kasan: guard release_free_meta() shadow access with kasan_arch_is_ready()

release_free_meta() accesses the shadow directly through the path

  kasan_slab_free
    __kasan_slab_free
      kasan_release_object_meta
        release_free_meta
          kasan_mem_to_shadow

There are no kasan_arch_is_ready() guards here, allowing an oops when the
shadow is not initialized.  The oops can be seen on a Power8 KVM guest.

This patch adds the guard to release_free_meta(), as it's the first level
that specifically requires the shadow.

It is safe to put the guard at the start of this function, before the
stack put: only kasan_save_free_info() can initialize the saved stack,
which itself is guarded with kasan_arch_is_ready() by its caller
poison_slab_object().  If the arch becomes ready before
release_free_meta() then we will not observe KASAN_SLAB_FREE_META in the
object's shadow, so we will not put an uninitialized stack either.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 63b85ac56a64 ("kasan: stop leaking stack trace handles")
Signed-off-by: Benjamin Gray <[email protected]>
Reviewed-by: Andrey Konovalov <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 3 insertions, 0 deletions

diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index df6627f62402..032bf3e98c24 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -522,6 +522,9 @@ static void release_alloc_meta(struct kasan_alloc_meta *meta)
 
 static void release_free_meta(const void *object, struct kasan_free_meta *meta)
 {
+	if (!kasan_arch_is_ready())
+		return;
+
 	/* Check if free meta is valid. */
 	if (*(u8 *)kasan_mem_to_shadow(object) != KASAN_SLAB_FREE_META)
 		return;