From 637241a900cbd982f744d44646b48a273d609b34 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Wed, 12 Jun 2013 14:04:39 -0700 Subject: 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 Reported-by: Christian Kujau Tested-by: Josh Boyer Cc: Kay Sievers Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/syslog.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'include/linux/syslog.h') diff --git a/include/linux/syslog.h b/include/linux/syslog.h index 38911391a139..98a3153c0f96 100644 --- 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); -- cgit v1.2.3