Re: [PATCH] static_call,x86: Robustify trampoline patching

Peter Zijlstra
2021-11-02T15:15:48Z

On Tue, Nov 02, 2021 at 01:57:44PM +0100, Peter Zijlstra wrote:

> So how insane is something like this, have each function:
>
> foo.cfi:
> endbr64
> xorl $0xdeadbeef, %r10d
> jz foo
> ud2
> nop # make it 16 bytes
> foo:
> # actual function text goes here
>
>
> And for each hash have two thunks:
>
>
> # arg: r11
> # clobbers: r10, r11
> __x86_indirect_cfi_deadbeef:
> movl -9(%r11), %r10 # immediate in foo.cfi
> xorl $0xdeadbeef, %r10 # our immediate
> jz 1f
> ud2
> 1: ALTERNATIVE_2 "jmp *%r11",
> "jmp __x86_indirect_thunk_r11", X86_FEATURE_RETPOLINE
> "lfence; jmp *%r11", X86_FEATURE_RETPOLINE_AMD
>
>
>
> # arg: r11
> # clobbers: r10, r11
> __x86_indirect_ibt_deadbeef:
> movl $0xdeadbeef, %r10
> subq $0x10, %r11
> ALTERNATIVE "", "lfence", X86_FEATURE_RETPOLINE
> jmp *%r11
>

These two thunks could of course be one big alternative.

> And have the actual indirect callsite look like:
>
> # r11 - &foo
> ALTERNATIVE_2 "cs call __x86_indirect_thunk_r11",
> "cs call __x86_indirect_cfi_deadbeef", X86_FEATURE_CFI
> "cs call __x86_indirect_ibt_deadbeef", X86_FEATURE_IBT

Also simplifying this.

> Although if the compiler were to emit:
>
> cs call __x86_indirect_cfi_deadbeef
>
> we could probaly fix it up from there.
>
>
> Then we can at runtime decide between:
>
> {!cfi, cfi, ibt} x {!retpoline, retpoline, retpoline-amd}
>