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Re: RFC: stack/heap collision vulnerability and mitigation with GCC


On 19/06/17 20:04, Jeff Law wrote:
> On 06/19/2017 11:50 AM, Joseph Myers wrote:
>> On Mon, 19 Jun 2017, Jeff Law wrote:
>>
>>> A key point to remember is that you can never have an allocation
>>> (potentially using more than one allocation site) which is larger than a
>>> page without probing the page.
>>
>> There's a platform ABI issue here.  At least some kernel fixes for these 
>> stack issues, as I understand it, increase the size of the stack guard to 
>> more than a single page.  It would be possible to define the ABI to 
>> require such a larger guard for protection and so reduce the number of 
>> (non-alloca/VLA-using) functions that need probes generated, depending on 
>> whether a goal is to achieve security on kernels without such a fix.  
>> (Thinking in terms of how to get to enabling such probes by default.)
> On 32 bit platforms we don't have a lot of address space left, so we
> have to be careful about creating too large of a guard.
> 
> On 64 bit platforms we have a lot more freedom and I suspect larger
> guards, mandated by the ABI would be useful, if for no other reason than
> allowing us to allocate more stack without probing.   A simple array of
> PATH_MAX characters triggers probing right now.   I suspect (but didn't
> bother to confirm) that PATH_MAX array are what causes git to have so
> many large stacks.
> 
> Also if we look at something like ppc and aarch64, we've currently got
> the PROBE_INTERVAL set to 4k.  But in reality they're using much larger
> page sizes.  So we could improve things there as well.
> 

There are aarch64 linux systems using 4k pages for compatibility with
existing aarch32 binaries.

R.

> 
> jeff
> 


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