kmsg: honor dmesg_restrict sysctl on /dev/kmsg

The dmesg_restrict sysctl currently covers the syslog method for access
dmesg, however /dev/kmsg isn't covered by the same protections.  Most
people haven't noticed because util-linux dmesg(1) defaults to using the
syslog method for access in older versions.  With util-linux dmesg(1)
defaults to reading directly from /dev/kmsg.

To fix /dev/kmsg, let's compare the existing interfaces and what they
allow:

 - /proc/kmsg allows:
  - open (SYSLOG_ACTION_OPEN) if CAP_SYSLOG since it uses a destructive
    single-reader interface (SYSLOG_ACTION_READ).
  - everything, after an open.

 - syslog syscall allows:
  - anything, if CAP_SYSLOG.
  - SYSLOG_ACTION_READ_ALL and SYSLOG_ACTION_SIZE_BUFFER, if
    dmesg_restrict==0.
  - nothing else (EPERM).

The use-cases were:
 - dmesg(1) needs to do non-destructive SYSLOG_ACTION_READ_ALLs.
 - sysklog(1) needs to open /proc/kmsg, drop privs, and still issue the
   destructive SYSLOG_ACTION_READs.

AIUI, dmesg(1) is moving to /dev/kmsg, and systemd-journald doesn't
clear the ring buffer.

Based on the comments in devkmsg_llseek, it sounds like actions besides
reading aren't going to be supported by /dev/kmsg (i.e.
SYSLOG_ACTION_CLEAR), so we have a strict subset of the non-destructive
syslog syscall actions.

To this end, move the check as Josh had done, but also rename the
constants to reflect their new uses (SYSLOG_FROM_CALL becomes
SYSLOG_FROM_READER, and SYSLOG_FROM_FILE becomes SYSLOG_FROM_PROC).
SYSLOG_FROM_READER allows non-destructive actions, and SYSLOG_FROM_PROC
allows destructive actions after a capabilities-constrained
SYSLOG_ACTION_OPEN check.

 - /dev/kmsg allows:
  - open if CAP_SYSLOG or dmesg_restrict==0
  - reading/polling, after open

Addresses https://bugzilla.redhat.com/show_bug.cgi?id=903192

[akpm@linux-foundation.org: use pr_warn_once()]
Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Christian Kujau <lists@nerdbynature.de>
Tested-by: Josh Boyer <jwboyer@redhat.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 50 insertions, 41 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index fa36e1494420..8212c1aef125 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -363,6 +363,53 @@ static void log_store(int facility, int level,
         log_next_seq++;
 }
 
+#ifdef CONFIG_SECURITY_DMESG_RESTRICT
+int dmesg_restrict = 1;
+#else
+int dmesg_restrict;
+#endif
+
+static int syslog_action_restricted(int type)
+{
+        if (dmesg_restrict)
+                return 1;
+        /*
+         * Unless restricted, we allow "read all" and "get buffer size"
+         * for everybody.
+         */
+        return type != SYSLOG_ACTION_READ_ALL &&
+               type != SYSLOG_ACTION_SIZE_BUFFER;
+}
+
+static int check_syslog_permissions(int type, bool from_file)
+{
+        /*
+         * If this is from /proc/kmsg and we've already opened it, then we've
+         * already done the capabilities checks at open time.
+         */
+        if (from_file && type != SYSLOG_ACTION_OPEN)
+                return 0;
+
+        if (syslog_action_restricted(type)) {
+                if (capable(CAP_SYSLOG))
+                        return 0;
+                /*
+                 * For historical reasons, accept CAP_SYS_ADMIN too, with
+                 * a warning.
+                 */
+                if (capable(CAP_SYS_ADMIN)) {
+                        pr_warn_once("%s (%d): Attempt to access syslog with "
+                                     "CAP_SYS_ADMIN but no CAP_SYSLOG "
+                                     "(deprecated).\n",
+                                 current->comm, task_pid_nr(current));
+                        return 0;
+                }
+                return -EPERM;
+        }
+        return security_syslog(type);
+}
+
+
 /* /dev/kmsg - userspace message inject/listen interface */
 struct devkmsg_user {
         u64 seq;
@@ -620,7 +667,8 @@ static int devkmsg_open(struct inode *inode, struct file *file)
         if ((file->f_flags & O_ACCMODE) == O_WRONLY)
                 return 0;
 
-        err = security_syslog(SYSLOG_ACTION_READ_ALL);
+        err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
+                                       SYSLOG_FROM_READER);
         if (err)
                 return err;
 
@@ -813,45 +861,6 @@ static inline void boot_delay_msec(int level)
 }
 #endif
 
-#ifdef CONFIG_SECURITY_DMESG_RESTRICT
-int dmesg_restrict = 1;
-#else
-int dmesg_restrict;
-#endif
-
-static int syslog_action_restricted(int type)
-{
-        if (dmesg_restrict)
-                return 1;
-        /* Unless restricted, we allow "read all" and "get buffer size" for everybody */
-        return type != SYSLOG_ACTION_READ_ALL && type != SYSLOG_ACTION_SIZE_BUFFER;
-}
-
-static int check_syslog_permissions(int type, bool from_file)
-{
-        /*
-         * If this is from /proc/kmsg and we've already opened it, then we've
-         * already done the capabilities checks at open time.
-         */
-        if (from_file && type != SYSLOG_ACTION_OPEN)
-                return 0;
-
-        if (syslog_action_restricted(type)) {
-                if (capable(CAP_SYSLOG))
-                        return 0;
-                /* For historical reasons, accept CAP_SYS_ADMIN too, with a warning */
-                if (capable(CAP_SYS_ADMIN)) {
-                        printk_once(KERN_WARNING "%s (%d): "
-                                 "Attempt to access syslog with CAP_SYS_ADMIN "
-                                 "but no CAP_SYSLOG (deprecated).\n",
-                                 current->comm, task_pid_nr(current));
-                        return 0;
-                }
-                return -EPERM;
-        }
-        return 0;
-}
-
 #if defined(CONFIG_PRINTK_TIME)
 static bool printk_time = 1;
 #else
@@ -1249,7 +1258,7 @@ out:
 
 SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
 {
-        return do_syslog(type, buf, len, SYSLOG_FROM_CALL);
+        return do_syslog(type, buf, len, SYSLOG_FROM_READER);
 }
 
 /*