Re: [PATCH v5 00/15] x86: Add support for Clang CFI

Peter Zijlstra
2021-10-29T20:04:24Z

On Wed, Oct 27, 2021 at 08:50:17AM -0700, Sami Tolvanen wrote:
> On Wed, Oct 27, 2021 at 7:18 AM Ard Biesheuvel <ardb@kernel.org> wrote:

> > > /*
> > > * Turns a Clang CFI jump-table entry into an actual function pointer.
> > > * These jump-table entries are simply jmp.d32 instruction with their
> > > * relative offset pointing to the actual function, therefore decode the
> > > * instruction to find the real function.
> > > */
> > > static __always_inline void *nocfi_ptr(void *func)
> > > {
> > > union text_poke_insn insn = *(union text_poke_insn *)func;
> > >
> > > return func + sizeof(insn) + insn.disp;
> > > }
> > >
> > > But really, that wants to be a compiler intrinsic.
> >
> > Agreed. We could easily do something similar on arm64, but I'd prefer
> > to avoid that too.
>
> I'll see what we can do. Note that the compiler built-in we previously
> discussed would have semantics similar to function_nocfi(). It would
> return the raw function address from a symbol name, but it wouldn't
> decode the address from an arbitrary pointer, so this would require
> something different.

So I had a bit of a peek at what clang generates:

3fa4: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 3fa7: R_X86_64_32S __SCK__x86_pmu_handle_irq
3fab: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 3fae: R_X86_64_32S __SCT__x86_pmu_handle_irq.cfi_jt
3fb2: e8 00 00 00 00 call 3fb7 <init_hw_perf_events+0x1dc> 3fb3: R_X86_64_PLT32 __static_call_update-0x4

So this then gives the trampoline jump table entry to
__static_call_update(), with the result that it will rewrite the
jump-table entry, not the trampoline!

Now it so happens that the trampoline looks *exactly* like the
jump-table entry (one jmp.d32 instruction), so in that regards it'll
again 'work'.

But this is all really, as in *really*, wrong. And I'm really sad I'm
the one to have to discover this, even though I've mentioned
static_call()s being tricky in previous reviews.