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    SubjectRe: [PATCH v2 2/3] mm: add LSM hook for writes to readonly memory
    On Thu, Sep 29, 2016 at 01:32:56AM +0200, Jann Horn wrote:
    > On Wed, Sep 28, 2016 at 04:22:53PM -0700, Andy Lutomirski wrote:
    > > On Wed, Sep 28, 2016 at 3:54 PM, Jann Horn <jann@thejh.net> wrote:
    > > > SELinux attempts to make it possible to whitelist trustworthy sources of
    > > > code that may be mapped into memory, and Android makes use of this feature.
    > > > To prevent an attacker from bypassing this by modifying R+X memory through
    > > > /proc/$pid/mem or PTRACE_POKETEXT, it is necessary to call a security hook
    > > > in check_vma_flags().
    > >
    > > If selinux policy allows PTRACE_POKETEXT, is it really so bad for that
    > > to result in code execution?
    >
    > Have a look at __ptrace_may_access():
    >
    > /* Don't let security modules deny introspection */
    > if (same_thread_group(task, current))
    > return 0;
    >
    > This means thread A can attach to thread B and poke its memory, and SELinux
    > can't do anything about it.
    >
    > I guess another perspective on this would be that it's a problem that
    > interfaces usable for poking user memory are subject to introspection rules
    > (as opposed to e.g. /proc/self/maps, where it is actually useful).

    Ugh, I'm talking nonsense, ptrace() doesn't work on threads. (/proc/$pid/mem
    works though). And then, ptrace-ish APIs aside, there are those weird
    devices that do DMA with force=1.
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