riscv/signal: save and restore the shadow stack on a signal

riscv/signal: save and restore the shadow stack on a signal

Save the shadow stack pointer in the sigcontext structure when
delivering a signal.  Restore the shadow stack pointer from sigcontext
on sigreturn.

As part of the save operation, the kernel uses the 'ssamoswap'
instruction to save a snapshot of the current shadow stack on the
shadow stack itself (this can be called a "save token"). During
restore on sigreturn, the kernel retrieves the save token from the top
of the shadow stack and validates it. This ensures that user mode
can't arbitrarily pivot to any shadow stack address without having a
token and thus provides a strong security assurance during the window
between signal delivery and sigreturn.

Use an ABI-compatible way of saving/restoring the shadow stack pointer
into the signal stack. This follows the 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 for the extension. Then, the extension body contains the
new architectural states in the form defined by uapi.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de>
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-17-b55691eacf4f@rivosinc.com
[pjw@kernel.org: cleaned patch description, code comments; resolved checkpatch warning]
Signed-off-by: Paul Walmsley <pjw@kernel.org>