2021-05-14 14:35:40

by Alexander Potapenko

Subject: [PATCH] kasan: slab: always reset the tag in get_freepointer_safe()

With CONFIG_DEBUG_PAGEALLOC enabled, the kernel should also untag the
object pointer, as done in get_freepointer().

Failing to do so reportedly leads to SLUB freelist corruptions that
manifest as boot-time crashes.

Cc: Andrew Morton <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Elliot Berman <[email protected]>
Cc: [email protected]
Signed-off-by: Alexander Potapenko <[email protected]>
---
mm/slub.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/mm/slub.c b/mm/slub.c
index feda53ae62ba..9a4f59e5b0c2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -301,6 +301,7 @@ static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
if (!debug_pagealloc_enabled_static())
return get_freepointer(s, object);

+ object = kasan_reset_tag(object);
freepointer_addr = (unsigned long)object + s->offset;
copy_from_kernel_nofault(&p, (void **)freepointer_addr, sizeof(p));
return freelist_ptr(s, p, freepointer_addr);
--
2.31.1.751.gd2f1c929bd-goog

2021-05-15 01:48:15

by Alexander Potapenko

Subject: Re: [PATCH] kasan: slab: always reset the tag in get_freepointer_safe()

On Fri, May 14, 2021 at 9:22 AM Alexander Potapenko <[email protected]> wrote:
>
> With CONFIG_DEBUG_PAGEALLOC enabled, the kernel should also untag the
> object pointer, as done in get_freepointer().
>
> Failing to do so reportedly leads to SLUB freelist corruptions that
> manifest as boot-time crashes.
>
> Cc: Andrew Morton <[email protected]>
> Cc: Marco Elver <[email protected]>
> Cc: Vincenzo Frascino <[email protected]>
> Cc: Andrey Ryabinin <[email protected]>
> Cc: Andrey Konovalov <[email protected]>
> Cc: Elliot Berman <[email protected]>
> Cc: [email protected]
> Signed-off-by: Alexander Potapenko <[email protected]>
> ---
> mm/slub.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index feda53ae62ba..9a4f59e5b0c2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -301,6 +301,7 @@ static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
> if (!debug_pagealloc_enabled_static())
> return get_freepointer(s, object);
>
> + object = kasan_reset_tag(object);
> freepointer_addr = (unsigned long)object + s->offset;
> copy_from_kernel_nofault(&p, (void **)freepointer_addr, sizeof(p));
> return freelist_ptr(s, p, freepointer_addr);
> --
> 2.31.1.751.gd2f1c929bd-goog
>


--
Alexander Potapenko
Software Engineer

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