linux-kernel - Re: [PATCH] arm64: mte: Move MTE TCF0 check in entry-common

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Date:   Thu, 8 Apr 2021 18:03:54 +0100
From:   Will Deacon <will@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH] arm64: mte: Move MTE TCF0 check in entry-common

On Thu, Apr 08, 2021 at 04:06:23PM +0100, Mark Rutland wrote:
> On Thu, Apr 08, 2021 at 03:56:04PM +0100, Will Deacon wrote:
> > On Thu, Apr 08, 2021 at 03:37:23PM +0100, Vincenzo Frascino wrote:
> > > diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> > > index 9d3588450473..837d3624a1d5 100644
> > > --- a/arch/arm64/kernel/entry-common.c
> > > +++ b/arch/arm64/kernel/entry-common.c
> > > @@ -289,10 +289,16 @@ asmlinkage void noinstr enter_from_user_mode(void)
> > >  	CT_WARN_ON(ct_state() != CONTEXT_USER);
> > >  	user_exit_irqoff();
> > >  	trace_hardirqs_off_finish();
> > > +
> > > +	/* Check for asynchronous tag check faults in user space */
> > > +	check_mte_async_tcf0();
> > >  }
> > 
> > Is enter_from_user_mode() always called when we enter the kernel from EL0?
> > afaict, some paths (e.g. el0_irq()) only end up calling it if
> > CONTEXT_TRACKING or TRACE_IRQFLAGS are enabled.
> 
> Currently everything that's in {enter,exit}_from_user_mode() only
> matters when either CONTEXT_TRACKING or TRACE_IRQFLAGS is selected (and
> expands to an empty stub otherwise).
> 
> We could drop the ifdeffery in user_{enter,exit}_irqoff() to have them
> called regardless, or add CONFIG_MTE to the list.

I'm always in favour of dropping ifdeffery if it's getting in the way.

> > >  asmlinkage void noinstr exit_to_user_mode(void)
> > >  {
> > > +	/* Ignore asynchronous tag check faults in the uaccess routines */
> > > +	clear_mte_async_tcf0();
> > > +
> > 
> > and this one seems to be called even less often.
> 
> This is always done in ret_to_user, so (modulo ifdeferry above) all
> returns to EL0 call this.

Right, I was just saying that if you disabled those CONFIG options then this
isn't called _at all_ whereas I think enter_from_user_mode() still is on
some paths.

Will