s390: add stack for machine check handler

The previous code used the normal kernel stack for machine checks.
This is problematic when a machine check interrupts a system call
or interrupt handler right at the beginning where registers are set up.

Assume system_call is interrupted at the first instruction and a machine
check is triggered. The machine check handler is called, checks the PSW
to see whether it is coming from user space, notices that it is already
in kernel mode but %r15 still contains the user space stack. This would
lead to a kernel crash.

There are basically two ways of fixing that: Either using the 'critical
cleanup' approach which compares the address in the PSW to see whether
it is already at a point where the stack has been set up, or use an extra
stack for the machine check handler.

For simplicity, we will go with the second approach and allocate an extra
stack. This adds some memory overhead for large systems, but usually large
system have plenty of memory so this isn't really a concern. But it keeps
the mchk stack setup simple and less error prone.

Fixes: 0b0ed657fe00 ("s390: remove critical section cleanup from entry.S")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Cc: <stable@kernel.org> # v5.8+
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

1 files changed, 16 insertions, 2 deletions

diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
index 6b004940c4dc..60da976eee6f 100644
--- a/arch/s390/kernel/setup.c
+++ b/arch/s390/kernel/setup.c
@@ -338,7 +338,7 @@ int __init arch_early_irq_init(void)
 	return 0;
 }
 
-static int __init async_stack_realloc(void)
+static int __init stack_realloc(void)
 {
 	unsigned long old, new;
 
@@ -348,9 +348,16 @@ static int __init async_stack_realloc(void)
 		panic("Couldn't allocate async stack");
 	WRITE_ONCE(S390_lowcore.async_stack, new + STACK_INIT_OFFSET);
 	free_pages(old, THREAD_SIZE_ORDER);
+
+	old = S390_lowcore.mcck_stack - STACK_INIT_OFFSET;
+	new = stack_alloc();
+	if (!new)
+		panic("Couldn't allocate machine check stack");
+	WRITE_ONCE(S390_lowcore.mcck_stack, new + STACK_INIT_OFFSET);
+	memblock_free(old, THREAD_SIZE);
 	return 0;
 }
-early_initcall(async_stack_realloc);
+early_initcall(stack_realloc);
 
 void __init arch_call_rest_init(void)
 {
@@ -372,6 +379,7 @@ void __init arch_call_rest_init(void)
 static void __init setup_lowcore_dat_off(void)
 {
 	unsigned long int_psw_mask = PSW_KERNEL_BITS;
+	unsigned long mcck_stack;
 	struct lowcore *lc;
 
 	if (IS_ENABLED(CONFIG_KASAN))
@@ -439,6 +447,12 @@ static void __init setup_lowcore_dat_off(void)
 	lc->restart_data = 0;
 	lc->restart_source = -1UL;
 
+	mcck_stack = (unsigned long)memblock_alloc(THREAD_SIZE, THREAD_SIZE);
+	if (!mcck_stack)
+		panic("%s: Failed to allocate %lu bytes align=0x%lx\n",
+		      __func__, THREAD_SIZE, THREAD_SIZE);
+	lc->mcck_stack = mcck_stack + STACK_INIT_OFFSET;
+
 	/* Setup absolute zero lowcore */
 	mem_assign_absolute(S390_lowcore.restart_stack, lc->restart_stack);
 	mem_assign_absolute(S390_lowcore.restart_fn, lc->restart_fn);