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

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Date:   Mon, 14 Feb 2022 23:25:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Joao Moreira <joao@...rdrivepizza.com>,
        Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
        hjl.tools@...il.com, Josh Poimboeuf <jpoimboe@...hat.com>,
        andrew.cooper3@...rix.com, LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        llvm@...ts.linux.dev
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

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@...radead.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.