[PATCH v2 2/2] powerpc/64/interrupt: Prevent NMI PMI causing a dangerous warning

[Nicholas Piggin <npiggin@gmail.com>]

NMI PMIs really should not return using the normal interrupt_return
function. If such a PMI hits in code returning to user with the context
switched to user mode, this warning can fire. This was enough to cause
crashes when reproducing on 64s, because another perf interrupt would
hit while reporting bug, and that would cause another bug, and so on
until smashing the stack.

Work around that particular crash for now by just disabling that context
warning for PMIs. This is a hack and not a complete fix, there could be
other such problems lurking in corners. But it does fix the known crash.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kernel/exceptions-64e.S |  7 +++++++
 arch/powerpc/kernel/exceptions-64s.S |  7 +++++++
 arch/powerpc/kernel/interrupt.c      | 12 +++++++++---
 3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/exceptions-64e.S b/arch/powerpc/kernel/exceptions-64e.S
index 930e36099015..2f68fb2ee4fc 100644
--- a/arch/powerpc/kernel/exceptions-64e.S
+++ b/arch/powerpc/kernel/exceptions-64e.S
@@ -813,6 +813,13 @@ kernel_dbg_exc:
 	EXCEPTION_COMMON(0x260)
 	CHECK_NAPPING()
 	addi	r3,r1,STACK_FRAME_OVERHEAD
+	/*
+	 * XXX: Returning from performance_monitor_exception taken as a
+	 * soft-NMI (Linux irqs disabled) may be risky to use interrupt_return
+	 * and could cause bugs in return or elsewhere. That case should just
+	 * restore registers and return. There is a workaround for one known
+	 * problem in interrupt_exit_kernel_prepare().
+	 */
 	bl	performance_monitor_exception
 	b	interrupt_return
 
diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 5381a43e50fe..7ca55ac7bb87 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -2357,6 +2357,13 @@ EXC_VIRT_END(performance_monitor, 0x4f00, 0x20)
 EXC_COMMON_BEGIN(performance_monitor_common)
 	GEN_COMMON performance_monitor
 	addi	r3,r1,STACK_FRAME_OVERHEAD
+	/*
+	 * XXX: Returning from performance_monitor_exception taken as a
+	 * soft-NMI (Linux irqs disabled) may be risky to use interrupt_return
+	 * and could cause bugs in return or elsewhere. That case should just
+	 * restore registers and return. There is a workaround for one known
+	 * problem in interrupt_exit_kernel_prepare().
+	 */
 	bl	performance_monitor_exception
 	b	interrupt_return_srr
 
diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index f9db0a172401..7bc93367de68 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -374,10 +374,16 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
 	if (regs_is_unrecoverable(regs))
 		unrecoverable_exception(regs);
 	/*
-	 * CT_WARN_ON comes here via program_check_exception,
-	 * so avoid recursion.
+	 * CT_WARN_ON comes here via program_check_exception, so avoid
+	 * recursion.
+	 *
+	 * Skip the assertion on PMIs to work around a problem caused by NMI
+	 * PMIs incorrectly taking this interrupt return path, it's possible
+	 * for this to hit after interrupt exit to user switches context to
+	 * user. See also the comment in the performance monitor handler in
+	 * exceptions-64e/s.S
 	 */
-	if (TRAP(regs) != INTERRUPT_PROGRAM)
+	if (TRAP(regs) != INTERRUPT_PROGRAM && TRAP(regs) != INTERRUPT_PERFMON)
 		CT_WARN_ON(ct_state() == CONTEXT_USER);
 
 	kuap = kuap_get_and_assert_locked();
-- 
2.37.2