Re: [PATCH] secretmem: Prevent secretmem_users from wrapping to zero

[Kees Cook <keescook@chromium.org>]

On Mon, Oct 25, 2021 at 07:16:34PM +0100, Matthew Wilcox (Oracle) wrote:
> Commit 110860541f44 ("mm/secretmem: use refcount_t instead of atomic_t")
> attempted to fix the problem of secretmem_users wrapping to zero and
> allowing suspend once again.  Prevent secretmem_users from wrapping to
> zero by forbidding new users if the number of users has wrapped from
> positive to negative.  This stops a long way short of reaching the
> necessary 4 billion users, so there's no need to be clever with special
> anti-wrap types or checking the return value from atomic_inc().

I still prefer refcount_t here because it provides deterministic
saturation, but the risk right now is so narrow ("don't hibernate"),
I'm not going to fight for it. I think it'd be fine to use it initialized
to 1, and have the removal check for == 0 as a failure state, which would
deterministically cover the underflow case too.

-Kees

> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Jordy Zomer <jordy@pwning.systems>
> Cc: Kees Cook <keescook@chromium.org>,
> Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
>  mm/secretmem.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/secretmem.c b/mm/secretmem.c
> index 030f02ddc7c1..c2dda408bb36 100644
> --- a/mm/secretmem.c
> +++ b/mm/secretmem.c
> @@ -203,6 +203,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned int, flags)
>  
>  	if (flags & ~(SECRETMEM_FLAGS_MASK | O_CLOEXEC))
>  		return -EINVAL;
> +	if (atomic_read(&secretmem_users) < 0)
> +		return -ENFILE;
>  
>  	fd = get_unused_fd_flags(flags & O_CLOEXEC);
>  	if (fd < 0)
> -- 
> 2.33.0
> 

-- 
Kees Cook