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Subject: Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification
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To:     Mark Rutland <mark.rutland@arm.com>,
        Dave Martin <Dave.Martin@arm.com>
Cc:     linux-kernel@vger.kernel.org, Andrew Jones <drjones@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
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        Kees Cook <keescook@chromium.org>,
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        Paul Elliott <paul.elliott@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sudakshina Das <sudi.das@arm.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will.deacon@arm.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Amit Kachhap <amit.kachhap@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org
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From:   Richard Henderson <richard.henderson@linaro.org>
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Date:   Fri, 11 Oct 2019 11:25:33 -0400
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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.


r~