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Date:   Mon, 31 Oct 2022 12:13:50 -0700
From:   Joao Moreira <joao@overdrivepizza.com>
To:     Kees Cook <keescook@chromium.org>
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 2022-10-18 22:19, Kees Cook wrote:
> 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.

Should I push for this within clang? I have the patch semi-ready (below) 
and would have some cycles this week for polishing it.

>> >
>> > 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.