Re: Patch 4/6 randomize the stack pointer

[John Richard Moser]

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Arjan van de Ven wrote:
> 
> The patch below replaces the existing 8Kb randomisation of the userspace
> stack pointer (which is currently only done for Hyperthreaded P-IVs) with a 
> more general randomisation over a 64Kb range.
> 

64k of stack randomization is trivial to evade.  Know the alignment, and
stick branch instructions (or a stream of no-ops if they align)
periodically so that....

[   ]|STACK---STACK---NONONOSHELLCODE
STACK---STACK---NONONOSHELLCODE
- ----------------------^
|
- -- You jump here in any case.

Now, in PaX, there's a randomization over something like a 256M range
for the stack IIRC.  Who does a 256M stack overflow?  It's several gigs
on 64 bit archs I think.  I think it's 16 bits with 4k (12 bit) pages,
so.... 28 bits... 268435456.... 256M, yes.  I'm pretty sure this is 24 +
12 on amd64 for example, so 64 gigs.

If your stack base is within 256M of ESP, you're safe.  It's also fairly
implausible--definitely non-trivial--to do a 256M stack buffer overflow.
 Programmatically it's no different; but in real life, that's 256M that
has to be in a corrupted jpeg, mp3 file, or network transmission.
Somebody's gonna notice.  64 gig doesn't happen.

Your patch 5/6 for mmap rand is also small.  1M is trivial, though I'd
imagine mmap() rand would pose a bit more confusion in some cases at
least, even for small ranges.

Still, this is a joke, like OpenBSD's stackgap.

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