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 09:57:16 -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 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?

>
>  /*
> + * 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)

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?

--Andy
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