Re: [PATCH] kmsg: honor dmesg_restrict sysctl on /dev/kmsg

From: Kees Cook
Date: Tue Apr 30 2013 - 14:54:23 EST

On Tue, Apr 30, 2013 at 11:35 AM, Josh Boyer <jwboyer@xxxxxxxxxx> wrote:
> On Tue, Apr 30, 2013 at 10:25:41AM -0700, Kees Cook wrote:
>> 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.
>> - 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
>> 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
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> Reported-by: Christian Kujau <lists@xxxxxxxxxxxxxxx>
>> Cc: Josh Boyer <jwboyer@xxxxxxxxxx>
>> Cc: Kay Sievers <kay@xxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
> So staring at this for a while, I think it looks correct. It's
> basically the same thing as the v3 I sent out, with the constant rename
> and no check in devkmsg_read, right?

Yeah, I just explicitly clarified the use-cases and reasoning. :)

> I'll try and get it tested here locally tomorrow.

Great; thanks!


Kees Cook
Chrome OS Security
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