Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 7 Feb 2018 11:43:37 -0800 | Subject | Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f |
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On Wed, Feb 7, 2018 at 11:28 AM, Borislav Petkov <bp@alien8.de> wrote: > On Wed, Feb 07, 2018 at 08:14:51PM +0100, Peter Zijlstra wrote: >> Then someone went and wrecked it. > > Yeah, note says UD0 didn't eat a ModRM byte on old CPUs. But then that > changed too. Fun stuff changing insn encoding underway. > > So if we opt for adding a ModRM byte, could a 0x90 NOP work so that it > doesn't shit itself on those old CPUs?
We could just also decide that the only thing that the modrm bytes of UD0 actually *affect* is how the CPU might act for a page-crossing instruction.
Because I think that's the only semantic difference: if it's a page-crosser, the instruction could take a page fault before raising the #UD.
Is there any other decode issue we might want to look out for?
Anyway, then we'd make the rule be:
- we promise to always pad up the following bytes (our extra warning information etc) so that we never have a missing page afterwards (this is presumably practically speaking already the case). It might still be a page-crossing instruction, but we won't take a page fault in kernel space (due to it being at the end of some text sectoin or whatever that changes the NX bit or due to DEBUG_PAGEALLOC having unmapped the next page).
- we special-case the decoder so that we don't get this warning
and then we just ignore the issue entirely.
Hmm?
Linus
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