target/riscv: fix access permission checks for CSR_SSP

Commit:8205bc1 ("target/riscv: introduce ssp and enabling controls for
zicfiss") introduced CSR_SSP but it mis-interpreted the spec on access
to CSR_SSP in M-mode. Gated to CSR_SSP is not gated via `xSSE`. But
rather rules clearly specified in section "22.2.1. Shadow Stack Pointer
(ssp) CSR access contr" in the priv spec.

Fixes: 8205bc1 ("target/riscv: introduce ssp and enabling controls
for zicfiss". Thanks to Adam Zabrocki for bringing this to attention.

Reported-by: Adam Zabrocki <[email protected]>
Signed-off-by: Deepak Gupta <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Alistair Francis <[email protected]>

Author: deepak0414

target/riscv/csr.c

@@ -192,6 +192,11 @@ static RISCVException cfi_ss(CPURISCVState *env, int csrno)
         return RISCV_EXCP_ILLEGAL_INST;
     }
 
+    /* If ext implemented, M-mode always have access to SSP CSR */
+    if (env->priv == PRV_M) {
+        return RISCV_EXCP_NONE;
+    }
+
     /* if bcfi not active for current env, access to csr is illegal */
     if (!cpu_get_bcfien(env)) {
 #if !defined(CONFIG_USER_ONLY)