Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification support

[Mark Rutland]

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.