Messages in this thread | | | Date | Tue, 15 Feb 2022 12:03:12 -0800 | From | Kees Cook <> | Subject | Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups |
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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
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