linux-kernel - Re: [PATCH v5 00/15] x86: Add support for Clang CFI

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Date:   Wed, 27 Oct 2021 14:46:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>, 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@...r.kernel.org,
        llvm@...ts.linux.dev, ardb@...nel.org
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI

On Wed, Oct 27, 2021 at 01:05:15PM +0100, Mark Rutland wrote:
> On Tue, Oct 26, 2021 at 10:16:22PM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 13, 2021 at 11:16:43AM -0700, Sami Tolvanen wrote:
> > > This series adds support for Clang's Control-Flow Integrity (CFI)
> > > checking to x86_64. With CFI, the compiler injects a runtime
> > > check before each indirect function call to ensure the target is
> > > a valid function with the correct static type. This restricts
> > > possible call targets and makes it more difficult for an attacker
> > > to exploit bugs that allow the modification of stored function
> > > pointers. For more details, see:
> > > 
> > >   https://clang.llvm.org/docs/ControlFlowIntegrity.html
> > 
> > So, if I understand this right, the compiler emits, for every function
> > two things: 1) the actual funcion and 2) a jump-table entry.
> > 
> > Then, every time the address of a function is taken, 2) is given instead
> > of the expected 1), right?
> 
> Yes, and we had to bodge around this with function_nocfi() to get the
> actual function address.

The patch set under consideration seems to have forgotten to provide one
for x86 :/

> Really there should be a compiler intrinsic or attribute for this, given
> the compiler has all the releveant information available. On arm64 we
> had to us inine asm to generate the addres...

Agreed, this *really* shouldn't be an arch asm hack trying to undo
something the compiler did.