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

Kees Cook
2022-02-08T23:42:38Z

On Thu, Dec 23, 2021 at 06:05:50PM -0800, joao@overdrivepizza.com wrote:
> On 2021-11-22 09:03, Peter Zijlstra wrote:
> > Objtool based IBT validation in 3 passes:
> >
> > --ibt-fix-direct:
> >
> > Detect and rewrite any code/reloc from a JMP/CALL instruction
> > to an ENDBR instruction. This is basically a compiler bug since
> > neither needs the ENDBR and decoding it is a pure waste of time.
> >
> > --ibt:
> >
> > Report code relocs that are not JMP/CALL and don't point to ENDBR
> >
> > There's a bunch of false positives, for eg. static_call_update()
> > and copy_thread() and kprobes. But most of them were due to
> > _THIS_IP_ which has been taken care of with the prior annotation.
> >
> > --ibt-seal:
> >
> > Find and NOP superfluous ENDBR instructions. Any function that
> > doesn't have it's address taken should not have an ENDBR
> > instruction. This removes about 1-in-4 ENDBR instructions.
> >
>
> I did some experimentation with compiler-based implementation for two of the
> features described here (plus an additional one). Before going into details,
> just a quick highlight that the compiler has limited visibility over
> handwritten assembly sources thus, depending on the feature, a
> compiler-based approach will not cover as much as objtool. All the
> observations below were made when compiling the kernel with defconfig, +
> CLANG-related options, + LTO sometimes. Here I used kernel revision
> 0fcfb00b28c0b7884635dacf38e46d60bf3d4eb1 with PeterZ's IBT Beginning patches
> applied on top (plus changes to Kbuild), thus, IBT was not really enforced.
> Tests consisted mostly of Clang's synthetics tests + booting a compiled
> kernel.
>
> Prototypes of the features are available in:
> https://github.com/lvwr/llvm/tree/joao/ibt -- I fixed as many corner cases I
> could find while trying it out, but I believe some might still be haunting.
> Also, I'm not very keen to Kbuild internals nor to however the kernel
> patches itself during runtime, so I may have missed some details.
>
> Finally, I'm interested in general feedback about this... better ways of
> implementing, alternative approaches, new possible optimizations and
> everything. I should be AFK for a few days in the next weeks, but I'll be
> glad to discuss this in January and then. Happy Holidays :)
>
> The features:
>
> -mibt-seal:
>
> Add ENDBRs exclusively to address-taken functions regardless of its linkage
> visibility. Only make sense together with LTO.
>
> Observations: Reduced drastically the number of ENDBRs placed in the kernel
> binary (From 44383 to 33192), but still missed some that were later fixed by
> objtool (Number of fixes by objtool reduced from 11730 to 540). I did not
> investigate further why these superfluous ENDBRs were still left in the
> binary, but at this point my hypotheses spin around (i) false-positive
> address-taken conclusions by the compiler, possibly due to things like
> exported symbols and such; (ii) assembly sources which are invisible to the
> compiler (although this would more likely hide address taken functions);
> (iii) other binary level transformations done by objtool.
>
> Runtime testing: The kernel was verified to properly boot after being
> compiled with -mibt-seal (+ LTO).
>
> Note: This feature was already submitted for upstreaming with the
> llvm-project: https://reviews.llvm.org/D116070

Ah nice; I see this has been committed now.

Given that IBT will need to work with both Clang and gcc, I suspect the
objtool approach will still end up needing to do all the verification.

(And as you say, it has limited visibility into assembly.)

-Kees

--
Kees Cook