Messages in this thread | | | Date | Thu, 18 Jun 2020 23:18:23 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/7] x86/entry: Fix #UD vs WARN more |
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On Thu, Jun 18, 2020 at 12:29:50PM -0700, Andy Lutomirski wrote: > > > On Jun 18, 2020, at 12:02 PM, Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Thu, Jun 18, 2020 at 11:36:53AM -0700, Andy Lutomirski wrote: > > > >> I wasn't imagining going far down the rabbit hole at all -- I think > >> that, at most, we should cover the path for when the fault wasn't a > >> BUG/WARN in the first place. I admit that, for #UD in particular, > >> this isn't a big deal, but if it were a different vector, this could > >> matter. > > > > Right, so there's 3 cases for ud2: > > > > - WARN; ud2, bug_entry, recovers > > - BUG; ud2, bug_entry, dies > > - UBSAN; ud2, !bug_entry, dies > > 4. The #UD matches an extable entry. I don’t know whether this ever happens for real.
#UD yes, ud2 instruction, not so much.
> The failure is still a bit farfetched: we’d need an extable to hit in > an inconsistent state where we blow up due to a lack of entry > handling.
Right, by noinstr checking the instruction is actually ud2 I think we mostly good. There really aren't that many places that emit ud2.
> But I think you might need some IRQ fiddling. With your patch, a WARN > with IRQs on will execute the printk code with IRQs off without > lockstep handling, and an appropriately configured debugging kernel > may get a recursive splat. Or if irq tracing somehow notices that > IRQs got turned off, the warning recovery might return back to an IF=1 > context with IRQs traced as off. > > So maybe also do an untraced cond_local_irq_enable()? After all, if > we’re trying to report a bug from IRQs on, it should be okay to have > IRQs on while reporting it. It might even work better than having IRQs > off.
Yes, very good point. Now I want to go look at the old code... I'll frob something tomorrow, brain is pretty fried by now.
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