Subject: Re: [PATCH] seccomp: Improve performance by optimizing memory barrier

From: Leon Romanovsky <[email-masked]>
Date: Mon, 8 Feb 2021 08:43:36 +0200

On Mon, Feb 01, 2021 at 08:49:41PM +0800, wanghongzhe wrote:
> If a thread(A)'s TSYNC flag is set from seccomp(), then it will
> synchronize its seccomp filter to other threads(B) in same thread
> group. To avoid race condition, seccomp puts rmb() between
> reading the mode and filter in seccomp check patch(in B thread).
> As a result, every syscall's seccomp check is slowed down by the
> memory barrier.
>
> However, we can optimize it by calling rmb() only when filter is
> NULL and reading it again after the barrier, which means the rmb()
> is called only once in thread lifetime.
>
> The 'filter is NULL' conditon means that it is the first time
> attaching filter and is by other thread(A) using TSYNC flag.
> In this case, thread B may read the filter first and mode later
> in CPU out-of-order exection. After this time, the thread B's
> mode is always be set, and there will no race condition with the
> filter/bitmap.
>
> In addtion, we should puts a write memory barrier between writing
> the filter and mode in smp_mb__before_atomic(), to avoid
> the race condition in TSYNC case.
>
> Signed-off-by: wanghongzhe <[email-masked]>
> ---
>  kernel/seccomp.c | 31 ++++++++++++++++++++++---------
>  1 file changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 952dc1c90229..b944cb2b6b94 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -397,8 +397,20 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
>  			READ_ONCE(current->seccomp.filter);
>
>  	/* Ensure unexpected behavior doesn't result in failing open. */
> -	if (WARN_ON(f == NULL))
> -		return SECCOMP_RET_KILL_PROCESS;
> +	if (WARN_ON(f == NULL)) {
> +		/*
> +		 * Make sure the first filter addtion (from another
> +		 * thread using TSYNC flag) are seen.
> +		 */
> +		rmb();
> +
> +		/* Read again */
> +		f = READ_ONCE(current->seccomp.filter);
> +
> +		/* Ensure unexpected behavior doesn't result in failing open. */
> +		if (WARN_ON(f == NULL))
> +			return SECCOMP_RET_KILL_PROCESS;
> +	}

IMHO, double WARN_ON() for the fallback flow is too much.
Also according to the description, this "f == NULL" check is due to
races and not programming error which WARN_ON() are intended to catch.

Thanks