Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753442Ab0KJASc (ORCPT ); Tue, 9 Nov 2010 19:18:32 -0500 Received: from mx1.vsecurity.com ([209.67.252.12]:50178 "EHLO mx1.vsecurity.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752121Ab0KJASb (ORCPT ); Tue, 9 Nov 2010 19:18:31 -0500 Subject: [PATCH v2] Restrict unprivileged access to kernel syslog From: Dan Rosenberg To: linux-kernel@vger.kernel.org Cc: torvalds@linux-foundation.org, mingo@elte.hu, kees.cook@canonical.com Content-Type: text/plain; charset="UTF-8" Date: Tue, 09 Nov 2010 19:18:29 -0500 Message-ID: <1289348309.7380.31.camel@dan> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4958 Lines: 151 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. v2 adds CONFIG_SECURITY_RESTRICT_DMESG. When enabled, the default sysctl value is set to "1". When disabled, the default sysctl value is set to "0". Signed-off-by: Dan Rosenberg CC: Linus Torvalds CC: Ingo Molnar CC: Kees Cook CC: stable --- Documentation/sysctl/kernel.txt | 11 +++++++++++ include/linux/kernel.h | 1 + kernel/printk.c | 6 ++++++ kernel/sysctl.c | 9 +++++++++ security/Kconfig | 11 +++++++++++ security/commoncap.c | 2 ++ 6 files changed, 40 insertions(+), 0 deletions(-) diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt index 3894eaa..c6bac30 100644 --- 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 +214,16 @@ 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. By default, it is set to (0), resulting in no +restrictions. When set to (1), users must have CAP_SYS_ADMIN +to use dmesg(8). + +============================================================== + 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 450092c..f0d0088 100644 --- 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 b2ebaee..485d653 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -261,6 +261,12 @@ static inline void boot_delay_msec(void) } #endif +#ifdef CONFIG_SECURITY_RESTRICT_DMESG +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 c33a1ed..b65bf63 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), diff --git a/security/Kconfig b/security/Kconfig index bd72ae6..6ca390e 100644 --- a/security/Kconfig +++ b/security/Kconfig @@ -39,6 +39,17 @@ config KEYS_DEBUG_PROC_KEYS If you are unsure as to whether this is required, answer N. +config SECURITY_RESTRICT_DMESG + bool "Restrict unprivileged access to the kernel syslog" + 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 5e632b4..04b80f9 100644 --- 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; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/