linux-kernel - Re: [PATCH 2/2] rseq: Kill process when unknown flags are encountered in ABI structures

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Message-ID: <1021959527.83217.1658862705489.JavaMail.zimbra@efficios.com>
Date:   Tue, 26 Jul 2022 15:11:45 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api <linux-api@...r.kernel.org>,
        Peter Oskolkov <posk@...k.io>
Subject: Re: [PATCH 2/2] rseq: Kill process when unknown flags are
 encountered in ABI structures

----- On Jun 22, 2022, at 3:46 PM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:

> rseq_abi()->flags and rseq_abi()->rseq_cs->flags 29 upper bits are
> currently unused.
> 
> The current behavior when those bits are set is to ignore them. This is
> not an ideal behavior, because when future features will start using
> those flags, if user-space fails to correctly validate that the kernel
> indeed supports those flags (e.g. with a new sys_rseq flags bit) before
> using them, it may incorrectly assume that the kernel will handle those
> flags way when in fact those will be silently ignored on older kernels.
> 
> Validating that unused flags bits are cleared will allow a smoother
> transition when those flags will start to be used by allowing
> applications to fail early, and obviously, when they attempt to use the
> new flags on an older kernel that does not support them.

Hi Peter, as for the prior patch, would you consider pulling this through
the tip tree ?

Thanks,

Mathieu

> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> ---
> kernel/rseq.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 81d7dc80787b..bda8175f8f99 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -176,7 +176,7 @@ 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))
> +	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS) || cs_flags)
> 		return -EINVAL;
> 
> 	/* Get thread flags. */
> @@ -184,7 +184,7 @@ 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))
> +	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS) || flags)
> 		return -EINVAL;
> 
> 	/*
> --
> 2.30.2

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com