Re: [RFC][PATCH v2] procfs: Always expose /proc/<pid>/map_files/ and make it readable

From: Andrew Morton <akpm@linux-foundation.org>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Calvin Owens <calvinowens@fb.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Peter Feiner <pfeiner@google.com>,
	Grant Likely <grant.likely@secretlab.ca>,
	Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>,
	linux-kernel@vger.kernel.org, kernel-team@fb.com,
	Pavel Emelyanov <xemul@openvz.org>,
	linux-api@vger.kernel.org, Kees Cook <keescook@chromium.org>
Subject: Re: [RFC][PATCH v2] procfs: Always expose /proc/<pid>/map_files/ and make it readable
Date: Mon, 26 Jan 2015 15:43:46 -0800	[thread overview]
Message-ID: <20150126154346.c63c512e5821e9e0ea31f759@linux-foundation.org> (raw)
In-Reply-To: <20150126210054.GG651@moon>

On Tue, 27 Jan 2015 00:00:54 +0300 Cyrill Gorcunov <gorcunov@gmail.com> wrote:

> On Mon, Jan 26, 2015 at 02:47:31PM +0200, Kirill A. Shutemov wrote:
> > On Fri, Jan 23, 2015 at 07:15:44PM -0800, Calvin Owens wrote:
> > > Currently, /proc/<pid>/map_files/ is restricted to CAP_SYS_ADMIN, and
> > > is only exposed if CONFIG_CHECKPOINT_RESTORE is set. This interface
> > > is very useful for enumerating the files mapped into a process when
> > > the more verbose information in /proc/<pid>/maps is not needed.

This is the main (actually only) justification for the patch, and it it
far too thin.  What does "not needed" mean.  Why can't people just use
/proc/pid/maps?

> > > This patch moves the folder out from behind CHECKPOINT_RESTORE, and
> > > removes the CAP_SYS_ADMIN restrictions. Following the links requires
> > > the ability to ptrace the process in question, so this doesn't allow
> > > an attacker to do anything they couldn't already do before.
> > > 
> > > Signed-off-by: Calvin Owens <calvinowens@fb.com>
> > 
> > Cc +linux-api@
> 
> Looks good to me, thanks! Though I would really appreciate if someone
> from security camp take a look as well.

hm, who's that.  Kees comes to mind.

And reviewers' task would be a heck of a lot easier if they knew what
/proc/pid/map_files actually does.  This:

akpm3:/usr/src/25> grep -r map_files Documentation 
akpm3:/usr/src/25> 

does not help.

The 640708a2cff7f81 changelog says:

:     This one behaves similarly to the /proc/<pid>/fd/ one - it contains
:     symlinks one for each mapping with file, the name of a symlink is
:     "vma->vm_start-vma->vm_end", the target is the file.  Opening a symlink
:     results in a file that point exactly to the same inode as them vma's one.
:     
:     For example the ls -l of some arbitrary /proc/<pid>/map_files/
:     
:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80403000-7f8f80404000 -> /lib64/libc-2.5.so
:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f8061e000-7f8f80620000 -> /lib64/libselinux.so.1
:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80826000-7f8f80827000 -> /lib64/libacl.so.1.1.0
:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80a2f000-7f8f80a30000 -> /lib64/librt-2.5.so
:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80a30000-7f8f80a4c000 -> /lib64/ld-2.5.so

afacit this info is also available in /proc/pid/maps, so things
shouldn't get worse if the /proc/pid/map_files permissions are at least
as restrictive as the /proc/pid/maps permissions.  Is that the case? 
(Please add to changelog).

There's one other problem here: we're assuming that the map_files
implementation doesn't have bugs.  If it does have bugs then relaxing
permissions like this will create new vulnerabilities.  And the
map_files implementation is surprisingly complex.  Is it bug-free?