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

Kees Cook
2022-02-15T23:06:03Z

On Tue, Feb 15, 2022 at 10:05:50PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 15, 2022 at 12:03:12PM -0800, Kees Cook wrote:
> > On Tue, Feb 15, 2022 at 08:56:03AM -0800, Sami Tolvanen wrote:
> > > On Mon, Feb 14, 2022 at 2:25 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > > > On Mon, Feb 14, 2022 at 01:38:18PM -0800, Sami Tolvanen wrote:
> > > > > 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.
> > >
> > > Sure, but wouldn't that require us to generate something like
> > > __bug_table, so we know where the CFI specific traps are?
> >
> > It also means the trap handler needs to do a bunch of instruction
> > decoding to find the address that was going to be jumped to, etc.
>
> arch/x86/kernel/alternative.c:apply_retpolines() has all that, since we
> need to to know that to re-write the thunk-call.

Ah, okay, well that makes things easier. :)

> > > > > 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.
> > >
> > > Yes, CFI failures are not recoverable in that code. In fact, LLVM
> > > assumes that the llvm.trap intrinsic (i.e. ud2) never returns, but I
> > > suppose we could just use an int3 instead. I assume that's sufficient
> > > to stop speculation?
> >
> > Peter, is there a reason you want things in the specific order of:
> >
> > cmp, je-to-call, trap, call
> >
> > Isn't it more run-time efficient to have an out-of-line failure of
> > the form:
> >
> > cmp, jne-to-trap, call, ...code..., trap, jmp-to-call
> >
> > I thought the static label stuff allowed the "default out of line"
> > option, as far as pessimizing certain states, etc? The former is certainly
> > code-size smaller, though, yes, but doesn't it waste space in the cache
> > line for the unlikely case, etc?
>
> Mostly so that we can deduce the address of the trap from the retpoline
> site, also the above has a fairly high chance of using jcc.d32 which is
> actually larger than jcc.d8+ud2.

Ah, yeah, that's an interesting point.

Still, I worry about finding ways to convinces Clang to emit precisely
cmp/je/trap/call, but I guess we'll catch it immediately if it doesn't.
:P

--
Kees Cook