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

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Date:   Wed, 27 Oct 2021 12:17:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        "x86@...nel.org" <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-hardening@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI

On Wed, Oct 27, 2021 at 10:02:56AM +0000, David Laight wrote:
> From: Peter Zijlstra
> > Sent: 26 October 2021 21:16
> > 
> > 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?
> > 
> > But how does this work with things like static_call(), which we give a
> > function address (now a jump-table entry) and use that to write direct
> > call instructions?
> > 
> > Should not this jump-table thingy get converted to an actual function
> > address somewhere around arch_static_call_transform() ? This also seems
> > relevant for arm64 (which already has CLANG_CFI supported) given:
> > 
> >   https://lkml.kernel.org/r/20211025122102.46089-3-frederic@kernel.org
> > 
> > Or am I still not understanding this CFI thing?
> 
> From what I remember the compiler adds code prior to every jump indirect
> to check that the function address is in the list of valid functions
> (with a suitable prototype - or some similar check).

It definitely mucks about with the address too; see here:

  https://lkml.kernel.org/r/YW6a67fGzM2AyHot@hirez.programming.kicks-ass.net

I'm thinking static_call() wants the real actual function address before
it writes instructions.