[PATCH 2/3] tweak: Allow disabling k_spec() function in drivers/tty/vt/keyboard.c

[Tetsuo Handa]

syzbot is reporting unexpected kernel reboots [1]. This seems to be
caused by triggering Ctrl-Alt-Del event via k_spec() function in
drivers/tty/vt/keyboard.c file, for the console output includes normal
restart sequence. Therefore, allow disabling only k_spec() function
in order to allow fuzzers to examine the remaining part in that file.

[1] https://syzkaller.appspot.com/bug?id=321861b1588b44d064b779b92293c5d55cfe8430

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
 drivers/tty/vt/keyboard.c | 2 ++
 lib/Kconfig.tewak         | 7 +++++++
 2 files changed, 9 insertions(+)

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 15d33fa0c925..f08855c4c5ba 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -633,6 +633,8 @@ static void k_spec(struct vc_data *vc, unsigned char value, char up_flag)
 	     kbd->kbdmode == VC_OFF) &&
 	     value != KVAL(K_SAK))
 		return;		/* SAK is allowed even in raw mode */
+	if (IS_ENABLED(CONFIG_TWEAK_DISABLE_KBD_K_SPEC_HANDLER))
+		return;
 	fn_handler[value](vc);
 }
 
diff --git a/lib/Kconfig.tewak b/lib/Kconfig.tewak
index a5ce0db67f28..a1d038bcc2a5 100644
--- a/lib/Kconfig.tewak
+++ b/lib/Kconfig.tewak
@@ -10,4 +10,11 @@ menuconfig TWEAK_KERNEL_BEHAVIOR
 
 if TWEAK_KERNEL_BEHAVIOR
 
+config TWEAK_DISABLE_KBD_K_SPEC_HANDLER
+       bool "Disable k_spec() function in drivers/tty/vt/keyboard.c"
+       help
+	 k_spec() function allows triggering e.g. Ctrl-Alt-Del event.
+	 Such event is annoying for fuzz testing which wants to test
+	 kernel code without rebooting the system.
+
 endif # TWEAK_KERNEL_BEHAVIOR
-- 
2.18.2