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

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2015-01-30 20:58, Calvin Owens wrote:
> On Thursday 01/29 at 17:30 -0800, Kees Cook wrote:
>> 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.
>
> If you haven't done close() on a mmapped file, doesn't fd/* allow the
> same access to the corresponding regions of memory? Or am I missing
> something?
>
> But that said, I can't think of any reason making it MODE_ATTACH would
> be a problem. Would you rather that be enforced on follow_link() like
> the original patch did, or enforce it for the whole directory?
>
Whole directory would probably be better, as even just the mapped ranges 
could be considered sensitive information.  Ideally, the check should be 
done on both follow_link(), and the directory itself.



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