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Date: Wed, 24 Apr 2013 15:50:32 -0400
From: Josh Boyer <jwboyer@...hat.com>
To: Kay Sievers <kay@...y.org>, Kees Cook <keescook@...omium.org>
Cc: 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 01:58:35PM -0400, Josh Boyer wrote:
> 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.
Yeah, that seems to work. So, comments or Reviewed-by/Acked-by on it
would be welcome.
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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