[PATCH] kcsan: avoid unintended access checking in NMIs

[Marco Elver <elver@google.com>]

If a watcher deliberately disables interrupts (either by user choice, or
because we're dealing with a scoped reordered access) to avoid detecting
any data races in interrupts, NMIs are still able to fire.

When we set up a watchpoint on a scoped reordered access, we disabled
interrupts because the same CPU cannot observe reordering of its own
accesses. To ensure we observe no false positives from NMIs, disable
access checking for interrupt contexts as well.

Fixes: 69562e4983d9 ("kcsan: Add core support for a subset of weak memory modeling")
Signed-off-by: Marco Elver <elver@google.com>
---
 kernel/kcsan/core.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 8a7baf4e332e..2db82661cd60 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -585,8 +585,14 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
 	 * information is lost if dirtied by KCSAN.
 	 */
 	kcsan_save_irqtrace(current);
-	if (!interrupt_watcher)
+	if (!interrupt_watcher) {
 		local_irq_save(irq_flags);
+		/*
+		 * NMIs can still fire, disable checking for all interrupt
+		 * contexts.
+		 */
+		raw_cpu_ptr(&kcsan_cpu_ctx)->disable_count++;
+	}
 
 	watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
 	if (watchpoint == NULL) {
@@ -699,8 +705,10 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
 	atomic_long_dec(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
 
 out_unlock:
-	if (!interrupt_watcher)
+	if (!interrupt_watcher) {
+		raw_cpu_ptr(&kcsan_cpu_ctx)->disable_count--;
 		local_irq_restore(irq_flags);
+	}
 	kcsan_restore_irqtrace(current);
 	ctx->disable_scoped--;
 
-- 
2.55.0.229.g6434b31f56-goog