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

    > 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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