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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 21 Oct 2021 17:48:55 +0200
Message-ID: <CACT4Y+YxOynnn1DPUe_7HE_Dc+302YnhnO0a8awqG0rSV9-JMw@mail.gmail.com>
Subject: Re: [PATCH] mm/secretmem: Avoid letting secretmem_users drop to zero
To:     Kees Cook <keescook@chromium.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
        Jordy Zomer <jordy@pwning.systems>, linux-mm@kvack.org,
        James Bottomley <james.bottomley@hansenpartnership.com>,
        David Hildenbrand <david@redhat.com>,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
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On Thu, 21 Oct 2021 at 17:40, Kees Cook <keescook@chromium.org> wrote:
>
> Quoting Dmitry: "refcount_inc() needs to be done before fd_install().
> After fd_install() finishes, the fd can be used by userspace and we can
> have secret data in memory before the refcount_inc().
>
> A straightforward mis-use where a user will predict the returned fd
> in another thread before the syscall returns and will use it to store
> secret data is somewhat dubious because such a user just shoots themself
> in the foot.
>
> But a more interesting mis-use would be to close the predicted fd and
> decrement the refcount before the corresponding refcount_inc, this way
> one can briefly drop the refcount to zero while there are other users
> of secretmem."
>
> Move fd_install() after refcount_inc().
>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Jordy Zomer <jordy@pwning.systems>
> Cc: linux-mm@kvack.org
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Link: https://lore.kernel.org/lkml/CACT4Y+b1sW6-Hkn8HQYw_SsT7X3tp-CJNh2ci0wG3ZnQz9jjig@mail.gmail.com
> Fixes: 9a436f8ff631 ("PM: hibernate: disable when there are active secretmem users")
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
>  mm/secretmem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/secretmem.c b/mm/secretmem.c
> index 1fea68b8d5a6..924d84ba481f 100644
> --- a/mm/secretmem.c
> +++ b/mm/secretmem.c
> @@ -217,8 +217,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned int, flags)
>
>         file->f_flags |= O_LARGEFILE;
>
> -       fd_install(fd, file);
>         refcount_inc(&secretmem_users);
> +       fd_install(fd, file);
>         return fd;
>
>  err_put_fd:
> --
> 2.30.2
>