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

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>

diff --git a/fs/proc/kmsg.c b/fs/proc/kmsg.c
index bd4b5a740ff1b9de6492f5da9adaf7e500d35b25..bdfabdaefdceab967df948cd3509a39990d3fe8b 100644 (file)
--- a/fs/proc/kmsg.c
+++ b/fs/proc/kmsg.c
@@ -21,12 +21,12 @@ extern wait_queue_head_t log_wait;
 
 static int kmsg_open(struct inode * inode, struct file * file)
 {
-       return do_syslog(SYSLOG_ACTION_OPEN, NULL, 0, SYSLOG_FROM_FILE);
+       return do_syslog(SYSLOG_ACTION_OPEN, NULL, 0, SYSLOG_FROM_PROC);
 }
 
 static int kmsg_release(struct inode * inode, struct file * file)
 {
-       (void) do_syslog(SYSLOG_ACTION_CLOSE, NULL, 0, SYSLOG_FROM_FILE);
+       (void) do_syslog(SYSLOG_ACTION_CLOSE, NULL, 0, SYSLOG_FROM_PROC);
        return 0;
 }
 
@@ -34,15 +34,15 @@ 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(SYSLOG_ACTION_SIZE_UNREAD, NULL, 0, SYSLOG_FROM_FILE))
+           !do_syslog(SYSLOG_ACTION_SIZE_UNREAD, NULL, 0, SYSLOG_FROM_PROC))
                return -EAGAIN;
-       return do_syslog(SYSLOG_ACTION_READ, buf, count, SYSLOG_FROM_FILE);
+       return do_syslog(SYSLOG_ACTION_READ, buf, count, SYSLOG_FROM_PROC);
 }
 
 static unsigned int kmsg_poll(struct file *file, poll_table *wait)
 {
        poll_wait(file, &log_wait, wait);
-       if (do_syslog(SYSLOG_ACTION_SIZE_UNREAD, NULL, 0, SYSLOG_FROM_FILE))
+       if (do_syslog(SYSLOG_ACTION_SIZE_UNREAD, NULL, 0, SYSLOG_FROM_PROC))
                return POLLIN | POLLRDNORM;
        return 0;
 }

diff --git a/include/linux/syslog.h b/include/linux/syslog.h
index 38911391a139a37376e2053008d6e7bb49559cd5..98a3153c0f964e9e5a2d9402d15e9fa8788e2d57 100644 (file)
--- a/include/linux/syslog.h
+++ b/include/linux/syslog.h
@@ -44,8 +44,8 @@
 /* Return size of the log buffer */
 #define SYSLOG_ACTION_SIZE_BUFFER   10
 
-#define SYSLOG_FROM_CALL 0
-#define SYSLOG_FROM_FILE 1
+#define SYSLOG_FROM_READER           0
+#define SYSLOG_FROM_PROC             1
 
 int do_syslog(int type, char __user *buf, int count, bool from_file);
 

diff --git a/kernel/printk.c b/kernel/printk.c
index 8cceacef8bead66a8a432a5caca3729ef31af5a3..84e01c9ba2bab457c6df045bbaab04362b072150 100644 (file)
--- 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)) {
+                       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 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);
 }
 
 /*