linux-kernel - Re: [PATCH v3a] vsprintf: Check real user/group id for %pK

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Message-ID: <20131014122154.GA7341@dztty>
Date:	Mon, 14 Oct 2013 13:21:54 +0100
From:	Djalal Harouni <tixxdz@...ndz.org>
To:	Ryan Mallon <rmallon@...il.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Joe Perches <joe@...ches.com>,
	Andrew Morton <akpm@...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>,
	George Spelvin <linux@...izon.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3a] vsprintf: Check real user/group id for %pK

On Mon, Oct 14, 2013 at 11:17:06AM +0100, Djalal Harouni wrote:
> On Fri, Oct 11, 2013 at 02:19:14PM +1100, Ryan Mallon wrote:
> > On 11/10/13 13:20, Eric W. Biederman wrote:
> > > Joe Perches <joe@...ches.com> writes:
> > > 
> > >> 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.
> > > 
> > > Sigh.  This is all wrong.  The only correct thing to test is
> > > file->f_cred.  Aka the capabilities of the program that opened the
> > > file.
> > > 
> > > Which means that the interface to %pK in the case of kptr_restrict is
> > > broken as it has no way to be passed the information it needs to make
> > > a sensible decision.
> > 
> > Would it make sense to add a struct file * to struct seq_file and set
> > that in seq_open? Then the capability check can be done against seq->file.
> For the "add a struct file * to struct seq_file" and set it during
> seq_open(), It was proposed by Linus, but Al Viro didn't like it:
> https://lkml.org/lkml/2013/9/25/765
> 
> I'm not sure if this will work for you: you can make seq_file->private
> cache some data, by calling single_open()... at ->open(), later check it
> during read()...
> 
> 
> As noted by Eric, I'll also go for the capability check at ->open(), if it
> does not break some userspace. BTW the CAP_SYSLOG check should do the job
> 
> Checks during read() are not sufficient, since the design allows passing
> file descriptors and dup() stdin/stdout of suid-execve.
> 
> 
> IMO: unprivileged code should not get that file descriptor, so ->open()
> should fail.
> If this will break userspace then allow open() and cache result for read()
> 
> 
> Can you emulate the behaviour of kptr_restrict=1 ? If so:
> 1) perform check during open() and cache data
> 2) during read() check kptr_restrict==1
>    check the cached value and if opener had CAP_SYSLOG  if so:
>    print something like this: 00000000 T startup_32
Sorry, I mean if the opener didn't have CAP_SYSLOG

-- 
Djalal Harouni
http://opendz.org
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