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Date:   Tue, 18 Oct 2022 22:19:11 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Joao Moreira <joao@overdrivepizza.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        Sami Tolvanen <samitolvanen@google.com>,
        linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
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On Tue, Oct 18, 2022 at 09:48:42PM -0700, Joao Moreira wrote:
> > > Is it useful to get the compiler to emit 0xcc with
> > > -fpatchable-function-entry under any circumstance? I can probably
> > > change
> > > that quickly if needed/useful.
> > 
> > Having it emit 0xcc for the bytes in front of the symbol might be
> > interesting. It would mean a few kernel changes, but nothing too hard.
> > 
> > That is, -fpatchable-function-entry=N,M gets us N-M bytes in at the
> > start of the symbol and M bytes in front of it. The N-M bytes at the
> > start of the function *are* executed and should obviously not become
> > 0xcc (GCC keeps them 0x90 while LLVM makes them large NOPs).
> 
> Uhum, all makes sense. I drafted something here:
> 
> https://github.com/lvwr/llvm-project/commits/joao/int3
> 
> Let me know if this works for you or if there is something that should be
> tweaked, like adding a specific flag and such. This currently emits 0xcc
> instead of 0x90 for the nops before the function entry symbol for kernel
> code on x86-64. It seems to be working (see generated snippet below), but
> let me know otherwise:
> 
> Generated with -fpatchable-function-entry=10,5
> 
> Disassembly of section .text:
> 
> 0000000000000000 <save_processor_state-0x5>:
>    0:   cc                      int3
>    1:   cc                      int3
>    2:   cc                      int3
>    3:   cc                      int3
>    4:   cc                      int3
> 
> 0000000000000005 <save_processor_state>:
>    5:   0f 1f 44 00 08          nopl   0x8(%rax,%rax,1)
>    a:   41 57                   push   %r15
>    c:   41 56                   push   %r14

Cool! I like that. Assuming objtool doesn't freak out, that seems like a
nice way to go.

-- 
Kees Cook