On Fri, Feb 22, 2013 at 01:18:57PM -0500, Josh Boyer wrote:
> Originally, the addition of dmesg_restrict covered both the syslog
> method of accessing dmesg, as well as /dev/kmsg itself. This was done
> indirectly by security_syslog calling cap_syslog before doing any LSM
> checks.
>
> However, commit 12b3052c3ee (capabilities/syslog: open code cap_syslog
> logic to fix build failure) moved the code around and pushed the checks
> into the caller itself. That seems to have inadvertently dropped the
> checks for dmesg_restrict on /dev/kmsg. Most people haven't noticed
> because util-linux dmesg(1) defaults to using the syslog method for
> access in older versions. With util-linux 2.22 and a kernel newer than
> 3.5, dmesg(1) defaults to reading directly from /dev/kmsg.
>
> Fix this by making an explicit check in the devkmsg_open function.
>
> This fixes https://bugzilla.redhat.com/show_bug.cgi?id=903192
>
> Reported-by: Christian Kujau <li...@nerdbynature.de>
> CC: sta...@vger.kernel.org
> Signed-off-by: Josh Boyer <jwbo...@redhat.com>
> ---
> kernel/printk.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/kernel/printk.c b/kernel/printk.c
> index f24633a..398ef9a 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -615,6 +615,9 @@ static int devkmsg_open(struct inode *inode, struct file
> *file)
> struct devkmsg_user *user;
> int err;
>
> + if (dmesg_restrict && !capable(CAP_SYSLOG))
> + return -EACCES;
> +
I think this should use check_syslog_permissions() instead, as done for
/proc/kmsg and the syslog syscall.
err = check_syslog_permissions(SYSLOG_ACTION_OPTION, SYSLOG_FROM_FILE);
if (err)
return err;
And going forward we should probably think about dropping the CAP_SYS_ADMIN
backward-compat code in check_syslog_permissions.
> /* write-only does not need any file context */
> if ((file->f_flags & O_ACCMODE) == O_WRONLY)
> return 0;
-Kees
--
Kees Cook
Chrome OS Security
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