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

Peter Zijlstra
2022-05-16T18:31:01Z

On Mon, May 16, 2022 at 10:15:00AM -0700, Sami Tolvanen wrote:
> 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?

Well, yes, but also that combination isn't very interesting. See,

https://lore.kernel.org/all/20220420004241.2093-1-joao@overdrivepizza.com/T/#m5d67fb010d488b2f8eee33f1eb39d12f769e4ad2

and the patch I did down-thread:

https://lkml.kernel.org/r/YoJKhHluN4n0kZDm@hirez.programming.kicks-ass.net

If we have IBT, then FineIBT is a much better option than kCFI+IBT.
Removing that superfluous endbr also shrinks the whole thing by 4 bytes.

So I'm fine with the compiler generating working code for that
combination; but please get me an option to supress it in order to save
those pointless bytes. All this CFI stuff is enough bloat as it is.

> > > ; 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.

Yes, please fix that. Again see that same patch for why this is a
problem. Objtool can trivially find retpoline calls, but finding this
kCFI gadget is going to be hard work. If you ensure they're
unconditionally stuck together, then the problem goes away find one,
finds the other.