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Date: Wed, 10 Jun 2020 18:24:20 +0300
From: Alexander Popov <>
To: Kees Cook <>
Cc: Jann Horn <>, Elena Reshetova
 <>, Emese Revfy <>,
 Miguel Ojeda <>,
 Masahiro Yamada <>,
 Michal Marek <>,
 Andrew Morton <>,
 Masahiro Yamada <>,
 Thiago Jung Bauermann <>,
 Luis Chamberlain <>, Jessica Yu <>,
 Sven Schnelle <>, Iurii Zaikin <>,
 Catalin Marinas <>, Will Deacon <>,
 Vincenzo Frascino <>,
 Thomas Gleixner <>, Peter Collingbourne <>,
 Naohiro Aota <>, Alexander Monakov <>,
 Mathias Krause <>, PaX Team <>,
 Brad Spengler <>, Laura Abbott <>,
 Florian Weimer <>,
 Kernel Hardening <>,, the arch/x86 maintainers <>,
 Linux ARM <>,
 kernel list <>,,
Subject: Re: [PATCH 1/5] gcc-plugins/stackleak: Exclude alloca() from the
 instrumentation logic

On 09.06.2020 21:39, Kees Cook wrote:
> On Thu, Jun 04, 2020 at 06:23:38PM +0300, Alexander Popov wrote:
>> On 04.06.2020 17:01, Jann Horn wrote:
>>> On Thu, Jun 4, 2020 at 3:51 PM Alexander Popov <> wrote:
>>>> Some time ago Variable Length Arrays (VLA) were removed from the kernel.
>>>> The kernel is built with '-Wvla'. Let's exclude alloca() from the
>>>> instrumentation logic and make it simpler. The build-time assertion
>>>> against alloca() is added instead.
>>> [...]
>>>> +                       /* Variable Length Arrays are forbidden in the kernel */
>>>> +                       gcc_assert(!is_alloca(stmt));
>>> There is a patch series from Elena and Kees on the kernel-hardening
>>> list that deliberately uses __builtin_alloca() in the syscall entry
>>> path to randomize the stack pointer per-syscall - see
>>> <>.
>> Thanks, Jann.
>> At first glance, leaving alloca() handling in stackleak instrumentation logic
>> would allow to integrate stackleak and this version of random_kstack_offset.
> Right, it seems there would be a need for this coverage to remain,
> otherwise the depth of stack erasure might be incorrect.
> It doesn't seem like the other patches strictly depend on alloca()
> support being removed, though?

Ok, I will leave alloca() support, reorganize the patch series and send v2.

>> Kees, Elena, did you try random_kstack_offset with upstream stackleak?
> I didn't try that combination yet, no. It seemed there would likely
> still be further discussion about the offset series first (though the
> thread has been silent -- I'll rebase and resend it after rc2).

Ok, please add me to CC list.

Best regards,

>> It looks to me that without stackleak erasing random_kstack_offset can be
>> weaker. I mean, if next syscall has a bigger stack randomization gap, the data
>> on thread stack from the previous syscall is not overwritten and can be used. Am
>> I right?
> That's correct. I think the combination is needed, but I don't think
> they need to be strictly tied together.
>> Another aspect: CONFIG_STACKLEAK_METRICS can be used for guessing kernel stack
>> offset, which is bad. It should be disabled if random_kstack_offset is on.
> Agreed.

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