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Message-ID: <20191011154043.GG33537@lakrids.cambridge.arm.com>
Date: Fri, 11 Oct 2019 16:40:43 +0100
From: Mark Rutland <mark.rutland@....com>
To: Dave Martin <Dave.Martin@....com>
Cc: Richard Henderson <richard.henderson@...aro.org>,
Paul Elliott <paul.elliott@....com>,
Peter Zijlstra <peterz@...radead.org>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
Yu-cheng Yu <yu-cheng.yu@...el.com>,
Amit Kachhap <amit.kachhap@....com>,
Vincenzo Frascino <vincenzo.frascino@....com>,
linux-arch@...r.kernel.org, Eugene Syromiatnikov <esyr@...hat.com>,
Szabolcs Nagy <szabolcs.nagy@....com>,
"H.J. Lu" <hjl.tools@...il.com>, Andrew Jones <drjones@...hat.com>,
Kees Cook <keescook@...omium.org>,
Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
Kristina Martšenko <kristina.martsenko@....com>,
Mark Brown <broonie@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-arm-kernel@...ts.infradead.org,
Florian Weimer <fweimer@...hat.com>,
linux-kernel@...r.kernel.org, Sudakshina Das <sudi.das@....com>
Subject: Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification
support
On Fri, Oct 11, 2019 at 04:32:26PM +0100, Dave Martin wrote:
> On Fri, Oct 11, 2019 at 11:25:33AM -0400, Richard Henderson wrote:
> > On 10/11/19 11:10 AM, Mark Rutland wrote:
> > > On Thu, Oct 10, 2019 at 07:44:33PM +0100, Dave Martin wrote:
> > >> @@ -730,6 +730,11 @@ static void setup_return
> > >> regs->regs[29] = (unsigned long)&user->next_frame->fp;
> > >> regs->pc = (unsigned long)ka->sa.sa_handler;
> > >>
> > >> + if (system_supports_bti()) {
> > >> + regs->pstate &= ~PSR_BTYPE_MASK;
> > >> + regs->pstate |= PSR_BTYPE_CALL;
> > >> + }
> > >> +
> > >
> > > I think we might need a comment as to what we're trying to ensure here.
> > >
> > > I was under the (perhaps mistaken) impression that we'd generate a
> > > pristine pstate for a signal handler, and it's not clear to me that we
> > > must ensure the first instruction is a target instruction.
> >
> > I think it makes sense to treat entry into a signal handler as a call. Code
> > that has been compiled for BTI, and whose page has been marked with PROT_BTI,
> > will already have the pauth/bti markup at the beginning of the signal handler
> > function; we might as well verify that.
> >
> > Otherwise sigaction becomes a hole by which an attacker can force execution to
> > start at any arbitrary address.
>
> Ack, that's the intended rationale -- I also outlined this in the commit
> message.
Ah, sorry. I evidently did not read that thoroughly enough.
> Does this sound reasonable?
>
>
> Either way, I feel we should do this: any function in a PROT_BTI page
> should have a suitable landing pad. There's no reason I can see why
> a protection given to any other callback function should be omitted
> for a signal handler.
>
> Note, if the signal handler isn't in a PROT_BTI page then overriding
> BTYPE here will not trigger a Branch Target exception.
>
> I'm happy to drop a brief comment into the code also, once we're
> agreed on what the code should be doing.
So long as there's a comment as to why, I have no strong feelings here.
:)
Thanks,
Mark.
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