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SubjectRe: [RFC] perf: mmap2 not covering VM_CLONE regions

* Peter Zijlstra <> wrote:

> On Wed, Oct 02, 2013 at 01:23:16PM +0200, Peter Zijlstra wrote:
> > So the only thing I can come up with is something like the below;
> > supposedly the sha hash mixing a boot time random seed and the mm
> > pointer is enough to avoid it being a data leak.
> That is, right until it becomes feasible to run 2^64 SHA1 computations.
> I've actually no idea how hard that is given todays GPU assisted
> efforts.

Well, here are the possible cryptanalytic attacks I can think of:

- differential, because here you don't just have access to the hash
value but you can essentially feed highly correlated plaintext to the
hash at will, by starting/stopping threads, knowing their typical mm
pointer differences, etc.

I.e. less than 2^64, potentially a lot less.

- then there are timing attacks, and someone having access to a PMU
context and who can trigger this SHA1 computation arbitrarily in task
local context can run very accurate and low noise timing attacks...

I don't think the kernel's sha_transform() is hardened against timing
attacks, it's performance optimized so it has variable execution time
highly dependent on plaintext input - which leaks information about the

But then again, how realistic is an attack? All that effort just to
recover the raw kernel data pointer value of a struct mm? Dunno whether we
should worry about it.



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