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

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:

quoted

On Fri, Oct 11, 2019 at 04:44:45PM +0100, Dave Martin wrote:

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On Fri, Oct 11, 2019 at 04:40:43PM +0100, Mark Rutland wrote:

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On Fri, Oct 11, 2019 at 04:32:26PM +0100, Dave Martin wrote:

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On Fri, Oct 11, 2019 at 11:25:33AM -0400, Richard Henderson wrote:

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On 10/11/19 11:10 AM, Mark Rutland wrote:

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On Thu, Oct 10, 2019 at 07:44:33PM +0100, Dave Martin wrote:

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@@ -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.

quoted

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.

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