[PATCH] powerpc/kexec: Fix radix to hash kexec

[Balbir Singh]

This patch fixes a crash seen while doing a kexec from
radix mode to hash mode. Key 0 is special in hash and
used in the RPN by default, we set the key values to 0
today. In radix mode key 0 is used to control
supervisor<->user access. In hash key 0 is used by default,
so the first instruction after the switch causes a crash on
kexec.

This patch fixes
(3b10d00 powerpc/mm/radix: Prevent kernel execution of user space)
The commit above introduced setting of IAMR and AMOR values to
prevent execution of user mode instructions from supervisor mode.
We need to clean up these SPR's on kexec.

Reported-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Signed-off-by: Balbir Singh <bsingharora at gmail.com>
---

NOTES:
 This patch can be made power9 specific, but I've chosen not
 to do so, I could add it in a feature section if recommended.
 Since this is only for kexec resetting them might be helpful
 when we fully implement keys as well.

 arch/powerpc/kernel/misc_64.S | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/powerpc/kernel/misc_64.S b/arch/powerpc/kernel/misc_64.S
index c119044..39d2b58 100644
--- a/arch/powerpc/kernel/misc_64.S
+++ b/arch/powerpc/kernel/misc_64.S
@@ -614,6 +614,16 @@ _GLOBAL(kexec_sequence)
 	li	r0,0
 	std	r0,16(r1)
 
+	/*
+	 * This is the best time to turn AMR/IAMR off.
+	 * key 0 is used in radix for supervisor<->user
+	 * protection, but on hash key 0 is reserved
+	 * ideally we want to enter with a clean state.
+	 * NOTE, we rely on r0 being 0 from above.
+	 */
+	mtspr	SPRN_IAMR,r0
+	mtspr	SPRN_AMOR,r0
+
 	/* save regs for local vars on new stack.
 	 * yes, we won't go back, but ...
 	 */
-- 
2.9.4