lib/stackdepot.c: fix global out-of-bounds in stack_slabs

Walter Wu has reported a potential case in which init_stack_slab() is
called after stack_slabs[STACK_ALLOC_MAX_SLABS - 1] has already been
initialized.  In that case init_stack_slab() will overwrite
stack_slabs[STACK_ALLOC_MAX_SLABS], which may result in a memory
corruption.

Link: http://lkml.kernel.org/r/20200218102950.260263-1-glider@google.com
Fixes: cd11016e5f521 ("mm, kasan: stackdepot implementation. Enable stackdepot for SLAB")
Signed-off-by: Alexander Potapenko <glider@google.com>
Reported-by: Walter Wu <walter-zh.wu@mediatek.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index ed717dd..81c69c0 100644 (file)
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -83,15 +83,19 @@ static bool init_stack_slab(void **prealloc)
                return true;
        if (stack_slabs[depot_index] == NULL) {
                stack_slabs[depot_index] = *prealloc;
+               *prealloc = NULL;
        } else {
-               stack_slabs[depot_index + 1] = *prealloc;
+               /* If this is the last depot slab, do not touch the next one. */
+               if (depot_index + 1 < STACK_ALLOC_MAX_SLABS) {
+                       stack_slabs[depot_index + 1] = *prealloc;
+                       *prealloc = NULL;
+               }
                /*
                 * This smp_store_release pairs with smp_load_acquire() from
                 * |next_slab_inited| above and in stack_depot_save().
                 */
                smp_store_release(&next_slab_inited, 1);
        }
-       *prealloc = NULL;
        return true;
 }