linux-kernel - Re: [PATCH] Restrict unprivileged access to kernel syslog

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Date:	Tue, 9 Nov 2010 12:23:50 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Dan Rosenberg <drosenberg@...curity.com>
Cc:	linux-kernel@...r.kernel.org, security@...nel.org,
	stable@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH] Restrict unprivileged access to kernel syslog


* Dan Rosenberg <drosenberg@...curity.com> wrote:

> The kernel syslog contains debugging information that is often useful
> during exploitation of other vulnerabilities, such as kernel heap
> addresses.  Rather than futilely attempt to sanitize hundreds (or
> thousands) of printk statements and simultaneously cripple useful
> debugging functionality, it is far simpler to create an option that
> prevents unprivileged users from reading the syslog.
> 
> This patch, loosely based on grsecurity's GRKERNSEC_DMESG, creates the
> dmesg_restrict sysctl.  When set to "0", the default, no restrictions
> are enforced.  When set to "1", only users with CAP_SYS_ADMIN can read
> the kernel syslog via dmesg(8) or other mechanisms.
> 
> Signed-off-by: Dan Rosenberg <drosenberg@...curity.com>
> ---
>  Documentation/sysctl/kernel.txt |   11 +++++++++++
>  include/linux/kernel.h          |    1 +
>  kernel/printk.c                 |    2 ++
>  kernel/sysctl.c                 |    9 +++++++++
>  security/commoncap.c            |    2 ++
>  5 files changed, 25 insertions(+), 0 deletions(-)

> +int dmesg_restrict = 0;

The initialization to zero is implicit, no need to write it out.

Also, it would also be useful to have a CONFIG_SECURITY_RESTRICT_DMESG=y option 
introduced by your patch as well, which flag allows a distro or user to disable 
unprivileged syslog reading via the kernel config.

Thanks,

	Ingo
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