Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

Peter Zijlstra
2022-05-16T09:54:59Z

On Fri, May 13, 2022 at 01:21:58PM -0700, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG, the compiler injects a type preamble
> immediately before each function and a check to validate the target
> function type before indirect calls:
>
> ; type preamble
> __cfi_function:
> int3
> int3
> mov <id>, %eax
> int3
> int3
> function:
> ...

When I enable CFI_CLANG and X86_KERNEL_IBT I get:

0000000000000c80 <__cfi_io_schedule_timeout>:
c80: cc int3
c81: cc int3
c82: b8 b5 b1 39 b3 mov $0xb339b1b5,%eax
c87: cc int3
c88: cc int3

0000000000000c89 <io_schedule_timeout>:
c89: f3 0f 1e fa endbr64

That seems unfortunate. Would it be possible to get an additional
compiler option to suppress the endbr for all symbols that get a __cfi_
preaamble?

Also, perhaps s/CFI_CLANG/KERNEL_CFI/ or somesuch, so that GCC might
also implement this same scheme (in time)?

> ; indirect call check
> cmpl    <id>, -6(%r11)
> je .Ltmp1
> ud2
> .Ltmp1:
> call __x86_indirect_thunk_r11

The first one I try and find looks like:

26: 41 81 7b fa a6 96 9e 38 cmpl $0x389e96a6,-0x6(%r11)
2e: 74 02 je 32 <__traceiter_sched_kthread_stop+0x29>
30: 0f 0b ud2
32: 4c 89 f6 mov %r14,%rsi
35: e8 00 00 00 00 call 3a <__traceiter_sched_kthread_stop+0x31> 36: R_X86_64_PLT32 __x86_indirect_thunk_r11-0x4

This must not be. If I'm to rewrite that lot to:

movl $\hash, %r10d
sub $9, %r11
call *%r11
.nop 4

Then there must not be spurious instruction in between the ud2 and the
indirect call/retpoline thing.