Re: [RFC PATCH for 4.18 2/2] rseq: check that rseq->rseq_cs padding is zero

[Will Deacon <will.deacon@arm.com>]

Hi Mathieu,

On Thu, Jun 28, 2018 at 12:23:59PM -0400, Mathieu Desnoyers wrote:
> On 32-bit kernels, the rseq->rseq_cs_padding field is never read by the
> kernel. However, 64-bit kernels dealing with 32-bit compat tasks read the
> full 64-bit in its entirety, and terminates the offending process with
> a segmentation fault if the upper 32 bits are set due to failure of
> copy_from_user().
> 
> Ensure that both 32-bit and 64-bit kernels dealing with 32-bit tasks end
> up terminating offending tasks with a segmentation fault if the upper
> 32-bit padding bits (rseq->rseq_cs_padding) are set by adding an explicit
> check that padding is zero on 32-bit kernels.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Paul Turner <pjt@google.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Andy Lutomirski <luto@amacapital.net>
> CC: Andi Kleen <andi@firstfloor.org>
> CC: Dave Watson <davejwatson@fb.com>
> CC: Chris Lameter <cl@linux.com>
> CC: Ingo Molnar <mingo@redhat.com>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Ben Maurer <bmaurer@fb.com>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Josh Triplett <josh@joshtriplett.org>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Andrew Morton <akpm@linux-foundation.org>
> CC: Russell King <linux@arm.linux.org.uk>
> CC: Catalin Marinas <catalin.marinas@arm.com>
> CC: Will Deacon <will.deacon@arm.com>
> CC: Michael Kerrisk <mtk.manpages@gmail.com>
> CC: Boqun Feng <boqun.feng@gmail.com>
> CC: linux-api@vger.kernel.org
> ---
>  kernel/rseq.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 4ba582046fcd..b038f35a60d6 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -112,6 +112,29 @@ static int rseq_reset_rseq_cpu_id(struct task_struct *t)
>  	return 0;
>  }
>  
> +#ifndef __LP64__
> +/*
> + * Ensure that padding is zero.
> + */
> +static int check_rseq_cs_padding(struct task_struct *t)
> +{
> +	unsigned long pad;
> +	int ret;
> +
> +	ret = __get_user(pad, &t->rseq->rseq_cs_padding);
> +	if (ret)
> +		return ret;
> +	if (pad)
> +		return -EFAULT;
> +	return 0;
> +}
> +#else
> +static int check_rseq_cs_padding(struct task_struct *t)
> +{
> +	return 0;
> +}
> +#endif

I'm still not sure how this works with a 64-bit kernel and a compat (32-bit)
task. The check_rseq_cs_padding() will return 0 regardless of the upper bits
of the rseq_cs field, whereas a native 32-bit kernel would actually go and
check them.

What am I missing here?

Will