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Date: Wed, 24 Apr 2013 13:43:31 -0400
From: Josh Boyer <jwboyer@...hat.com>
To: Kees Cook <keescook@...omium.org>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Eric Paris <eparis@...hat.com>,
"kzak@...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>
Subject: Re: [PATCH v2] kmsg: Honor dmesg_restrict sysctl on /dev/kmsg
On Tue, Apr 09, 2013 at 11:48:20AM -0400, Josh Boyer 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
So this does fix that bug. But then it introduced this one:
https://bugzilla.redhat.com/show_bug.cgi?id=952655
Basically, dmesg(1) now always falls back to using the syslog interface
instead of /dev/kmsg because nothing is granting CAP_SYSLOG for normal
users. That was somewhat intentional based on the feedback from Kees
and Eric, but it does present a problem.
If we want to keep the existing open behavior for /dev/kmsg, and still
honor dmesg_restrict, we basically need it to fail in devkmsg_read.
With the current functions we have, that won't work so we'll either need
to hack that up or just have devkmsg_read call syslog_action_restricted
instead.
josh
> 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>
> ---
>
> v2: Rework patch based on code from Eric Paris, add check in devkmsg_read as
> suggested by Kees Cook.
>
> kernel/printk.c | 91 +++++++++++++++++++++++++++++----------------------------
> 1 file changed, 47 insertions(+), 44 deletions(-)
>
> diff --git a/kernel/printk.c b/kernel/printk.c
> index abbdd9e..5541095 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_FILE);
> + 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_OPEN, 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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