Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

Peter Zijlstra
2022-02-14T22:26:29Z

On Mon, Feb 14, 2022 at 01:38:18PM -0800, Sami Tolvanen wrote:
> On Fri, Feb 11, 2022 at 5:38 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > I think we'll end up with something related to KCFI, but with distinct
> > differences:
> >
> > - 32bit immediates for smaller code
>
> Sure, I don't see issues with that. Based on a quick test with
> defconfig, this reduces vmlinux size by 0.30%.
>
> > - __kcfi_check_fail() is out for smaller code
>
> I'm fine with adding a trap mode that's used by default, but having
> more helpful diagnostics when something fails is useful even in
> production systems in my experience. This change results in a vmlinux
> that's another 0.92% smaller.

You can easily have the exception generate a nice warning, you can even
have it continue. You really don't need a call for that.

> > Which then yields:
> >
> > caller:
> > cmpl $0xdeadbeef, -0x4(%rax) # 7 bytes
> > je 1f # 2 bytes
> > ud2 # 2 bytes
> > 1: call __x86_indirect_thunk_rax # 5 bytes
>
> Note that the compiler might not emit this *exact* sequence of
> instructions. For example, Clang generates this for events_sysfs_show
> with the modified KCFI patch:
>
> 2274: cmpl $0x4d7bed9e,-0x4(%r11)
> 227c: jne 22c0 <events_sysfs_show+0x6c>
> 227e: call 2283 <events_sysfs_show+0x2f>
> 227f: R_X86_64_PLT32 __x86_indirect_thunk_r11-0x4
> ...
> 22c0: ud2
>
> In this case the function has two indirect calls and Clang seems to
> prefer to emit just one ud2.

That will not allow you to recover from the exception. UD2 is not an
unconditional fail. It should have an out-going edge in this case too.

Heck, most of the WARN_ON() things are UD2 instructions.

Also, you really should add a CS prefix to the retpoline thunk call if
you insist on using r11 (or any of the higher regs).

> > .align 16
> > .byte 0xef, 0xbe, 0xad, 0xde # 4 bytes
> > func:
> > endbr # 4 bytes
>
> Here func is no longer aligned to 16 bytes, in case that's important.

The idea was to have the hash and the endbr in the same cacheline.

> > Did I miss anything? Got anything wrong?
>
> How would you like to deal with the 4-byte hashes in objtool? We
> either need to annotate all function symbols in the kernel, or we need
> a way to distinguish the hashes from random instructions, so we can
> also have functions that don't have a type hash.

Easiest would be to create a special section with all the hash offsets
in I suppose. A bit like -mfentry-section=name.