linux-kernel - Re: [PATCH v4] vsprintf: Check real user/group id for %pK

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Date:	Tue, 15 Oct 2013 13:51:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ryan Mallon <rmallon@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	eldad@...refinery.com, Jiri Kosina <jkosina@...e.cz>,
	jgunthorpe@...idianresearch.com,
	Dan Rosenberg <dan.j.rosenberg@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	George Spelvin <linux@...izon.com>,
	Joe Perches <joe@...ches.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Landley <rob@...dley.net>, Ingo Molnar <mingo@...e.hu>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH v4] vsprintf: Check real user/group id for %pK

On Tue, 15 Oct 2013 09:38:48 +1100 Ryan Mallon <rmallon@...il.com> wrote:

> Some setuid binaries will allow reading of files which have read
> permission by the real user id. This is problematic with files which
> use %pK because the file access permission is checked at open() time,
> but the kptr_restrict setting is checked at read() time. If a setuid
> binary opens a %pK file as an unprivileged user, and then elevates
> permissions before reading the file, then kernel pointer values may be
> leaked.
> 
> This happens for example with the setuid pppd application on Ubuntu 12.04:
> 
>   $ head -1 /proc/kallsyms
>   00000000 T startup_32
> 
>   $ pppd file /proc/kallsyms
>   pppd: In file /proc/kallsyms: unrecognized option 'c1000000'
> 
> This will only leak the pointer value from the first line, but other
> setuid binaries may leak more information.
> 
> Fix this by adding a check that in addition to the current process
> having CAP_SYSLOG, that effective user and group ids are equal to the
> real ids. If a setuid binary reads the contents of a file which uses
> %pK then the pointer values will be printed as NULL if the real user
> is unprivileged.
> 
> Update the sysctl documentation to reflect the changes, and also
> correct the documentation to state the kptr_restrict=0 is the default.
> 
> This is a only temporary solution to the issue. The correct solution
> is to do the permission check at open() time on files, and to replace
> %pK with a function which checks the open() time permission. %pK uses
> in printk should be removed since no sane permission check can be
> done, and instead protected by using dmesg_restrict.

I grabbed this and queued it for 3.13-rc1, marked for backporting into
-stable.  Given the amount of churn on this one I think it would be
imprudent to put it into mainline immediately.

I haven't been following the discussion very closely, so if anyone
thinks it should be ungrabbed, please speak up.

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