[tip:x86/irq] x86/irq/64: Limit IST stack overflow check to #DB stack

[tip-bot for Thomas Gleixner <tipbot@xxxxxxxxx>]

Commit-ID:  7dbcf2b0b770eeb803a416ee8dcbef78e6389d40
Gitweb:     https://git.kernel.org/tip/7dbcf2b0b770eeb803a416ee8dcbef78e6389d40
Author:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Sun, 14 Apr 2019 17:59:38 +0200
Committer:  Borislav Petkov <bp@xxxxxxx>
CommitDate: Wed, 17 Apr 2019 12:06:56 +0200

x86/irq/64: Limit IST stack overflow check to #DB stack

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@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Mitsuo Hayasaka <mitsuo.hayasaka.hu@xxxxxxxxxxx>
Cc: Nicolai Stange <nstange@xxxxxxx>
Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Cc: x86-ml <x86@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190414160143.682135110@xxxxxxxxxxxxx
---
 arch/x86/kernel/irq_64.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/irq_64.c b/arch/x86/kernel/irq_64.c
index 0469cd078db1..b50ac9c7397b 100644
--- 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(struct pt_regs *regs)
 		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;