linux-kernel - Re: [RFC/RFT] CFI: Add support for gcc CFI in aarch64

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Message-ID: <Y7PtfkuE3KimY4Gq@FVFF77S0Q05N>
Date:   Tue, 3 Jan 2023 08:55:26 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dan Li <ashimida.1990@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Marco Elver <elver@...gle.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Song Liu <song@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uros Bizjak <ubizjak@...il.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Aaron Tomlin <atomlin@...hat.com>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Yuntao Wang <ytcoode@...il.com>,
        Changbin Du <changbin.du@...el.com>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [RFC/RFT] CFI: Add support for gcc CFI in aarch64

On Mon, Dec 19, 2022 at 04:04:55PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 19, 2022 at 05:32:04AM -0800, Dan Li wrote:
> > Hi Peter,
> > 
> > On 12/19, Peter Zijlstra wrote:
> > > On Sun, Dec 18, 2022 at 10:17:58PM -0800, Dan Li wrote:
> > > 
> > > > 1. When a typeid mismatch is detected, the cfi_check_failed function
> > > >    will be called instead of the brk instruction. This function needs
> > > >    to be implemented by the compiler user.
> > > >    If there are user mode programs or other systems that want to use
> > > >    this feature, it may be more convenient to use a callback (so this
> > > >    compilation option is set to -fsanitize=cfi instead of kcfi).
> > > 
> > > This is not going to be acceptible for x86_64.
> > 
> > I'm not familiar enough with the x86_64 platform, could you please
> > tell me why this is not acceptable? Is there a similar situation
> > on the arm64 platform?
> 
> Mostly because the call would be a 5 byte instruction while the trap
> (UD2) is only 2 bytes.
> 
> I suspect Argh64 has a similar problem if the to be called function is
> outside the immediate range (26 bits or thereabout), in which case you
> end up with a multi-instruction sequence to construct the call target or
> so.

Either that or a direct branc to a PLT.

> A trap is always a single instruction.

Indeed.

I strongly prefer the BRK for the reasons I've given in my other reply, which
include code size.

Thanks,
Mark.