[PATCH V3] firmware: google: Test spinlock on panic path to avoid lockups

[Guilherme G. Piccoli]

Currently the gsmi driver registers a panic notifier as well as
reboot and die notifiers. The callbacks registered are called in
atomic and very limited context - for instance, panic disables
preemption and local IRQs, also all secondary CPUs (not executing
the panic path) are shutdown.

With that said, taking a spinlock in this scenario is a dangerous
invitation for lockup scenarios. So, fix that by checking if the
spinlock is free to acquire in the panic notifier callback - if not,
bail-out and avoid a potential hang.

Fixes: 74c5b31c6618 ("driver: Google EFI SMI")
Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
Cc: David Gow <davidgow@xxxxxxxxxx>
Cc: Julius Werner <jwerner@xxxxxxxxxxxx>
Reviewed-by: Evan Green <evgreen@xxxxxxxxxxxx>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx>
---


This is a re-submission of the patch - it was in a series [0], but
Greg suggested me to resubmit individually in order it gets picked
by the relevant maintainers, instead of asking them to merge
individual patches from a series. Notice I've trimmed a bit the CC
list, it was bigger due to the patch being in a series...

This is truly the V3 of the patch, below is the diff between versions:

V3:
- added Evan's review tag - thanks!

V2:
- do not use spin_trylock anymore, to avoid messing with
non-panic paths; now we just check the spinlock state in
the panic notifier before taking it. Thanks Evan for the review!

[0] https://lore.kernel.org/lkml/20220719195325.402745-4-gpiccoli@xxxxxxxxxx/


 drivers/firmware/google/gsmi.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/firmware/google/gsmi.c b/drivers/firmware/google/gsmi.c
index adaa492c3d2d..3ef5f3c0b4e4 100644
--- a/drivers/firmware/google/gsmi.c
+++ b/drivers/firmware/google/gsmi.c
@@ -681,6 +681,14 @@ static struct notifier_block gsmi_die_notifier = {
 static int gsmi_panic_callback(struct notifier_block *nb,
 			       unsigned long reason, void *arg)
 {
+	/*
+	 * Perform the lock check before effectively trying
+	 * to acquire it on gsmi_shutdown_reason() to avoid
+	 * potential lockups in atomic context.
+	 */
+	if (spin_is_locked(&gsmi_dev.lock))
+		return NOTIFY_DONE;
+
 	gsmi_shutdown_reason(GSMI_SHUTDOWN_PANIC);
 	return NOTIFY_DONE;
 }
-- 
2.37.2