taint: add taint for randstruct

Since the randstruct plugin can intentionally produce extremely unusual
kernel structure layouts (even performance pathological ones), some
maintainers want to be able to trivially determine if an Oops is coming
from a randstruct-built kernel, so as to keep their sanity when
debugging.  This adds the new flag and initializes taint_mask
immediately when built with randstruct.

Link: http://lkml.kernel.org/r/1519084390-43867-4-git-send-email-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/kernel.txt

@@ -991,6 +991,7 @@ ORed together. The letters are seen in "Tainted" line of Oops reports.
  16384 (L): A soft lockup has previously occurred on the system.
  32768 (K): The kernel has been live patched.
  65536 (X): Auxiliary taint, defined and used by for distros.
+131072 (T): The kernel was built with the struct randomization plugin.
 
 ==============================================================
 

include/linux/kernel.h

@@ -562,7 +562,8 @@ extern enum system_states {
 #define TAINT_SOFTLOCKUP		14
 #define TAINT_LIVEPATCH			15
 #define TAINT_AUX			16
-#define TAINT_FLAGS_COUNT		17
+#define TAINT_RANDSTRUCT		17
+#define TAINT_FLAGS_COUNT		18
 
 struct taint_flag {
 	char c_true;	/* character printed when tainted */

kernel/panic.c

@@ -34,7 +34,8 @@
 #define PANIC_BLINK_SPD 18
 
 int panic_on_oops = CONFIG_PANIC_ON_OOPS_VALUE;
-static unsigned long tainted_mask;
+static unsigned long tainted_mask =
+	IS_ENABLED(CONFIG_GCC_PLUGIN_RANDSTRUCT) ? (1 << TAINT_RANDSTRUCT) : 0;
 static int pause_on_oops;
 static int pause_on_oops_flag;
 static DEFINE_SPINLOCK(pause_on_oops_lock);
@@ -325,6 +326,7 @@ const struct taint_flag taint_flags[TAINT_FLAGS_COUNT] = {
 	[ TAINT_SOFTLOCKUP ]		= { 'L', ' ', false },
 	[ TAINT_LIVEPATCH ]		= { 'K', ' ', true },
 	[ TAINT_AUX ]			= { 'X', ' ', true },
+	[ TAINT_RANDSTRUCT ]		= { 'T', ' ', true },
 };
 
 /**