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, 5 insertions, 5 deletions

diff --git a/fs/proc/kmsg.c b/fs/proc/kmsg.c
index bd4b5a740ff1..bdfabdaefdce 100644
--- 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;
 }