debugfs: Restrict debugfs when the kernel is locked down

Disallow opening of debugfs files that might be used to muck around when
the kernel is locked down as various drivers give raw access to hardware
through debugfs.  Given the effort of auditing all 2000 or so files and
manually fixing each one as necessary, I've chosen to apply a heuristic
instead.  The following changes are made:

 (1) chmod and chown are disallowed on debugfs objects (though the root dir
     can be modified by mount and remount, but I'm not worried about that).

 (2) When the kernel is locked down, only files with the following criteria
     are permitted to be opened:

	- The file must have mode 00444
	- The file must not have ioctl methods
	- The file must not have mmap

 (3) When the kernel is locked down, files may only be opened for reading.

Normal device interaction should be done through configfs, sysfs or a
miscdev, not debugfs.

Note that this makes it unnecessary to specifically lock down show_dsts(),
show_devs() and show_call() in the asus-wmi driver.

I would actually prefer to lock down all files by default and have the
the files unlocked by the creator.  This is tricky to manage correctly,
though, as there are 19 creation functions and ~1600 call sites (some of
them in loops scanning tables).

Signed-off-by: David Howells <[email protected]>
cc: Andy Shevchenko <[email protected]>
cc: [email protected]
cc: [email protected]
cc: Matthew Garrett <[email protected]>
cc: Thomas Gleixner <[email protected]>
Cc: Greg KH <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Signed-off-by: Matthew Garrett <[email protected]>
Signed-off-by: James Morris <[email protected]>

security/lockdown/lockdown.c

@@ -30,6 +30,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_TIOCSSERIAL] = "reconfiguration of serial port IO",
 	[LOCKDOWN_MODULE_PARAMETERS] = "unsafe module parameters",
 	[LOCKDOWN_MMIOTRACE] = "unsafe mmio",
+	[LOCKDOWN_DEBUGFS] = "debugfs access",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_KCORE] = "/proc/kcore access",
 	[LOCKDOWN_KPROBES] = "use of kprobes",