linux-hardening - Re: [PATCH] mm/secretmem: Avoid letting secretmem

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Date:   Thu, 21 Oct 2021 12:47:34 -0400 (EDT)
From:   Jordy Zomer <jordy@...ing.systems>
To:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        David Hildenbrand <david@...hat.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] mm/secretmem: Avoid letting secretmem_users drop to
 zero

Reviewed-by: Jordy Zomer <jordy@...ing.systems>

That's a nice find, Dmitry! Thanks. Also, thank you for writing a patch in such a short period of time, Kees:)

Looks good to me!

> On 10/21/2021 11:40 AM Kees Cook <keescook@...omium.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@...nel.org>
> Cc: Jordy Zomer <jordy@...ing.systems>
> Cc: linux-mm@...ck.org
> Reported-by: Dmitry Vyukov <dvyukov@...gle.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@...omium.org>
> ---
>  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