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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).

Linus

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