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

[Deepak Gupta]

On Fri, May 24, 2024 at 05:46:16PM +0800, Andy Chiu wrote:
>Hi Deepak,
>
>On Thu, Apr 4, 2024 at 7:42 AM Deepak Gupta <debug at rivosinc.com> 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 at rivosinc.com>
>> ---
>>  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.

Yeah this is a problem if program was built with older kernel/old toolchain
(or cfi unaware toolchain). Thanks.

>
>>         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.

I think following how vector does would be cleaner.
Let me munch on this a little bit.

>
>Cheers,
>Andy