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>

1 files changed, 4 insertions, 3 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 17463ca2e229..809cf9a258a0 100644
--- a/kernel/printk.c
+++ b/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);
 }
 
 /*