Re: [PATCH v3 22/29] riscv sigcontext: adding cfi state field in sigcontext

[Andy Chiu]

Hi Deepak,

On Thu, Apr 4, 2024 at 7:42 AM Deepak Gupta <debug@xxxxxxxxxxxx> wrote:
>
> Shadow stack needs to be saved and restored on signal delivery and signal
> return.
>
> sigcontext embedded in ucontext is extendible. Adding cfi state in there
> which can be used to save cfi state before signal delivery and restore
> cfi state on sigreturn
>
> Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> ---
>  arch/riscv/include/uapi/asm/sigcontext.h | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/arch/riscv/include/uapi/asm/sigcontext.h b/arch/riscv/include/uapi/asm/sigcontext.h
> index cd4f175dc837..5ccdd94a0855 100644
> --- a/arch/riscv/include/uapi/asm/sigcontext.h
> +++ b/arch/riscv/include/uapi/asm/sigcontext.h
> @@ -21,6 +21,10 @@ struct __sc_riscv_v_state {
>         struct __riscv_v_ext_state v_state;
>  } __attribute__((aligned(16)));
>
> +struct __sc_riscv_cfi_state {
> +       unsigned long ss_ptr;   /* shadow stack pointer */
> +       unsigned long rsvd;             /* keeping another word reserved in case we need it */
> +};
>  /*
>   * Signal context structure
>   *
> @@ -29,6 +33,7 @@ struct __sc_riscv_v_state {
>   */
>  struct sigcontext {
>         struct user_regs_struct sc_regs;
> +       struct __sc_riscv_cfi_state sc_cfi_state;

I am concerned about this change as this could potentially break uabi.
Let's say there is a pre-CFI program running on this kernel. It
receives a signal so the kernel lays out the sig-stack as presented in
this structure. If the program accesses sc_fpregs, it would now get
sc_cfi_state. As the offset has changed, and the pre-CFI program has
not been re-compiled.

>         union {
>                 union __riscv_fp_state sc_fpregs;
>                 struct __riscv_extra_ext_header sc_extdesc;
> --
> 2.43.2
>

There may be two ways to deal with this. One is to use a different
signal ABI for CFI-enabled programs. This may complicate the user
space because new programs will have to determine whether it should
use the CFI-ABI at run time. Another way is to follow what Vector does
for signal stack. It adds a way to introduce new extensions on signal
stack without impacting ABI.

Please let me know if I misunderstand anything, thanks.

Cheers,
Andy