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Subject[PATCH v2] kernel/printk: prevent deadlock at unexpected call kmsg_dump in NMI context
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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@yandex-team.ru>
Link: 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;

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