linux-kernel - Re: [PATCH 1/2] stackleak: Update for arm64

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Message-ID: <874lm2d96q.fsf@e105548-lin.cambridge.arm.com>
Date:   Tue, 27 Feb 2018 10:21:49 +0000
From:   Richard Sandiford <richard.sandiford@....com>
To:     Alexander Popov <alex.popov@...ux.com>
Cc:     Will Deacon <will.deacon@....com>,
        Laura Abbott <labbott@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/2] stackleak: Update for arm64

Hi Alexander,

Sorry for the slow reply, been caught up in an office move.

Alexander Popov <alex.popov@...ux.com> writes:
> Hello Will, Richard and GCC folks!
>
> On 22.02.2018 19:58, Will Deacon wrote:
>> On Tue, Feb 20, 2018 at 05:13:02PM -0800, Laura Abbott wrote:
>>>
>>> arm64 has another layer of indirection in the RTL.
>>> Account for this in the plugin.
>>>
>>> Signed-off-by: Laura Abbott <labbott@...hat.com>
>>> ---
>>>  scripts/gcc-plugins/stackleak_plugin.c | 5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c
>>> index 6fc991c98d8b..7dfaa027423f 100644
>>> --- a/scripts/gcc-plugins/stackleak_plugin.c
>>> +++ b/scripts/gcc-plugins/stackleak_plugin.c
>>> @@ -244,6 +244,11 @@ static unsigned int stackleak_final_execute(void)
>>>  		 * that insn.
>>>  		 */
>>>  		body = PATTERN(insn);
>>> +		/* arm64 is different */
>>> +		if (GET_CODE(body) == PARALLEL) {
>>> +			body = XEXP(body, 0);
>>> +			body = XEXP(body, 0);
>>> +		}
>> 
>> Like most kernel developers, I don't know the first thing about GCC internals
>> so I asked our GCC team and Richard (CC'd) reckons this should be:
>> 
>> 	if (GET_CODE(body) == PARALLEL)
>> 		body = XVECEXP(body, 0, 0);
>> 
>> instead of the hunk above. Can you give that a go instead, please?
>
> Thanks a lot!
>
> Would you be so kind to take a look at the whole STACKLEAK plugin?
> http://www.openwall.com/lists/kernel-hardening/2018/02/16/4
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/gcc-plugin/stackleak&id=57a0a6763b12e82dd462593d0f42be610e93cdc9
>
> It's not very big. I documented it in detail. I would be really grateful for the
> review!

Looks good to me FWIW.  Just a couple of minor things:

> +	/*
> +	 * 1. Loop through the GIMPLE statements in each of cfun basic blocks.
> +	 * cfun is a global variable which represents the function that is
> +	 * currently processed.
> +	 */
> +	FOR_EACH_BB_FN(bb, cfun) {
> +		for (gsi = gsi_start_bb(bb); !gsi_end_p(gsi); gsi_next(&gsi)) {
> +			gimple stmt;
> +
> +			stmt = gsi_stmt(gsi);
> +
> +			/* Leaf function is a function which makes no calls */
> +			if (is_gimple_call(stmt))
> +				is_leaf = false;

It's probably not going to matter in practice, but it might be worth
emphasising in the comments that this leafness is only approximate.
It will sometimes be a false positive, because we could still
end up creating calls to libgcc functions from non-call statements
(or for target-specific reasons).  It can also be a false negative,
since call statements can be to built-in or internal functions that
end up being open-coded.

> +	/*
> +	 * The stackleak_final pass should be executed before the "final" pass,
> +	 * which turns the RTL (Register Transfer Language) into assembly.
> +	 */
> +	PASS_INFO(stackleak_final, "final", 1, PASS_POS_INSERT_BEFORE);

This might be too late, since it happens e.g. after addresses have
been calculated for branch ranges, and after machine-specific passes
(e.g. bundling on ia64).

The stack size is final after reload, so inserting the pass after that
might be better.

Thanks,
Richard