Re: [RFC PATCH] rseq: Use pr_warn_once() when deprecated/unknown ABI flags are encountered

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Sep 29, 2022 at 10:12:27AM -0400, Mathieu Desnoyers wrote:
> These commits use WARN_ON_ONCE() and kill the offending processes when
> deprecated and unknown flags are encountered:
> 
> commit c17a6ff93213 ("rseq: Kill process when unknown flags are encountered in ABI structures")
> commit 0190e4198e47 ("rseq: Deprecate RSEQ_CS_FLAG_NO_RESTART_ON_* flags")
> 
> The WARN_ON_ONCE() triggered by userspace input prevents use of
> Syzkaller to fuzz the rseq system call.
> 
> Replace this WARN_ON_ONCE() by pr_warn_once() messages which contain
> actually useful information.
> 
> Reported-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Thanks for this!

I've set off a Syzkaller run with this applied, and I'll be able to tell you in
a day or two whether that's made it possible to spot anything more interesting.

Regardless, I think this is a good change, so FWIW:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  kernel/rseq.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index bda8175f8f99..d38ab944105d 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -171,12 +171,27 @@ static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
>  	return 0;
>  }
>  
> +static bool rseq_warn_flags(const char *str, u32 flags)
> +{
> +	u32 test_flags;
> +
> +	if (!flags)
> +		return false;
> +	test_flags = flags & RSEQ_CS_NO_RESTART_FLAGS;
> +	if (test_flags)
> +		pr_warn_once("Deprecated flags (%u) in %s ABI structure", test_flags, str);
> +	test_flags = flags & ~RSEQ_CS_NO_RESTART_FLAGS;
> +	if (test_flags)
> +		pr_warn_once("Unknown flags (%u) in %s ABI structure", test_flags, str);
> +	return true;
> +}
> +
>  static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
>  {
>  	u32 flags, event_mask;
>  	int ret;
>  
> -	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS) || cs_flags)
> +	if (rseq_warn_flags("rseq_cs", cs_flags))
>  		return -EINVAL;
>  
>  	/* Get thread flags. */
> @@ -184,7 +199,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
>  	if (ret)
>  		return ret;
>  
> -	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS) || flags)
> +	if (rseq_warn_flags("rseq", flags))
>  		return -EINVAL;
>  
>  	/*
> -- 
> 2.30.2
>