linux-kernel - Re: [PATCH 1/9] procfs: use flags to deny or allow access to /proc/<pid>/$entry

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Message-ID: <20140526172101.GA6380@dztty>
Date:	Mon, 26 May 2014 18:21:01 +0100
From:	Djalal Harouni <tixxdz@...ndz.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/9] procfs: use flags to deny or allow access to
 /proc/<pid>/$entry

On Mon, May 26, 2014 at 09:57:16AM -0700, Andy Lutomirski wrote:
> On Mon, May 26, 2014 at 6:27 AM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> > Add the deny or allow flags, so we can perform proper permission checks
> > and set the result accordingly. These flags are needed in case we have
> > to cache the result of permission checks that are done during ->open()
> > time. Later during ->read(), we can decide to allow or deny the read().
> >
> > The pid entries that need these flags are:
> > /proc/<pid>/stat
> > /proc/<pid>/wchan
> > /proc/<pid>/maps  (will be handled in next patches).
> >
> > These files are world readable, userspace depend on that. To prevent
> > ASLR leaks and to avoid breaking userspace, we follow this scheme:
> >
> > a) Perform permission checks during ->open()
> > b) Cache the result of a) and return success
> > c) Recheck the cached result during ->read()
> 
> Why is (c) needed?
In order to not break these entries, some of them are world readable.

So we perform the re-check that *single* cached integer, in order to
allow access for the non-sensitive, and block or pad with zeros the
sensitive.


> >
> >  /*
> > + * Flags used to deny or allow current to access /proc/<pid>/$entry
> > + * after proper permission checks.
> > + */
> > +enum {
> > +       PID_ENTRY_DENY  = 0,    /* Deny access */
> > +       PID_ENTRY_ALLOW = 1,    /* Allow access */
> > +};
> 
> I think this would be less alarming if this were:
> 
> #define PID_ENTRY_DENY ((void *)1UL)
> #define PID_ENTRY_ALLOW ((void *)2UL)
Hmm,

I would like to keep it enum, enum is type-safe and I want to follow the
semantics of /proc/pid/stat and others:

check the patches and you will see that by making the variable 1 or 0 it
follows what's currently done, and IMHO 0 or 1 is more intuitive in this
case!

> Also, I don't like DENY and ALLOW.  It's not denying and allowing.
> How about PID_ENTRY_OPENER_MAY_PTRACE and
> PID_ENTRY_OPENER_MAY_NOT_PTRACE?
Hm, Ok I'll perhaps change this! will see what other thinks!

Thank you!

> --Andy

-- 
Djalal Harouni
http://opendz.org
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