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

On Fri, Oct 18, 2019 at 12:05:52PM +0100, Mark Rutland wrote:

On Fri, Oct 11, 2019 at 05:42:00PM +0100, Dave Martin wrote:

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On Fri, Oct 11, 2019 at 05:01:13PM +0100, Dave Martin wrote:

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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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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.

OK, I guess we can park this discussion for now.

Cheers
---Dave

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