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From:   Vitaly Wool <vitalywool@gmail.com>
Date:   Tue, 2 Jul 2019 10:45:46 +0300
Message-ID: <CAMJBoFPbRcdZ+NnX17OQ-sOcCwe+ZAsxcDJoR0KDkgBY9WXvpg@mail.gmail.com>
Subject: Re: [PATCH] mm/z3fold: Fix z3fold_buddy_slots use after free
To:     Henry Burns <henryburns@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        Vitaly Vul <vitaly.vul@sony.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Xidong Wang <wangxidong_97@163.com>,
        Shakeel Butt <shakeelb@google.com>,
        Jonathan Adams <jwadams@google.com>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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Hi Henry,

On Mon, Jul 1, 2019 at 8:31 PM Henry Burns <henryburns@google.com> wrote:
>
> Running z3fold stress testing with address sanitization
> showed zhdr->slots was being used after it was freed.
>
> z3fold_free(z3fold_pool, handle)
>   free_handle(handle)
>     kmem_cache_free(pool->c_handle, zhdr->slots)
>   release_z3fold_page_locked_list(kref)
>     __release_z3fold_page(zhdr, true)
>       zhdr_to_pool(zhdr)
>         slots_to_pool(zhdr->slots)  *BOOM*

Thanks for looking into this. I'm not entirely sure I'm all for
splitting free_handle() but let me think about it.

> Instead we split free_handle into two functions, release_handle()
> and free_slots(). We use release_handle() in place of free_handle(),
> and use free_slots() to call kmem_cache_free() after
> __release_z3fold_page() is done.

A little less intrusive solution would be to move backlink to pool
from slots back to z3fold_header. Looks like it was a bad idea from
the start.

Best regards,
   Vitaly