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author | Kees Cook <keescook@chromium.org> | 2013-06-12 14:04:39 -0700 |
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committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2013-06-20 12:01:29 -0700 |
commit | 3864881bab5235d7e1e7a49298370fbc26d99be5 (patch) | |
tree | e8f5d4576a2043f3dc2fe7c8fb04b2c316644018 /kernel | |
parent | 67842e50729b9f9c9ebd1449735f2f3d129e2325 (diff) | |
download | lwn-3864881bab5235d7e1e7a49298370fbc26d99be5.tar.gz lwn-3864881bab5235d7e1e7a49298370fbc26d99be5.zip |
kmsg: honor dmesg_restrict sysctl on /dev/kmsg
commit 637241a900cbd982f744d44646b48a273d609b34 upstream.
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'kernel')
-rw-r--r-- | kernel/printk.c | 91 |
1 files changed, 50 insertions, 41 deletions
diff --git a/kernel/printk.c b/kernel/printk.c index abbdd9e2ac82..0e4eba6ae6b4 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -368,6 +368,53 @@ 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; @@ -624,7 +671,8 @@ 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; @@ -817,45 +865,6 @@ 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 @@ -1253,7 +1262,7 @@ 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); } /* |