Messages in this thread | | | Date | Tue, 18 Oct 2022 22:19:11 -0700 | From | Kees Cook <> | 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
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