Re: [kernel-hardening] Re: vDSO maximum stack usage, stack probes, and -fstack-check

[Andy Lutomirski <luto@kernel.org>]

On Fri, Nov 10, 2017 at 9:16 PM, Hector Martin 'marcan'
<marcan@marcan.st> wrote:
> On 2017-11-11 07:04, Andy Lutomirski wrote:
>>> On Nov 10, 2017, at 8:36 AM, Hector Martin 'marcan' <marcan@marcan.st> wrote:
>>>
>>>> On 2017-11-11 01:02, Hector Martin 'marcan' wrote:
>>>> Not entirely sure what's going on here.
>>>
>>> Actually, if you think about it, it doesn't matter that it skips the
>>> first page, since it's probing one page more. That just means the caller
>>> will have probed the previous page. So ultimately you're just probing
>>> ahead of where you need to, but that should be OK.
>>>
>>
>> The whole point is to touch the stack pages in order.  Also, I see no
>> guarantee that the function would touch the intermediate page before
>> clobbering the probed page.  You're seeing exactly that behavior, in
>> fact.
>
> Only because Go is not C and is not compiled like this. If all the code
> is GCC-compiled C code and built with -fstack-check, it should always
> probe stack pages in order except for potentially the second page in the
> stack, which may be touched after the third page (but hopefully your
> stack is at least two pages long to begin with).

If you're generating code to improve stack overflow, assuming that you
have at least two pages left seems like an *awful* assumption to make.

>
> AIUI -fstack-check was not intended for stack clash protection (the
> latter isn't even in a GCC release yet), but in most circumstances it
> seems to me like it's an effective mitigation if all code is compiled
> with it. Qualys mentioned it as such in their advisory. This is probably
> why Gentoo Hardened enables it by default globally in their toolchain.
>

Gentoo Hardened should seriously consider turning it back off.  Do you
happen to know what exactly Gentoo does to cause the vdso to get build
with -fstack-check?  I'll write a patch to either fail the build or to
force it off.