kptr_restrict for hiding kernel pointers from unprivileged users

Add the %pK printk format specifier and the /proc/sys/kernel/kptr_restrict
sysctl.

The %pK format specifier is designed to hide exposed kernel pointers,
specifically via /proc interfaces.  Exposing these pointers provides an
easy target for kernel write vulnerabilities, since they reveal the
locations of writable structures containing easily triggerable function
pointers.  The behavior of %pK depends on the kptr_restrict sysctl.

If kptr_restrict is set to 0, no deviation from the standard %p behavior
occurs.  If kptr_restrict is set to 1, the default, if the current user
(intended to be a reader via seq_printf(), etc.) does not have CAP_SYSLOG
(currently in the LSM tree), kernel pointers using %pK are printed as 0's.
 If kptr_restrict is set to 2, kernel pointers using %pK are printed as
0's regardless of privileges.  Replacing with 0's was chosen over the
default "(null)", which cannot be parsed by userland %p, which expects
"(nil)".

[akpm@linux-foundation.org: check for IRQ context when !kptr_restrict, save an indent level, s/WARN/WARN_ONCE/]
[akpm@linux-foundation.org: coding-style fixup]
[randy.dunlap@oracle.com: fix kernel/sysctl.c warning]
Signed-off-by: Dan Rosenberg <drosenberg@vsecurity.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: James Morris <jmorris@namei.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Thomas Graf <tgraf@infradead.org>
Cc: Eugene Teo <eugeneteo@kernel.org>
Cc: Kees Cook <kees.cook@canonical.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: David S. Miller <davem@davemloft.net>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Eric Paris <eparis@parisplace.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/kernel.txt

@@ -34,6 +34,7 @@ show up in /proc/sys/kernel:
 - hotplug
 - java-appletviewer           [ binfmt_java, obsolete ]
 - java-interpreter            [ binfmt_java, obsolete ]
+- kptr_restrict
 - kstack_depth_to_print       [ X86 only ]
 - l2cr                        [ PPC only ]
 - modprobe                    ==> Documentation/debugging-modules.txt
@@ -261,6 +262,19 @@ This flag controls the L2 cache of G3 processor boards. If
 
 ==============================================================
 
+kptr_restrict:
+
+This toggle indicates whether restrictions are placed on
+exposing kernel addresses via /proc and other interfaces.  When
+kptr_restrict is set to (0), there are no restrictions.  When
+kptr_restrict is set to (1), the default, kernel pointers
+printed using the %pK format specifier will be replaced with 0's
+unless the user has CAP_SYSLOG.  When kptr_restrict is set to
+(2), kernel pointers printed using %pK will be replaced with 0's
+regardless of privileges.
+
+==============================================================
+
 kstack_depth_to_print: (X86 only)
 
 Controls the number of words to print when dumping the raw

include/linux/printk.h

@@ -83,6 +83,7 @@ extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
 
 extern int printk_delay_msec;
 extern int dmesg_restrict;
+extern int kptr_restrict;
 
 /*
  * Print a one-time message (analogous to WARN_ONCE() et al):

kernel/sysctl.c

@@ -24,6 +24,7 @@
 #include <linux/slab.h>
 #include <linux/sysctl.h>
 #include <linux/signal.h>
+#include <linux/printk.h>
 #include <linux/proc_fs.h>
 #include <linux/security.h>
 #include <linux/ctype.h>
@@ -710,6 +711,15 @@ static struct ctl_table kern_table[] = {
 		.extra1		= &zero,
 		.extra2		= &one,
 	},
+	{
+		.procname	= "kptr_restrict",
+		.data		= &kptr_restrict,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &two,
+	},
 #endif
 	{
 		.procname	= "ngroups_max",

lib/vsprintf.c

@@ -936,6 +936,8 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
 	return string(buf, end, uuid, spec);
 }
 
+int kptr_restrict = 1;
+
 /*
  * Show a '%p' thing.  A kernel extension is that the '%p' is followed
  * by an extra set of alphanumeric characters that are extended format
@@ -979,6 +981,7 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
  *       Implements a "recursive vsnprintf".
  *       Do not use this feature without some mechanism to verify the
  *       correctness of the format string and va_list arguments.
+ * - 'K' For a kernel pointer that should be hidden from unprivileged users
  *
  * Note: The difference between 'S' and 'F' is that on ia64 and ppc64
  * function pointers are really function descriptors, which contain a
@@ -1035,6 +1038,25 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 		return buf + vsnprintf(buf, end - buf,
 				       ((struct va_format *)ptr)->fmt,
 				       *(((struct va_format *)ptr)->va));
+	case 'K':
+		/*
+		 * %pK cannot be used in IRQ context because its test
+		 * for CAP_SYSLOG would be meaningless.
+		 */
+		if (in_irq() || in_serving_softirq() || in_nmi()) {
+			if (spec.field_width == -1)
+				spec.field_width = 2 * sizeof(void *);
+			return string(buf, end, "pK-error", spec);
+		} else if ((kptr_restrict == 0) ||
+			 (kptr_restrict == 1 &&
+			  has_capability_noaudit(current, CAP_SYSLOG)))
+			break;
+
+		if (spec.field_width == -1) {
+			spec.field_width = 2 * sizeof(void *);
+			spec.flags |= ZEROPAD;
+		}
+		return number(buf, end, 0, spec);
 	}
 	spec.flags |= SMALL;
 	if (spec.field_width == -1) {