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SubjectRe: [RFC] perf: mmap2 not covering VM_CLONE regions
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On Wed, Oct 2, 2013 at 7:29 PM, Kees Cook <keescook@chromium.org> wrote:
> On Wed, Oct 2, 2013 at 10:20 AM, Stephane Eranian <eranian@google.com> wrote:
>> On Wed, Oct 2, 2013 at 7:14 PM, Kees Cook <keescook@chromium.org> wrote:
>>> On Wed, Oct 2, 2013 at 6:37 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>> On Wed, Oct 02, 2013 at 03:13:16PM +0200, Stephane Eranian wrote:
>>>>> On Wed, Oct 2, 2013 at 3:01 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>>> > On Wed, Oct 02, 2013 at 02:59:32PM +0200, Stephane Eranian wrote:
>>>>> >> On Wed, Oct 2, 2013 at 2:46 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>>> >> > On Wed, Oct 02, 2013 at 02:39:53PM +0200, Ingo Molnar wrote:
>>>>> >> >> - 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
>>>>> >> >> plaintext.
>>>>> >> >
>>>>> >> > Typical user doesn't have enough priv to profile kernel space; once you
>>>>> >> > do you also have enough priv to see kernel addresses outright (ie.
>>>>> >> > kallsyms etc..).
>>>>> >> >
>>>>> >> I was going to say just that. But that's not the default, paranoid level
>>>>> >> is at 1 by default and not 2. So I supposedly can still do:
>>>>> >>
>>>>> >> $ perf record -e cycles ......
>>>>> >>
>>>>> >> In per-thread mode and collect kernel level addresses.
>>>>> >
>>>>> > Oh right you are.. so yes that's a very viable avenue.
>>>>>
>>>>> You mean simply encoding the vma->vm_mm as the ino number, for instance.
>>>>
>>>> Nah.. I think Kees would very much shoot us on the spot for doing that.
>>>> But with the paranoid level defaulting to 1 the PMU attack on the kernel
>>>> SHA implenentation is feasible.
>>>
>>> We already have other kernel address leaks (e.g. heap addresses via
>>> INET_DIAG), and I'd like to avoid adding more. It'd be nice if there
>>> was a common way to uniquely mask these values that are really just
>>> "handles". We could use it both here and in the network code.
>>>
>>> Would it be possible to just have a "regular" incrementing handle,
>>> like fd, or are we talking about doing that map for all VMAs, which
>>> would make that mapping unfeasible due to storage needs?
>>>
>> All we need is a way to report that two vmas point to the same
>> vma->vm_mm, i..e, same physical data. If I understand what
>> you are suggesting, you'd add some sort of generation number
>> to the vm_mm. Each new vm_mm gets a new number. That
>> would work, I think. No kernel addresses reported directly nor
>> hashed.
>
> Right. Is that workable? It sounds like this handle is only needed at
> inspection time, though. Is this uniqueness test limited to a single
> process, or is this uniqueness test across processes?
>
Each time that vm_mm is allocated we would allocate a new generation
number.

Uniqueness is across processes. But that's by construction of the
address space.


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