[PATCH v3] powerpc/interrupt: Use early_radix_enabled() in NMI real-mode guard

[Venkat Rao Bagalkote <venkat88@linux.ibm.com>]

radix_enabled() uses a jump label which is only valid after
setup_feature_keys() is called. Before that point, on a pSeries
hash guest, early_check_vec5() clears MMU_FTR_TYPE_RADIX in
cur_cpu_spec->mmu_features, but the jump label has not yet been patched,
so radix_enabled() incorrectly returns true.

The dangerous usage window where it goes wrong in early_setup():

    early_setup:
        configure_exceptions();
        <exceptions can happen now, and handler enter/exit can be invoked>
        <those could use radix_enabled(), which returns stale true>
        setup_feature_keys();
        <jump labels set up; post this it is safe to use radix_enabled()>

If an NMI occurs in this window, !radix_enabled() evaluates to false in
DEFINE_INTERRUPT_HANDLER_NMI. The handler fails to skip NMI entry in real
mode and attempts to access memory outside the Real Mode Area (RMA),
hanging the boot.

Since common interrupt wrappers do not have the context of early or late,
using early_radix_enabled() is the safer option. It does a plain bitmask
check against cur_cpu_spec->mmu_features and is correct at all times.

Console logs from a pSeries HASH guest showing values across boot stages:

[    0.000000] DEBUG: after early_init_devtree: early_radix_enabled=0 radix_enabled=1 (mismatch means NMI real-mode check is unsafe!)
[    0.000000] DEBUG: after configure_exceptions (DANGEROUS WINDOW): early_radix_enabled=0 radix_enabled=1
[    0.000000] DEBUG: after setup_feature_keys (jump labels initialized): early_radix_enabled=0 radix_enabled=0 (should now match!)
[    0.057124] DEBUG: post secondary CPU bringup: early_radix_enabled=0 radix_enabled=0 (should match!)

Console logs from a RADIX guest showing values across boot stages:

[    0.000000] DEBUG: after early_init_devtree: early_radix_enabled=1 radix_enabled=1 (mismatch means NMI real-mode check is unsafe!)
[    0.000000] DEBUG: after configure_exceptions (DANGEROUS WINDOW): early_radix_enabled=1 radix_enabled=1
[    0.000000] DEBUG: after setup_feature_keys (jump labels initialized): early_radix_enabled=1 radix_enabled=1 (should now match!)
[    0.057016] DEBUG: post secondary CPU bringup: early_radix_enabled=1 radix_enabled=1 (should match!)

Add the missing #include <asm/mmu.h> in alphabetical order since
early_radix_enabled() is declared there.

Fixes: 8d0e21012743 ("powerpc/mce: Avoid nmi_enter/exit in real mode on pseries hash")
Signed-off-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Mukesh Kumar Chaurasiya <mkchauras@gmail.com>
---
v3:
 - Clarified early_setup execution flow and race window in changelog.
 - Explained why early_radix_enabled() is required for context-agnostic wrappers.
 - Added console logs for both HASH and RADIX guests in changelog.
v2:
 - Added Fixes: tag referencing commit 8d0e21012743.
 - Included <asm/mmu.h> in alphabetical order.
 - Added Reviewed-by tag from Mukesh.

 arch/powerpc/include/asm/interrupt.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/interrupt.h b/arch/powerpc/include/asm/interrupt.h
index 1b45a49e9bed..355f6bbf9894 100644
--- a/arch/powerpc/include/asm/interrupt.h
+++ b/arch/powerpc/include/asm/interrupt.h
@@ -70,6 +70,7 @@
 #include <linux/irq-entry-common.h>
 
 #include <asm/kprobes.h>
+#include <asm/mmu.h>
 #include <asm/runlatch.h>
 
 #ifdef CONFIG_PPC_IRQ_SOFT_MASK_DEBUG
@@ -290,7 +291,7 @@ interrupt_handler long func(struct pt_regs *regs)			\
 		state = irqentry_nmi_enter(regs);			\
 	} else if (IS_ENABLED(CONFIG_PPC_BOOK3S_64) &&			\
 		   firmware_has_feature(FW_FEATURE_LPAR) &&		\
-		   !radix_enabled()) {					\
+		   !early_radix_enabled()) {				\
 		/* no nmi_entry for a pseries hash guest		\
 		 * taking a real mode exception */			\
 	} else if (IS_ENABLED(CONFIG_KASAN)) {				\
@@ -307,7 +308,7 @@ interrupt_handler long func(struct pt_regs *regs)			\
 		irqentry_nmi_exit(regs, state);				\
 	} else if (IS_ENABLED(CONFIG_PPC_BOOK3S_64) &&			\
 		   firmware_has_feature(FW_FEATURE_LPAR) &&		\
-		   !radix_enabled()) {					\
+		   !early_radix_enabled()) {				\
 		/* no nmi_exit for a pseries hash guest			\
 		 * taking a real mode exception */			\
 	} else if (IS_ENABLED(CONFIG_KASAN)) {				\
-- 
2.45.2