Re: [PATCH] x86/ibt: Implement FineIBT
From: Joao Moreira
Date: Mon Oct 31 2022 - 15:14:14 EST
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.