[PATCH v3 5/7] efi/runtime-wrappers: bound the wait for EFI runtime service calls

[Breno Leitao <leitao@debian.org>]

When an EFI runtime service hangs in firmware, the efi_rts_wq worker is
stuck inside the call and cannot be cancelled. __efi_queue_work() then
waits on the completion forever while holding efi_runtime_lock, so every
later EFI caller is wedged until reboot; the only symptom is a "workqueue
lockup" and tasks piling up on the semaphore.

Replace wait_for_completion() with wait_for_completion_timeout() bounded
by EFI_RTS_TIMEOUT (120 seconds). On timeout, clear EFI_RUNTIME_SERVICES
and return EFI_ABORTED so later callers fail fast at the entry check
instead of each paying another 120 seconds. The wedged worker is
intentionally leaked and keeps ownership of efi_rts_work.

A worker that only starts running after the timeout would otherwise
dereference efi_rts_work.args, now pointing into the caller's freed stack
frame, and hand stale pointers to firmware. Park it with
efi_rts_park_worker() at the entry of efi_call_rts() when runtime
services are already disabled, before it touches args or enters firmware.

Known limitation: a worker already inside firmware when the timeout fires
still holds efi_rts_args pointing into the caller's stack frame; if
firmware unblocks afterwards and writes the output buffers, they land in
reused memory. Firmware hung this long rarely recovers; a follow-up could
bounce the buffers through kmalloc.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/firmware/efi/runtime-wrappers.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/runtime-wrappers.c b/drivers/firmware/efi/runtime-wrappers.c
index ae974edc0b04e..2ec5cbdf46d07 100644
--- a/drivers/firmware/efi/runtime-wrappers.c
+++ b/drivers/firmware/efi/runtime-wrappers.c
@@ -118,6 +118,14 @@ union efi_rts_args {
 
 struct efi_runtime_work efi_rts_work;
 
+/*
+ * Upper bound on how long we wait for a single EFI runtime service
+ * call to finish before declaring firmware wedged. Chosen to be longer
+ * than any plausible legitimate call (including UpdateCapsule on slow
+ * SPI-NOR) while still bounding userspace wait time.
+ */
+#define EFI_RTS_TIMEOUT		(120 * HZ)
+
 /*
  * efi_queue_work:	Queue EFI runtime service call and wait for completion
  * @_rts:		EFI runtime service function identifier
@@ -234,6 +242,9 @@ static void __nocfi efi_call_rts(struct work_struct *work)
 	efi_status_t status = EFI_NOT_FOUND;
 	unsigned long flags;
 
+	if (!efi_enabled(EFI_RUNTIME_SERVICES))
+		efi_rts_park_worker();
+
 	efi_runtime_lock_owner = current;
 
 	arch_efi_call_virt_setup();
@@ -355,7 +366,13 @@ static efi_status_t __efi_queue_work(enum efi_rts_ids id,
 		goto exit;
 	}
 
-	wait_for_completion(&efi_rts_work.efi_rts_comp);
+	if (!wait_for_completion_timeout(&efi_rts_work.efi_rts_comp,
+					 EFI_RTS_TIMEOUT)) {
+		pr_err("EFI runtime service %d wedged in firmware; disabling EFI runtime services\n",
+		       id);
+		clear_bit(EFI_RUNTIME_SERVICES, &efi.flags);
+		return EFI_ABORTED;
+	}
 
 	WARN_ON_ONCE(efi_rts_work.status == EFI_ABORTED);
 exit:

-- 
2.53.0-Meta