[RFC/RFT] CFI: Add support for gcc CFI in aarch64

[Peter Zijlstra]

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?