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

Kees Cook
2022-02-15T20:03:39Z

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.

> > > 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?

> > 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).
>
> I actually didn't touch the retpoline thunk call, that's exactly the
> code Clang normally generates.
>
> > > 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.
>
> OK, I'll take a look. With 64-bit hashes I was planning to use a known
> prefix, but that's not really an option with a 32-bit hash.

32-bit hashes would have both code size and runtime benefits: fewer
instructions for the compare therefore a smaller set of instructions
added.

--
Kees Cook