linux-kernel - Re: [PATCH v10 2/3] arm/syscalls: Check address limit on user-mode return

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Date:   Wed, 21 Jun 2017 10:08:15 +0100
From:   Will Deacon <will.deacon@....com>
To:     Thomas Garnier <thgarnie@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Petr Mladek <pmladek@...e.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Dave Hansen <dave.hansen@...el.com>,
        David Howells <dhowells@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Pratyush Anand <panand@...hat.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v10 2/3] arm/syscalls: Check address limit on user-mode
 return

On Tue, Jun 20, 2017 at 01:31:14PM -0700, Thomas Garnier wrote:
> On Tue, Jun 20, 2017 at 1:18 PM, Kees Cook <keescook@...omium.org> wrote:
> > On Wed, Jun 14, 2017 at 6:12 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> >> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> >> index eb5cd77bf1d8..e33c32d56193 100644
> >> --- a/arch/arm/kernel/entry-common.S
> >> +++ b/arch/arm/kernel/entry-common.S
> >> @@ -41,7 +41,9 @@ ret_fast_syscall:
> >>   UNWIND(.cantunwind    )
> >>         disable_irq_notrace                     @ disable interrupts
> >>         ldr     r1, [tsk, #TI_FLAGS]            @ re-check for syscall tracing
> >> -       tst     r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
> >> +       tst     r1, #_TIF_SYSCALL_WORK
> >> +       bne     fast_work_pending
> >> +       tst     r1, #_TIF_WORK_MASK
> >
> > (IIUC) MOV32 is 2 cycles (MOVW, MOVT), and each TST above is 1 cycle
> > and each BNE is 1 cycle (when not taken). So:
> >
> > mov32 r2, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
> > tst r1, r2
> > bne fast_work_pending
> >
> > is 4 cycles and tst, bne, tst, bne is also 4 cycles. Would mov32 be
> > more readable (since it keeps the flags together)?
> 
> I guess it would be more readable. Any opinion from the arm folks?

The mov32 sequence is probably better, but statically attributing cycles
on a per instruction basis is pretty futile on modern CPUs.

Will