[RFC PATCH 04/12] lib: sbi_hart: lock mwid CSR for RoT immutability

[Yu-Chien Peter Lin]

Lock the M-mode World ID (mwid) CSR during hart re-initialization to
enforce immutability of the WID established by the root-of-trust.

OpenSBI does not assign the WID value itself; it only sets MWID_LOCK
to freeze the value established by prior RoT stage. The MWID_LOCK bit
at XLEN-1 is sticky and makes the CSR read-only until reset, enforcing
a temporal security boundary per the RISC-V Worlds specification.

Signed-off-by: Yu-Chien Peter Lin <peter.lin at sifive.com>
---
 lib/sbi/sbi_hart.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/sbi/sbi_hart.c b/lib/sbi/sbi_hart.c
index cb0c66ea..4fbe46d7 100644
--- a/lib/sbi/sbi_hart.c
+++ b/lib/sbi/sbi_hart.c
@@ -804,6 +804,13 @@ int sbi_hart_reinit(struct sbi_scratch *scratch)
 	if (rc)
 		return rc;
 
+	/*
+	 * Assume MWID is restored by root-of-trust M-mode in previous
+	 * stage. Lock mwid so RoT-defined WID remains immutable.
+	 */
+	if (sbi_hart_has_extension(scratch, SBI_HART_EXT_SMWID))
+		csr_set(CSR_MWID, MWID_LOCK);
+
 	return 0;
 }
 
-- 
2.43.7