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

[Stephane Eranian]

On Wed, Oct 2, 2013 at 7:14 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Wed, Oct 2, 2013 at 6:37 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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.

> On the other hand, trying to hide kernel addresses from root is no
> easy task, especially given that while kptr_restrict has a "2" level,
> dmesg_restrict does not. Are these kernel-address-leaking perf modes
> already limited to a specific POSIX capability?
>
> -Kees
>
> --
> Kees Cook
> Chrome OS Security
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