syslog: distinguish between /proc/kmsg and syscalls

This allows the LSM to distinguish between syslog functions originating
from /proc/kmsg access and direct syscalls.  By default, the commoncaps
will now no longer require CAP_SYS_ADMIN to read an opened /proc/kmsg
file descriptor.  For example the kernel syslog reader can now drop
privileges after opening /proc/kmsg, instead of staying privileged with
CAP_SYS_ADMIN.  MAC systems that implement security_syslog have unchanged
behavior.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: James Morris <jmorris@namei.org>

fs/proc/kmsg.c

@@ -12,37 +12,37 @@
 #include <linux/poll.h>
 #include <linux/proc_fs.h>
 #include <linux/fs.h>
+#include <linux/syslog.h>
 
 #include <asm/uaccess.h>
 #include <asm/io.h>
 
 extern wait_queue_head_t log_wait;
 
-extern int do_syslog(int type, char __user *bug, int count);
-
 static int kmsg_open(struct inode * inode, struct file * file)
 {
-	return do_syslog(1,NULL,0);
+	return do_syslog(1, NULL, 0, SYSLOG_FROM_FILE);
 }
 
 static int kmsg_release(struct inode * inode, struct file * file)
 {
-	(void) do_syslog(0,NULL,0);
+	(void) do_syslog(0, NULL, 0, SYSLOG_FROM_FILE);
 	return 0;
 }
 
 static ssize_t kmsg_read(struct file *file, char __user *buf,
 			 size_t count, loff_t *ppos)
 {
-	if ((file->f_flags & O_NONBLOCK) && !do_syslog(9, NULL, 0))
+	if ((file->f_flags & O_NONBLOCK) &&
+	    !do_syslog(9, NULL, 0, SYSLOG_FROM_FILE))
 		return -EAGAIN;
-	return do_syslog(2, buf, count);
+	return do_syslog(2, buf, count, SYSLOG_FROM_FILE);
 }
 
 static unsigned int kmsg_poll(struct file *file, poll_table *wait)
 {
 	poll_wait(file, &log_wait, wait);
-	if (do_syslog(9, NULL, 0))
+	if (do_syslog(9, NULL, 0, SYSLOG_FROM_FILE))
 		return POLLIN | POLLRDNORM;
 	return 0;
 }

include/linux/security.h

@@ -76,7 +76,7 @@ extern int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 extern int cap_task_setscheduler(struct task_struct *p, int policy, struct sched_param *lp);
 extern int cap_task_setioprio(struct task_struct *p, int ioprio);
 extern int cap_task_setnice(struct task_struct *p, int nice);
-extern int cap_syslog(int type);
+extern int cap_syslog(int type, bool from_file);
 extern int cap_vm_enough_memory(struct mm_struct *mm, long pages);
 
 struct msghdr;
@@ -1349,6 +1349,7 @@ static inline void security_free_mnt_opts(struct security_mnt_opts *opts)
  *	logging to the console.
  *	See the syslog(2) manual page for an explanation of the @type values.
  *	@type contains the type of action.
+ *	@from_file indicates the context of action (if it came from /proc).
  *	Return 0 if permission is granted.
  * @settime:
  *	Check permission to change the system time.
@@ -1463,7 +1464,7 @@ struct security_operations {
 	int (*sysctl) (struct ctl_table *table, int op);
 	int (*quotactl) (int cmds, int type, int id, struct super_block *sb);
 	int (*quota_on) (struct dentry *dentry);
-	int (*syslog) (int type);
+	int (*syslog) (int type, bool from_file);
 	int (*settime) (struct timespec *ts, struct timezone *tz);
 	int (*vm_enough_memory) (struct mm_struct *mm, long pages);
 
@@ -1762,7 +1763,7 @@ int security_acct(struct file *file);
 int security_sysctl(struct ctl_table *table, int op);
 int security_quotactl(int cmds, int type, int id, struct super_block *sb);
 int security_quota_on(struct dentry *dentry);
-int security_syslog(int type);
+int security_syslog(int type, bool from_file);
 int security_settime(struct timespec *ts, struct timezone *tz);
 int security_vm_enough_memory(long pages);
 int security_vm_enough_memory_mm(struct mm_struct *mm, long pages);
@@ -2008,9 +2009,9 @@ static inline int security_quota_on(struct dentry *dentry)
 	return 0;
 }
 
-static inline int security_syslog(int type)
+static inline int security_syslog(int type, bool from_file)
 {
-	return cap_syslog(type);
+	return cap_syslog(type, from_file);
 }
 
 static inline int security_settime(struct timespec *ts, struct timezone *tz)

include/linux/syslog.h

@@ -0,0 +1,29 @@
+/*  Syslog internals
+ *
+ *  Copyright 2010 Canonical, Ltd.
+ *  Author: Kees Cook <kees.cook@canonical.com>
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2, or (at your option)
+ *  any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program; see the file COPYING.  If not, write to
+ *  the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#ifndef _LINUX_SYSLOG_H
+#define _LINUX_SYSLOG_H
+
+#define SYSLOG_FROM_CALL 0
+#define SYSLOG_FROM_FILE 1
+
+int do_syslog(int type, char __user *buf, int count, bool from_file);
+
+#endif /* _LINUX_SYSLOG_H */

kernel/printk.c

@@ -35,6 +35,7 @@
 #include <linux/kexec.h>
 #include <linux/ratelimit.h>
 #include <linux/kmsg_dump.h>
+#include <linux/syslog.h>
 
 #include <asm/uaccess.h>
 
@@ -273,14 +274,14 @@ static inline void boot_delay_msec(void)
  *	9 -- Return number of unread characters in the log buffer
  *     10 -- Return size of the log buffer
  */
-int do_syslog(int type, char __user *buf, int len)
+int do_syslog(int type, char __user *buf, int len, bool from_file)
 {
 	unsigned i, j, limit, count;
 	int do_clear = 0;
 	char c;
 	int error = 0;
 
-	error = security_syslog(type);
+	error = security_syslog(type, from_file);
 	if (error)
 		return error;
 
@@ -417,7 +418,7 @@ out:
 
 SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
 {
-	return do_syslog(type, buf, len);
+	return do_syslog(type, buf, len, SYSLOG_FROM_CALL);
 }
 
 /*

security/commoncap.c

@@ -27,6 +27,7 @@
 #include <linux/sched.h>
 #include <linux/prctl.h>
 #include <linux/securebits.h>
+#include <linux/syslog.h>
 
 /*
  * If a non-root user executes a setuid-root binary in
@@ -888,12 +889,16 @@ error:
 /**
  * cap_syslog - Determine whether syslog function is permitted
  * @type: Function requested
+ * @from_file: Whether this request came from an open file (i.e. /proc)
  *
  * Determine whether the current process is permitted to use a particular
  * syslog function, returning 0 if permission is granted, -ve if not.
  */
-int cap_syslog(int type)
+int cap_syslog(int type, bool from_file)
 {
+	/* /proc/kmsg can open be opened by CAP_SYS_ADMIN */
+	if (type != 1 && from_file)
+		return 0;
 	if ((type != 3 && type != 10) && !capable(CAP_SYS_ADMIN))
 		return -EPERM;
 	return 0;

security/security.c

@@ -203,9 +203,9 @@ int security_quota_on(struct dentry *dentry)
 	return security_ops->quota_on(dentry);
 }
 
-int security_syslog(int type)
+int security_syslog(int type, bool from_file)
 {
-	return security_ops->syslog(type);
+	return security_ops->syslog(type, from_file);
 }
 
 int security_settime(struct timespec *ts, struct timezone *tz)

security/selinux/hooks.c

@@ -76,6 +76,7 @@
 #include <linux/selinux.h>
 #include <linux/mutex.h>
 #include <linux/posix-timers.h>
+#include <linux/syslog.h>
 
 #include "avc.h"
 #include "objsec.h"
@@ -2049,11 +2050,11 @@ static int selinux_quota_on(struct dentry *dentry)
 	return dentry_has_perm(cred, NULL, dentry, FILE__QUOTAON);
 }
 
-static int selinux_syslog(int type)
+static int selinux_syslog(int type, bool from_file)
 {
 	int rc;
 
-	rc = cap_syslog(type);
+	rc = cap_syslog(type, from_file);
 	if (rc)
 		return rc;
 

security/smack/smack_lsm.c

@@ -157,12 +157,12 @@ static int smack_ptrace_traceme(struct task_struct *ptp)
  *
  * Returns 0 on success, error code otherwise.
  */
-static int smack_syslog(int type)
+static int smack_syslog(int type, bool from_file)
 {
 	int rc;
 	char *sp = current_security();
 
-	rc = cap_syslog(type);
+	rc = cap_syslog(type, from_file);
 	if (rc != 0)
 		return rc;