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

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Date:	Mon, 26 May 2014 11:06:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Djalal Harouni <tixxdz@...ndz.org>
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 10:21 AM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> 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.

What I mean is: why not just not re-check?  Is it to paper over the
lack of revoke.

>
>
>> >
>> >  /*
>> > + * 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:

It's not type-safe the way you're doing it, though.

--Andy
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