linux-hardening - Re: [PATCH] static_call,x86: Robustify trampoline patching

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Date:   Wed, 3 Nov 2021 12:32:20 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] static_call,x86: Robustify trampoline patching

On 11/3/21 01:35, Peter Zijlstra wrote:
> On Tue, Nov 02, 2021 at 05:20:05PM -0700, Andy Lutomirski wrote:
>> I think that's a big mistake -- any sane ENDBR-using scheme would
>> really prefer that ENDBR to be right next to the actual function body,
>> and really any scheme would benefit due to better cache locality.
> 
> Agreed, IBT/BTI want the landing pad in front of the actual function.
> 
>> But, more importantly, IMO any sane ENDBR-using scheme wants to
>> generate the indirect stub as part of code gen for the actual
>> function.
> 
> Sorta, I really want to be able to not have a landing pad for functions
> whose address is never taken. At that point it doesn't matter if it gets
> generated along with the function and then stripped/poisoned later, or
> generated later.

Stripping is conceptually straightforward even without LTO.

foo.indirect:
  ENDBR
foo:
  ...

and the linker learns (using magic function sections?) that, if 
foo.indirect is not referenced, then it should not be generated.  Or a 
very straightforward walk over all the relocations in an object to 
poison the unused .indirect entries could be done.  Making this work 
with DSOs, EXPORT_SYMBOL, etc will be somewhat nontrivial, but not 
impossible.

--Andy