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

Sami Tolvanen
2022-05-16T17:15:37Z

On Mon, May 16, 2022 at 2:54 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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?

What's the concern with the endbr? Dropping it would currently break
the CFI+IBT combination on newer hardware, no?

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

I'm fine with renaming the config.

> > ; 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.

With the current compiler patch, LLVM sets up function arguments after
the CFI check. if it's a problem, we can look into changing that.

Sami