Restrict unprivileged access to kernel syslog

The kernel syslog contains debugging information that is often useful
during exploitation of other vulnerabilities, such as kernel heap
addresses.  Rather than futilely attempt to sanitize hundreds (or
thousands) of printk statements and simultaneously cripple useful
debugging functionality, it is far simpler to create an option that
prevents unprivileged users from reading the syslog.

This patch, loosely based on grsecurity's GRKERNSEC_DMESG, creates the
dmesg_restrict sysctl.  When set to "0", the default, no restrictions are
enforced.  When set to "1", only users with CAP_SYS_ADMIN can read the
kernel syslog via dmesg(8) or other mechanisms.

[akpm@linux-foundation.org: explain the config option in kernel.txt]
Signed-off-by: Dan Rosenberg <drosenberg@vsecurity.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Eugene Teo <eugeneteo@kernel.org>
Acked-by: Kees Cook <kees.cook@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 15 insertions, 0 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index b2ebaee8c377..38e7d5868d60 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -261,6 +261,12 @@ static inline void boot_delay_msec(void)
 }
 #endif
 
+#ifdef CONFIG_SECURITY_DMESG_RESTRICT
+int dmesg_restrict = 1;
+#else
+int dmesg_restrict;
+#endif
+
 int do_syslog(int type, char __user *buf, int len, bool from_file)
 {
         unsigned i, j, limit, count;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c33a1edb799f..b65bf634035e 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -704,6 +704,15 @@ static struct ctl_table kern_table[] = {
         },
 #endif
         {
+                .procname       = "dmesg_restrict",
+                .data           = &dmesg_restrict,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = proc_dointvec_minmax,
+                .extra1         = &zero,
+                .extra2         = &one,
+        },
+        {
                 .procname       = "ngroups_max",
                 .data           = &ngroups_max,
                 .maxlen         = sizeof (int),