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authorKees Cook <keescook@chromium.org>2013-06-12 14:04:39 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-06-12 16:29:44 -0700
commit637241a900cbd982f744d44646b48a273d609b34 (patch)
tree0c8e84af9e6a37bd61f9fc9b7a668472df53df46 /kernel
parentcf7df378aa4ff7da3a44769b7ff6e9eef1a9f3db (diff)
downloadvexpress-lsk-637241a900cbd982f744d44646b48a273d609b34.tar.gz
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>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/printk.c91
1 files changed, 50 insertions, 41 deletions
diff --git a/kernel/printk.c b/kernel/printk.c
index fa36e149442..8212c1aef12 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);
}
/*