linux-kernel - [PATCH v4 2/3] twist: Allow disabling k_spec() function in drivers/tty/vt/keyboard.c

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Date:   Tue, 21 Apr 2020 22:19:50 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Matthew Garrett <mjg59@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Arnd Bergmann <arnd@...db.de>, Jiri Slaby <jslaby@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, syzkaller@...glegroups.com,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Subject: [PATCH v4 2/3] twist: Allow disabling k_spec() function in drivers/tty/vt/keyboard.c

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.

  [   97.727327][    T1] systemd-shutdown[1]: Unmounting file systems.
  [   97.734278][    T1] systemd-shutdown[1]: Remounting '/' read-only with options ''.
  [   97.747758][   T21] usb 2-1: device descriptor read/8, error -71
  [   97.747850][ T3116] usb 1-1: device descriptor read/8, error -71
  [   97.764818][    T1] EXT4-fs (sda1): re-mounted. Opts: 
  [   97.777551][    T1] systemd-shutdown[1]: Remounting '/' read-only with options ''.
  [   97.785448][    T1] EXT4-fs (sda1): re-mounted. Opts: 
  [   97.790920][    T1] systemd-shutdown[1]: All filesystems unmounted.
  [   97.797352][    T1] systemd-shutdown[1]: Deactivating swaps.
  [   97.803451][    T1] systemd-shutdown[1]: All swaps deactivated.
  [   97.809626][    T1] systemd-shutdown[1]: Detaching loop devices.
  [   97.890294][    T1] systemd-shutdown[1]: All loop devices detached.
  [   98.967832][ T3116] usb 1-1: device descriptor read/8, error -71
  [  100.108406][    T1] sd 0:0:1:0: [sda] Synchronizing SCSI cache
  [  100.116036][    T1] reboot: Restarting system
  [  100.120636][    T1] reboot: machine restart
  SeaBIOS (version 1.8.2-20200402_173431-google)
  Total RAM Size = 0x00000001e0000000 = 7680 MiB
  CPUs found: 2     Max CPUs supported: 2
  Comparing RSDP and RSDP

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@...ove.SAKURA.ne.jp>
---
 drivers/tty/vt/keyboard.c | 2 ++
 lib/Kconfig.twist         | 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_TWIST_DISABLE_KBD_K_SPEC_HANDLER))
+		return;
 	fn_handler[value](vc);
 }
 
diff --git a/lib/Kconfig.twist b/lib/Kconfig.twist
index a5ce0db67f28..a1d038bcc2a5 100644
--- a/lib/Kconfig.twist
+++ b/lib/Kconfig.twist
@@ -10,4 +10,11 @@ menuconfig TWIST_KERNEL_BEHAVIOR
 
 if TWIST_KERNEL_BEHAVIOR
 
+config TWIST_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 # TWIST_KERNEL_BEHAVIOR
-- 
2.18.2