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

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

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

quoted

@@ -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.


r~

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