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Commit eaf06b24 authored by Dan Rosenberg's avatar Dan Rosenberg Committed by Linus Torvalds
Browse files

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: default avatarDan Rosenberg <drosenberg@vsecurity.com>
Acked-by: default avatarIngo Molnar <mingo@elte.hu>
Acked-by: default avatarEugene Teo <eugeneteo@kernel.org>
Acked-by: default avatarKees Cook <kees.cook@canonical.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 203f40a5

Documentation/sysctl/kernel.txt

+14 −0
Original line number Diff line number Diff line
@@ -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

include/linux/kernel.h

+1 −0
Original line number Diff line number Diff line
@@ -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):

kernel/printk.c

+6 −0
Original line number Diff line number Diff line
@@ -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;

kernel/sysctl.c

+9 −0
Original line number Diff line number Diff line
@@ -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,

security/Kconfig

+12 −0
Original line number Diff line number Diff line
@@ -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