Messages in this thread | | | Date | Thu, 6 Jul 2023 18:00:14 +0900 | From | Masami Hiramatsu (Google) <> | Subject | Re: [PATCH 2/2] x86/retpoline,kprobes: Avoid treating rethunk as an indirect jump |
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On Thu, 6 Jul 2023 09:17:05 +0200 Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Jul 06, 2023 at 09:47:23AM +0900, Masami Hiramatsu wrote: > > > > > If I understand correctly, all indirect jump will be replaced with JMP_NOSPEC. > > > > If you read the insn_jump_into_range, I onlu jecks the jump code, not call. > > > > So the functions only have indirect call still allow optprobe. > > > > > > With the introduction of kCFI JMP_NOSPEC is no longer an equivalent to a > > > C indirect jump. > > > > If I understand correctly, kCFI is enabled by CFI_CLANG, and clang is not > > using jump-tables by default, so we can focus on gcc. In that case > > current check still work, correct? > > IIRC clang can use jump tables, but like GCC needs RETPOLINE=n and > IBT=n, so effectively nobody has them.
So if it requires RETPOLINE=n, current __indirect_thunk_start/end checking is not required, right? (that code is embraced with "#ifdef CONFIG_RETPOLINE")
> > The reason I did mention kCFI though is that kCFI has a larger 'indirect > jump' sequence, and I'm not sure we've thought about what can go > sideways if that's optprobed.
If I understand correctly, kCFI checks only indirect function call (check pointer), so no jump tables. Or does it use indirect 'jump' ?
> > I suspect the UD2 that's in there will go 'funny' if it's relocated into > an optprobe, as in, it'll not be recognised as a CFI fail.
UD2 can't be optprobed (kprobe neither) because it can change the dumped BUG address...
Thank you, -- Masami Hiramatsu (Google) <mhiramat@kernel.org>
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