[PATCH v4 4/9] lib: sbi_hart: lock mwid CSR for RoT immutability

[Yu-Chien Peter Lin <peter.lin@sifive.com>]

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@sifive.com>
Reviewed-by: Pawandeep Oza <pawandeep.oza@oss.qualcomm.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
---
 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 77ef1a70..611c97bf 100644
--- a/lib/sbi/sbi_hart.c
+++ b/lib/sbi/sbi_hart.c
@@ -705,6 +705,13 @@ 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


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