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    SubjectRe: [PATCH] arch/x86: fix out-of-bounds in get_wchan()
    On Mon, Sep 28, 2015 at 12:23 PM, Borislav Petkov <bp@alien8.de> wrote:
    > On Mon, Sep 28, 2015 at 11:49:18AM +0200, Dmitry Vyukov wrote:
    >> Original code did:
    >>
    >> if (p->thread.sp < stack || p->thread.sp >= stack+THREAD_SIZE)
    >> return 0;
    >> fp = *(u64 *)(p->thread.sp);
    >>
    >> p->thread.sp can change concurrently.
    >> So we could check that p->thread.sp is within stack bounds, but then
    >> dereference another value (which is already outside of bounds).
    >
    > Right, we do deref it. I realized that after hitting "Send" :\
    >
    > Which begs another, probably also stupid, question:
    >
    > What guarantees the task won't disappear after we've checked p?
    >
    > I.e., after this:
    >
    > if (!p || p == current || p->state == TASK_RUNNING)
    > return 0;


    I have not checked, but I would expect that it is caller's
    responsibility. There is generally no way to magically resurrect a
    pointer to a freed object passed in.


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