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Date:   Fri, 18 Oct 2019 12:05:52 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Dave Martin <Dave.Martin@arm.com>
Cc:     Paul Elliott <paul.elliott@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andrew Jones <drjones@redhat.com>,
        Amit Kachhap <amit.kachhap@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        linux-arch@vger.kernel.org, Eugene Syromiatnikov <esyr@redhat.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        "H.J. Lu" <hjl.tools@gmail.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>, Jann Horn <jannh@google.com>,
        Richard Henderson <richard.henderson@linaro.org>,
        Kristina =?utf-8?Q?Mart=C5=A1enko?= <kristina.martsenko@arm.com>,
        Mark Brown <broonie@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-arm-kernel@lists.infradead.org,
        Florian Weimer <fweimer@redhat.com>,
        linux-kernel@vger.kernel.org, Sudakshina Das <sudi.das@arm.com>
Subject: Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification
 support
Message-ID: <20191018110551.GB27759@lakrids.cambridge.arm.com>
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On Fri, Oct 11, 2019 at 05:42:00PM +0100, Dave Martin wrote:
> On Fri, Oct 11, 2019 at 05:01:13PM +0100, Dave Martin wrote:
> > On Fri, Oct 11, 2019 at 04:44:45PM +0100, Dave Martin wrote:
> > > On Fri, Oct 11, 2019 at 04:40:43PM +0100, Mark Rutland wrote:
> > > > 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.
> > > > :)
> > > 
> > > OK, I think it's worth a brief comment in the code either way, so I'll
> > > add something.
> > 
> > Hmm, come to think of it we do need special logic for a particular case
> > here:
> > 
> > If we are delivering a SIGILL here and the SIGILL handler was registered
> > with SA_NODEFER then we will get into a spin, repeatedly delivering
> > the BTI-triggered SIGILL to the same (bad) entry point.
> > 
> > Without SA_NODEFER, the SIGILL becomes fatal, which is the desired
> > behaviour, but we'll need to catch this recursion explicitly.
> > 
> > 
> > It's similar to the special force_sigsegv() case in
> > linux/kernel/signal.c...
> > 
> > Thoughts?
> 
> On second thought, maybe we don't need to do anything special.
> 
> A SIGSEGV handler registered with (SA_NODEFER & ~SA_RESETHAND) and that
> dereferences a duff address would spin similarly.
> 
> This SIGILL case doesn't really seem different.  Either way it's a
> livelock of the user task that doesn't compromise the kernel.  There
> are plenty of ways for such a livelock to happen.

That sounds reasonable to me.

Thanks,
Mark.