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    SubjectRe: FW: i386 Linux kernel DoS

    On Thu, 14 Nov 2002, Andrea Arcangeli wrote:
    > actually TF should cleared implicitly in the do_debug or it could get
    > the single step trap before you can clear TF explicitly in the entry.S.

    But that's fine. Getting a single step trap in the kernel is not a
    problem: the trap will clear TF/NT on the "recursive" kernel entry, and on
    the recursive "iret" nothing bad will happen.

    Remember: what is on the _stack_ doesn't matter. The only thing that
    matters is what is actually in the EFLAGS register itself.

    > but it's certainly zerocost to clear it explicitly there too just to
    > remeber it's one of the bits not cleared implicitly in hardware when
    > entering via lcall. However in 2.5 it seems the clear_TF in do_debug is
    > still missing.

    No, do_debug() already does

    /* Mask out spurious TF errors due to lazy TF clearing */
    if (condition & DR_STEP) {
    if ((regs->xcs & 3) == 0)
    goto clear_TF;

    which will make sure that we only get _one_ of these spurious (and
    harmless) TF traps if somebody tries to mess with us.

    So that is correct (and your patch is _not_ correct - it's not right
    checking what the EIP value is, since it doesn't matter. In fact, I think
    you could quite validly have "big" EIP values in user space by just
    creating interesting code segments).


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