[PATCH v22 17/28] riscv/signal: save and restore of shadow stack for signal

[Paul Walmsley]

On Thu, 23 Oct 2025, Deepak Gupta via B4 Relay wrote:

> From: Deepak Gupta <debug at rivosinc.com>
> 
> Save shadow stack pointer in sigcontext structure while delivering signal.
> Restore shadow stack pointer from sigcontext on sigreturn.
> 
> As part of save operation, kernel uses `ssamoswap` to save snapshot of
> current shadow stack on shadow stack itself (can be called as a save
> token). During restore on sigreturn, kernel retrieves token from top of
> shadow stack and validates it. This allows that user mode can't arbitrary
> pivot to any shadow stack address without having a token and thus provide
> strong security assurance between signaly delivery and sigreturn window.
> 
> Use ABI compatible way of saving/restoring shadow stack pointer into
> signal stack. This follows what Vector extension, where extra registers
> are placed in a form of extension header + extension body in the stack.
> The extension header indicates the size of the extra architectural
> states plus the size of header itself, and a magic identifier of the
> extension. Then, the extensions body contains the new architectural
> states in the form defined by uapi.
> 
> Signed-off-by: Andy Chiu <andy.chiu at sifive.com>
> Signed-off-by: Deepak Gupta <debug at rivosinc.com>

This patch causes some 'checkpatch.pl --strict' messages:

CHECK: Comparison to NULL could be written "!saved_shstk_ptr"
#271: FILE: arch/riscv/kernel/usercfi.c:186:
+	if (saved_shstk_ptr == NULL)

CHECK: Lines should not end with a '('
#300: FILE: arch/riscv/kernel/usercfi.c:215:
+		pr_info_ratelimited(

I've fixed them up here in the event that v22 goes in, but please do the 
same on your side in case a new version is needed.


- Paul