[PATCH 2/6] powerpc/64s/kuap: kuap_restore missing isync

[Nicholas Piggin]

Writing the AMR register is documented to require context synchronizing
operations before and after, for it to take effect as expected. The kuap
restore at interrupt exit time deliberately avoids the isync after the
AMR update because it only needs to take effect after the context
synchronizing rfid that soon follows. Add a comment for this.

The missing isync before the update doesn't have an obvious
justification, and seems it could theorietically allow a rogue user
access to leak past the AMR update. Add isyncs for these.

Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
 arch/powerpc/include/asm/book3s/64/kup-radix.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/book3s/64/kup-radix.h b/arch/powerpc/include/asm/book3s/64/kup-radix.h
index 3bcef989a35d..8dc5f292b806 100644
--- a/arch/powerpc/include/asm/book3s/64/kup-radix.h
+++ b/arch/powerpc/include/asm/book3s/64/kup-radix.h
@@ -16,7 +16,9 @@
 #ifdef CONFIG_PPC_KUAP
 	BEGIN_MMU_FTR_SECTION_NESTED(67)
 	ld	\gpr, STACK_REGS_KUAP(r1)
+	isync
 	mtspr	SPRN_AMR, \gpr
+	/* No isync required, see kuap_restore_amr() */
 	END_MMU_FTR_SECTION_NESTED_IFSET(MMU_FTR_RADIX_KUAP, 67)
 #endif
 .endm
@@ -62,8 +64,15 @@
 
 static inline void kuap_restore_amr(struct pt_regs *regs)
 {
-	if (mmu_has_feature(MMU_FTR_RADIX_KUAP))
+	if (mmu_has_feature(MMU_FTR_RADIX_KUAP)) {
+		isync();
 		mtspr(SPRN_AMR, regs->kuap);
+		/*
+		 * No isync required here because we are about to rfi
+		 * back to previous context before any user accesses
+		 * would be made, which is a CSI.
+		 */
+	}
 }
 
 static inline void kuap_check_amr(void)
-- 
2.23.0