powerpc/32: Fix hardlockup on vmap stack overflow

Since the commit c118c730 ("powerpc/32: Fix vmap stack - Do not
activate MMU before reading task struct") a vmap stack overflow
results in a hard lockup. This is because emergency_ctx is still
addressed with its virtual address allthough data MMU is not active
anymore at that time.

Fix it by using a physical address instead.

Fixes: c118c730 ("powerpc/32: Fix vmap stack - Do not activate MMU before reading task struct")
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ce30364fb7ccda489272af4a1612b6aa147e1d23.1637227521.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_32.h

@@ -202,11 +202,11 @@ _ASM_NOKPROBE_SYMBOL(\name\()_virt)
 	mfspr	r1, SPRN_SPRG_THREAD
 	lwz	r1, TASK_CPU - THREAD(r1)
 	slwi	r1, r1, 3
-	addis	r1, r1, emergency_ctx@ha
+	addis	r1, r1, emergency_ctx-PAGE_OFFSET@ha
 #else
-	lis	r1, emergency_ctx@ha
+	lis	r1, emergency_ctx-PAGE_OFFSET@ha
 #endif
-	lwz	r1, emergency_ctx@l(r1)
+	lwz	r1, emergency_ctx-PAGE_OFFSET@l(r1)
 	addi	r1, r1, THREAD_SIZE - INT_FRAME_SIZE
 	EXCEPTION_PROLOG_2 0 vmap_stack_overflow
 	prepare_transfer_to_handler