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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Wed, 13 Feb 2019 11:31:07 +0100
Message-ID: <CAAeHK+w-EWDivYTNiUAeSUVZVGOpUyxbbcC8_nMM1=CcpsJ9Ug@mail.gmail.com>
Subject: Re: [PATCH] slub: untag object before slab end
To:     Qian Cai <cai@lca.pw>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Feb 13, 2019 at 3:06 AM Qian Cai <cai@lca.pw> wrote:
>
> get_freepointer() could return NULL if there is no more free objects in
> the slab. However, it could return a tagged pointer (like
> 0x2200000000000000) with KASAN_SW_TAGS which would escape the NULL
> object checking in check_valid_pointer() and trigger errors below, so
> untag the object before checking for a NULL object there.

I think this solution is just masking the issue. get_freepointer()
shouldn't return tagged NULLs. Apparently when we save a freelist
pointer, the object where the pointer gets written is tagged
differently, than this same object when the pointer gets read. I found
one case where this happens (the last patch out my 5 patch series),
but apparently there are more.

>
> [   35.797667] BUG kmalloc-256 (Not tainted): Freepointer corrupt
> [   35.803584] -----------------------------------------------------------------------------
> [   35.803584]
> [   35.813368] Disabling lock debugging due to kernel taint
> [   35.818766] INFO: Allocated in build_sched_domains+0x28c/0x495c age=92 cpu=0 pid=1
> [   35.826443]  __kmalloc_node+0x4ac/0x508
> [   35.830343]  build_sched_domains+0x28c/0x495c
> [   35.834764]  sched_init_domains+0x184/0x1d8
> [   35.839012]  sched_init_smp+0x38/0xd4
> [   35.842732]  kernel_init_freeable+0x67c/0x1104
> [   35.847243]  kernel_init+0x18/0x2a4
> [   35.850790]  ret_from_fork+0x10/0x18
> [   35.854423] INFO: Freed in destroy_sched_domain+0xa0/0xcc age=11 cpu=0 pid=1
> [   35.861569]  destroy_sched_domain+0xa0/0xcc
> [   35.865814]  cpu_attach_domain+0x304/0xb34
> [   35.869971]  build_sched_domains+0x4654/0x495c
> [   35.874480]  sched_init_domains+0x184/0x1d8
> [   35.878724]  sched_init_smp+0x38/0xd4
> [   35.882443]  kernel_init_freeable+0x67c/0x1104
> [   35.886953]  kernel_init+0x18/0x2a4
> [   35.890495]  ret_from_fork+0x10/0x18
> [   35.894128] INFO: Slab 0x(____ptrval____) objects=85 used=0 fp=0x(____ptrval____) flags=0x7ffffffc000200
> [   35.903733] INFO: Object 0x(____ptrval____) @offset=38528 fp=0x(____ptrval____)
> [   35.903733]
> [   35.912637] Redzone (____ptrval____): bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
> [   35.922155] Redzone (____ptrval____): bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
> [   35.931672] Redzone (____ptrval____): bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
> [   35.941190] Redzone (____ptrval____): bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
> [   35.950707] Redzone (____ptrval____): bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
> [   35.960224] Redzone (____ptrval____): bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
> [   35.969741] Redzone (____ptrval____): bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
> [   35.979258] Redzone (____ptrval____): bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
> [   35.988776] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   35.998206] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.007636] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.017065] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.026494] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.035923] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.045353] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.054783] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.064212] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.073642] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.083071] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.092501] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.101930] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.111359] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.120788] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> [   36.130218] Object (____ptrval____): 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  kkkkkkkkkkkkkkk.
> [   36.139647] Redzone (____ptrval____): bb bb bb bb bb bb bb bb                          ........
> [   36.148462] Padding (____ptrval____): 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
> [   36.157979] Padding (____ptrval____): 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
> [   36.167496] Padding (____ptrval____): 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
> [   36.177021] CPU: 0 PID: 1 Comm: swapper/0 Tainted: G    B             5.0.0-rc6+ #41
> [   36.184854] Call trace:
> [   36.187328]  dump_backtrace+0x0/0x450
> [   36.191032]  show_stack+0x20/0x2c
> [   36.194385]  __dump_stack+0x20/0x28
> [   36.197911]  dump_stack+0xa0/0xfc
> [   36.201265]  print_trailer+0x1a8/0x1bc
> [   36.205057]  object_err+0x40/0x50
> [   36.208408]  check_object+0x214/0x2b8
> [   36.212111]  __free_slab+0x9c/0x31c
> [   36.215638]  discard_slab+0x78/0xa8
> [   36.219165]  kfree+0x918/0x980
> [   36.222259]  destroy_sched_domain+0xa0/0xcc
> [   36.226489]  cpu_attach_domain+0x304/0xb34
> [   36.230631]  build_sched_domains+0x4654/0x495c
> [   36.235125]  sched_init_domains+0x184/0x1d8
> [   36.239357]  sched_init_smp+0x38/0xd4
> [   36.243060]  kernel_init_freeable+0x67c/0x1104
> [   36.247555]  kernel_init+0x18/0x2a4
> [   36.251083]  ret_from_fork+0x10/0x18
>
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>
> Depends on slub-fix-slab_consistency_checks-kasan_sw_tags.patch.
>
>  mm/slub.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 4a61959e1887..2fd1cf39914c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -503,11 +503,11 @@ static inline int check_valid_pointer(struct kmem_cache *s,
>  {
>         void *base;
>
> +       object = kasan_reset_tag(object);
>         if (!object)
>                 return 1;
>
>         base = page_address(page);
> -       object = kasan_reset_tag(object);
>         object = restore_red_left(s, object);
>         if (object < base || object >= base + page->objects * s->size ||
>                 (object - base) % s->size) {
> --
> 2.17.2 (Apple Git-113)
>