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

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Date:   Sat, 30 Oct 2021 20:55:20 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Mark Rutland <mark.rutland@....com>, X86 ML <x86@...nel.org>,
        Kees Cook <keescook@...omium.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 Sat, 30 Oct 2021 at 20:03, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Sat, Oct 30, 2021 at 07:19:53PM +0200, Ard Biesheuvel wrote:
> > I just realized that arm64 has the exact same problem, which is not
> > being addressed by my v5 of the static call support patch.
>
> Yeah, it would.
>
> > As it turns out, the v11 Clang that I have been testing with is broken
> > wrt BTI landing pads, and omits them from the jump table entries.
> > Clang 12+ adds them properly, which means that both the jump table
> > entry and the static call trampoline may start with BTI C + direct
> > branch, and we also need additional checks to disambiguate.
>
> I'm not sure, why would the static_call trampoline need a BTI C ? The
> whole point of static_call() is to be a direct call, we should never
> have an indirect call to the trampoline, that would defeat the whole
> purpose.

This might happen when the distance between the caller and the
trampoline is more than 128 MB, in which case we emit a veneer that
uses an indirect call as well. So we definitely need the landing pad
in the trampoline.