[PATCH v2] kernel/printk: prevent deadlock at unexpected call kmsg_dump in NMI context

[Konstantin Khlebnikov]

Kernel message dumper - function kmsg_dump() is called on various oops or
panic paths which could happen in unpredictable context including NMI.

Panic in NMI is handled especially by stopping all other cpus with
smp_send_stop() and busting locks in printk_safe_flush_on_panic().

Other less-fatal cases shouldn't happen in NMI and cannot be handled.
But this might happen for example on oops in nmi context. In this case
dumper could deadlock on lockbuf_lock or break internal structures.

This patch catches kmsg_dump() called in NMI context except panic and
prints warning once.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Link: 156294329676.1745.2620297516210526183.stgit@buzz/">https://lore.kernel.org/lkml/156294329676.1745.2620297516210526183.stgit@buzz/ (v1)
---
 kernel/printk/printk.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1888f6a3b694..e711f64a1843 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3104,6 +3104,13 @@ void kmsg_dump(enum kmsg_dump_reason reason)
 	struct kmsg_dumper *dumper;
 	unsigned long flags;
 
+	/*
+	 * In NMI context only panic could be handled safely:
+	 * it stops other cpus and busts logbuf lock.
+	 */
+	if (WARN_ON_ONCE(reason != KMSG_DUMP_PANIC && in_nmi()))
+		return;
+
 	if ((reason > KMSG_DUMP_OOPS) && !always_kmsg_dump)
 		return;