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Date: Wed, 24 Apr 2013 13:58:35 -0400
From: Josh Boyer <jwboyer@...hat.com>
To: Kay Sievers <kay@...y.org>
Cc: Kees Cook <keescook@...omium.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Eric Paris <eparis@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Christian Kujau <lists@...dbynature.de>,
"# 3.4.x" <stable@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>, kzak@...hat.com
Subject: Re: [PATCH v2] kmsg: Honor dmesg_restrict sysctl on /dev/kmsg
On Wed, Apr 24, 2013 at 07:44:33PM +0200, Kay Sievers wrote:
> On Tue, Apr 9, 2013 at 6:33 PM, Kees Cook <keescook@...omium.org> wrote:
> > On Tue, Apr 9, 2013 at 8:48 AM, Josh Boyer <jwboyer@...hat.com> wrote:
> >> 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.
> >>
> >> Fix this by reworking all of the access methods to use the
> >> check_syslog_permissions function and adding checks to devkmsg_open and
> >> devkmsg_read.
> >>
> >> This fixes https://bugzilla.redhat.com/show_bug.cgi?id=903192
> >>
> >> Reported-by: Christian Kujau <lists@...dbynature.de>
> >> CC: stable@...r.kernel.org
> >> Signed-off-by: Eric Paris <eparis@...hat.com>
> >> Signed-off-by: Josh Boyer <jwboyer@...hat.com>
> >
> > Thanks!
> >
> > Acked-by: Kees Cook <keescook@...omium.org>
>
> If that's the version currently in Fedora, we just cannot do this.
> https://bugzilla.redhat.com/show_bug.cgi?id=952655
>
> /dev/kmsg is supposed, and was added, to be a sane alternative to
> syslog(). It is already used in dmesg(1) which is now broken with this
> patch.
>
> The access rules for /dev/kmsg should follow the access rules of
> syslog(), and not be any stricter.
I haven't tested it yet, but I think something like this should work
while still honoring dmesg_restrict. I'll test it out while the rest
of you debate things.
josh
From: Josh Boyer <jwboyer@...hat.com>
Date: Tue, 9 Apr 2013 11:08:13 -0400
Subject: [PATCH v3] 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.
Fix this by reworking all of the access methods to use the
check_syslog_permissions function and adding checks to devkmsg_open and
devkmsg_read.
This fixes https://bugzilla.redhat.com/show_bug.cgi?id=903192
Reported-by: Christian Kujau <lists@...dbynature.de>
CC: stable@...r.kernel.org
Signed-off-by: Eric Paris <eparis@...hat.com>
Signed-off-by: Josh Boyer <jwboyer@...hat.com>
---
v3: Allow devkmsg_open to work without CAP_SYSLOG, but still make
devkmsg_read honor dmesg_restrict
kernel/printk.c | 91 +++++++++++++++++++++++++++++----------------------------
1 file changed, 47 insertions(+), 44 deletions(-)
diff --git a/kernel/printk.c b/kernel/printk.c
index abbdd9e..2d7be05 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -368,6 +368,46 @@ 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)
+ goto ok;
+
+ if (syslog_action_restricted(type)) {
+ if (capable(CAP_SYSLOG))
+ goto ok;
+ /* 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));
+ goto ok;
+ }
+ return -EPERM;
+ }
+ok:
+ return security_syslog(type);
+}
+
/* /dev/kmsg - userspace message inject/listen interface */
struct devkmsg_user {
u64 seq;
@@ -443,10 +483,16 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
char cont = '-';
size_t len;
ssize_t ret;
+ int err;
if (!user)
return -EBADF;
+ err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
+ SYSLOG_FROM_CALL);
+ if (err)
+ return err;
+
ret = mutex_lock_interruptible(&user->lock);
if (ret)
return ret;
@@ -624,7 +670,7 @@ 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_FILE);
if (err)
return err;
@@ -817,45 +863,6 @@ 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
@@ -1131,10 +1138,6 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
if (error)
goto out;
- error = security_syslog(type);
- if (error)
- return error;
-
switch (type) {
case SYSLOG_ACTION_CLOSE: /* Close log */
break;
--
1.8.1.4
--
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