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>

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index 3894eaa23486f951ace787893740ac2850f7d6d9..209e1584c3dc25e8e1af61a3061c779a28f5d11e 100644 (file)
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -28,6 +28,7 @@ show up in /proc/sys/kernel:
 - core_uses_pid
 - ctrl-alt-del
 - dentry-state
+- dmesg_restrict
 - domainname
 - hostname
 - hotplug
@@ -213,6 +214,19 @@ to decide what to do with it.
 
 ==============================================================
 
+dmesg_restrict:
+
+This toggle indicates whether unprivileged users are prevented from using
+dmesg(8) to view messages from the kernel's log buffer.  When
+dmesg_restrict is set to (0) there are no restrictions.  When
+dmesg_restrict is set set to (1), users must have CAP_SYS_ADMIN to use
+dmesg(8).
+
+The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the default
+value of dmesg_restrict.
+
+==============================================================
+
 domainname & hostname:
 
 These files can be used to set the NIS/YP domainname and the

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index b526947bdf489f508c6905b2009005b2220a8643..fc3da9e4da199d2d89b76d0e0bb7388e069c820e 100644 (file)
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -293,6 +293,7 @@ extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
                                   unsigned int interval_msec);
 
 extern int printk_delay_msec;
+extern int dmesg_restrict;
 
 /*
  * Print a one-time message (analogous to WARN_ONCE() et al):

diff --git a/kernel/printk.c b/kernel/printk.c
index b2ebaee8c377d2aa92c073e772748242b5ce5c90..38e7d5868d60ccdfc8f13176f6021a4a712cdee1 100644 (file)
--- 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 c33a1edb799fda6db2e16fdf9d1d0401a9f78278..b65bf634035ed0684693b9c7c02b3229c8d68e0d 100644 (file)
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -703,6 +703,15 @@ static struct ctl_table kern_table[] = {
                .extra2         = &ten_thousand,
        },
 #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,

diff --git a/security/Kconfig b/security/Kconfig
index bd72ae6234947e424885f3808dbec54bddad244c..e80da955e6876aeb550bec75d3d99643020034eb 100644 (file)
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -39,6 +39,18 @@ config KEYS_DEBUG_PROC_KEYS
 
          If you are unsure as to whether this is required, answer N.
 
+config SECURITY_DMESG_RESTRICT
+       bool "Restrict unprivileged access to the kernel syslog"
+       default n
+       help
+         This enforces restrictions on unprivileged users reading the kernel
+         syslog via dmesg(8).
+
+         If this option is not selected, no restrictions will be enforced
+         unless the dmesg_restrict sysctl is explicitly set to (1).
+
+         If you are unsure how to answer this question, answer N.
+
 config SECURITY
        bool "Enable different security models"
        depends on SYSFS

diff --git a/security/commoncap.c b/security/commoncap.c
index 5e632b4857e443d8031eaa17c0e2bd7e877b3d14..04b80f9912bfc3357f1c4b51dd8877ca345df3cc 100644 (file)
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -895,6 +895,8 @@ int cap_syslog(int type, bool from_file)
 {
        if (type != SYSLOG_ACTION_OPEN && from_file)
                return 0;
+       if (dmesg_restrict && !capable(CAP_SYS_ADMIN))
+               return -EPERM;
        if ((type != SYSLOG_ACTION_READ_ALL &&
             type != SYSLOG_ACTION_SIZE_BUFFER) && !capable(CAP_SYS_ADMIN))
                return -EPERM;