[patch V3 02/32] x86/irq/64: Limit IST stack overflow check to #DB stack

Thomas Gleixner
2019-04-14T16:04:58Z

commit 37fe6a42b343 ("x86: Check stack overflow in detail") added a broad
check for the full exception stack area, i.e. it considers the full
exception stack area as valid.

That's wrong in two aspects:

1) It does not check the individual areas one by one

2) #DF, NMI and #MCE are not enabling interrupts which means that a
regular device interrupt cannot happen in their context. In fact if a
device interrupt hits one of those IST stacks that's a bug because some
code path enabled interrupts while handling the exception.

Limit the check to the #DB stack and consider all other IST stacks as
'overflow' or invalid.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
---
arch/x86/kernel/irq_64.c | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)

--- a/arch/x86/kernel/irq_64.c
+++ b/arch/x86/kernel/irq_64.c
@@ -26,9 +26,18 @@ int sysctl_panic_on_stackoverflow;
/*
* Probabilistic stack overflow check:
*
- * Only check the stack in process context, because everything else
- * runs on the big interrupt stacks. Checking reliably is too expensive,
- * so we just check from interrupts.
+ * Regular device interrupts can enter on the following stacks:
+ *
+ * - User stack
+ *
+ * - Kernel task stack
+ *
+ * - Interrupt stack if a device driver reenables interrupts
+ * which should only happen in really old drivers.
+ *
+ * - Debug IST stack
+ *
+ * All other contexts are invalid.
*/
static inline void stack_overflow_check(struct pt_regs *regs)
{
@@ -53,8 +62,8 @@ static inline void stack_overflow_check(
return;

oist = this_cpu_ptr(&orig_ist);
- estack_top = (u64)oist->ist[0] - EXCEPTION_STKSZ + STACK_TOP_MARGIN;
- estack_bottom = (u64)oist->ist[N_EXCEPTION_STACKS - 1];
+ estack_bottom = (u64)oist->ist[DEBUG_STACK];
+ estack_top = estack_bottom - DEBUG_STKSZ + STACK_TOP_MARGIN;
if (regs->sp >= estack_top && regs->sp <= estack_bottom)
return;