Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification support
From: Dave Martin <Dave.Martin@arm.com>
Date: 2019-10-11 15:32:33
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linux-arch, lkml
On Fri, Oct 11, 2019 at 11:25:33AM -0400, Richard Henderson wrote:
On 10/11/19 11:10 AM, Mark Rutland wrote:quoted
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.
Ack, that's the intended rationale -- I also outlined this in the commit message. 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. Cheers ---Dave _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel