_element(mempool_t *pool, void *element)
+{
+       if (pool->free == mempool_free_slab || pool->free == mempool_kfree) {
+               size_t size = ksize(element);
+               u8 *obj = element;
+               size_t i;
+
+               for (i = 0; i < size; i++) {
+                       u8 exp = (i < size - 1) ? POISON_FREE : POISON_END;
+
+                       if (obj[i] != exp) {
+                               poison_error(pool, element, size, i);
+                               return;
+                       }
+               }
+               memset(obj, POISON_INUSE, size);
+       }
+}
+
+static void poison_slab_element(mempool_t *pool, void *element)
+{
+       if (pool->alloc == mempool_alloc_slab ||
+           pool->alloc == mempool_kmalloc) {
+               size_t size = ksize(element);
+               u8 *obj = element;
+
+               memset(obj, POISON_FREE, size - 1);
+               obj[size - 1] = POISON_END;
+       }
+}
+#else /* CONFIG_DEBUG_SLAB || CONFIG_SLUB_DEBUG_ON */
+static inline void check_slab_element(mempool_t *pool, void *element)
+{
+}
+static inline void poison_slab_element(mempool_t *pool, void *element)
+{
+}
+#endif /* CONFIG_DEBUG_SLAB || CONFIG_SLUB_DEBUG_ON */
+
 static void add_element(mempool_t *pool, void *element)
 {
        BUG_ON(pool->curr_nr >= pool->min_nr);
+       poison_slab_element(pool, element);
        pool->elements[pool->curr_nr++] = element;
 }
 
 static void *remove_element(mempool_t *pool)
 {
-       BUG_ON(pool->curr_nr <= 0);
-       return pool->elements[--pool->curr_nr];
+       void *element = pool->elements[--pool->curr_nr];
+
+       BUG_ON(pool->curr_nr < 0);
+       check_slab_element(pool, element);
+       return element;
 }
 
 /**