powerpc/32: Statically initialise first emergency context

stable inclusion
from stable-5.10.38
commit 286b3ff9fd98eadeea5fde7985d464254c43064a
bugzilla: 51875
CVE: NA

--------------------------------

[ Upstream commit a4719f5b ]

The check of the emergency context initialisation in
vmap_stack_overflow is buggy for the SMP case, as it
compares r1 with 0 while in the SMP case r1 is offseted
by the CPU id.

Instead of fixing it, just perform static initialisation
of the first emergency context.
Signed-off-by: NChristophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4a67ba422be75713286dca0c86ee0d3df2eb6dfa.1615552867.git.christophe.leroy@csgroup.euSigned-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Acked-by: NWeilong Chen <chenweilong@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

arch/powerpc/kernel/head_32.h

@@ -338,11 +338,7 @@
 	lis	r1, emergency_ctx@ha
 #endif
 	lwz	r1, emergency_ctx@l(r1)
-	cmpwi	cr1, r1, 0
-	bne	cr1, 1f
-	lis	r1, init_thread_union@ha
-	addi	r1, r1, init_thread_union@l
-1:	addi	r1, r1, THREAD_SIZE - INT_FRAME_SIZE
+	addi	r1, r1, THREAD_SIZE - INT_FRAME_SIZE
 	EXCEPTION_PROLOG_2
 	SAVE_NVGPRS(r11)
 	addi	r3, r1, STACK_FRAME_OVERHEAD

arch/powerpc/kernel/setup_32.c

@@ -164,7 +164,7 @@ void __init irqstack_early_init(void)
 }
 
 #ifdef CONFIG_VMAP_STACK
-void *emergency_ctx[NR_CPUS] __ro_after_init;
+void *emergency_ctx[NR_CPUS] __ro_after_init = {[0] = &init_stack};
 
 void __init emergency_stack_init(void)
 {