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

[Kees Cook <keescook@chromium.org>]

On Tue, Jan 27, 2015 at 8:38 PM, Calvin Owens <calvinowens@fb.com> wrote:
> On Monday 01/26 at 15:43 -0800, Andrew Morton wrote:
>> 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?
>
> The biggest difference is that if you do something like this:
>
>         fd = open("/stuff", O_BLAH);
>         map = mmap(NULL, 4096, PROT_BLAH, MAP_SHARED, fd, 0);
>         close(fd);
>         unlink("/stuff");
>
> ...then map_files/ gives you a way to get a file descriptor for
> "/stuff", which you couldn't do with /proc/pid/maps.
>
> It's also something of a win if you just want to see what is mapped at a
> specific address, since you can just readlink() the symlink for the
> address range you care about and it will go grab the appropriate VMA and
> give you the answer. /proc/pid/maps requires walking the VMA tree, which
> is quite expensive for processes with many thousands of threads, even
> without the O(N^2) issue.
>
> (You have to know what address range you want though, since readdir() on
> map_files/ obviously has to walk the VMA tree just like /proc/N/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).
>
> Yes, the only difference is that you can follow the link as per above.
> I'll resend with a new message explaining that and the deletion thing.
>
>> 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?
>
> While I was messing with it I used it a good bit and didn't see any
> issues, although I didn't actively try to fuzz it or anything. I'd be
> happy to write something to test hammering it in weird ways if you like.
> I'm also happy to write testcases for namespaces.
>
> So far as security issues, as others have pointed out you can't follow
> the links unless you can ptrace the process in question, which seems
> like a pretty solid guarantee. As Cyrill pointed out in the discussion
> about the documentation, that's the same protection as /proc/N/fd/*, and
> those links function in the same way.

My concern here is that fd/* are connected as streams, and while that
has a certain level of badness as an external-to-the-process attacker,
PTRACE_MODE_READ is much weaker than PTRACE_MODE_ATTACH (which is
required for access to /proc/N/mem). Since these fds are the things
mapped into memory on a process, writing to them is a subset of access
to /proc/N/mem, and I don't feel that PTRACE_MODE_READ is sufficient.

-Kees

-- 
Kees Cook
Chrome OS Security