linux-kernel - Re: [PATCH v3 00/39] x86: Kernel IBT

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Date:   Fri, 4 Mar 2022 22:39:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v3 00/39] x86: Kernel IBT

On Fri, Mar 04, 2022 at 11:09:58AM -0800, Josh Poimboeuf wrote:
> On Thu, Mar 03, 2022 at 12:23:21PM +0100, Peter Zijlstra wrote:
> > Hi, another week, another series.
> > 
> > Since last time:
> > 
> >  - fixed and tested kexec (redgecomb)
> >  - s/4*HAS_KERNEL_IBT/ENDBR_INSN_SIZE/ (jpoimboe)
> >  - re-arranged Xen patches to avoid churn (andyhpp)
> >  - folded IBT_SEAL Kconfig and objtool options (jpoimboe)
> >  - dropped direct call/jmp rewrite from objtool (jpoimboe)
> >  - dropped UD1 poison (jpoimboe)
> >  - fixed kprobe selftests (masami,naveen)
> >  - fixed ftrace selftests (rostedt)
> >  - simplified CET/INT3 selftests (jpoimboe)
> >  - boot time msg on IBT (kees)
> >  - objtool WARN_FUNC sym+off fallback (jpoimboe)
> >  - picked up tags for unchanged patches
> >  - probably more
> > 
> > Supposedly clang-14-rc2 will work on this series, I'll validate the moment the
> > Debian package gets updated.
> > 
> > Patches go on top of tip/master + arm64/for-next/linkage. Also available here:
> > 
> >   git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/wip.ibt
> 
> I'm getting some warnings with CONFIG_X86_KERNEL_IBT=n:
> 
>   arch/x86/entry/entry_64.o: warning: objtool: irq_entries_start()+0x7: unreachable instruction
>   arch/x86/kernel/ftrace_64.o: warning: objtool: return_to_handler()+0x2a: unreachable instruction

Blergh, those are INT3 instructions, the first is the LDT stub padding
while the second is that INT3 you asked about earlier.

I can mark then all using SLS style rules, but that then triggers:

arch/x86/kernel/reboot.o: warning: objtool: native_machine_emergency_restart()+0x8f: BUG: why am I validating an ignored function?

which does horrible things on purpose to tickle a tripple fault in order
to reboot the machine.

Perhaps we should ignore INT3 by default, just like NOP ?