Messages in this thread | | | Date | Tue, 15 Feb 2022 22:05:50 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups |
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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.
> > > > 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.
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