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

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Message-ID: <Y6A/k7/KrCCDuux6@hirez.programming.kicks-ass.net>
Date:   Mon, 19 Dec 2022 11:40:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dan Li <ashimida.1990@...il.com>
Cc:     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>,
        Mark Rutland <mark.rutland@....com>,
        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 Sun, Dec 18, 2022 at 10:17:58PM -0800, Dan Li wrote:

> In the compiler part[4], there are some differences from Sami's
> implementation[3], mainly including:
> 
> 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.

> 2. A reserved typeid (such as 0x0U on the aarch64 platform) is always
>    inserted in front of functions that should not be called indirectly.
>    Functions that can be called indirectly will not use this hash value,
>    which prevents instructions/data before the function from being used
>    as a typeid by an attacker.
> 
> 3. Some bits are ignored in the typeid to avoid conflicts between the
>    typeid and the instruction set of a specific platform, thereby
>    preventing an attacker from bypassing the CFI check by using the
>    instruction as a typeid, such as on the aarch64 platform:
>    * If the following instruction sequence exists:
> 	  400620:       a9be7bfd        stp     x29, x30, [sp, #-32]!
> 	  400624:       910003fd        mov     x29, sp
> 	  400628:       f9000bf3        str     x19, [sp, #16]
>    * If the expected typeid of the indirect call is exactly 0x910003fd,
>      the attacker can jump to the next instruction position of any
>      "mov x29,sp" instruction (such as 0x400628 here).
> 
> 4. Insert a symbol __cfi_<function> before each function's typeid,
>    which may be helpful for fine-grained KASLR implementations (or not?).
> 
> 5. The current implementation of gcc only supports the aarch64 platform.

What, if any, are the plans for x86_64 support?