Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids

[Kees Cook]

On Fri, Nov 6, 2015 at 2:30 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Fri, Nov 6, 2015 at 1:59 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> Adding Ted, who might know how this all hooks together. (The context
>> is that a write() or truncate() on a setgid file clears the setgid,
>> but mmap writes don't.)
>>
>> On Wed, Nov 4, 2015 at 9:59 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Tue, Nov 3, 2015 at 10:58 PM, Willy Tarreau <w@xxxxxx> wrote:
>>>> On Tue, Nov 03, 2015 at 03:29:55PM -0800, Kees Cook wrote:
>>>>> Using "write" does kill the set-gid bit. I haven't looked at
>>>>> why.
>>>>> Al or anyone else, is there a meaningful distinction here?
>>>>
>>>> I remember this one, I got caught once while trying to put a shell into
>>>> a suid-writable file to get some privileges someone forgot to offer me :-)
>>>>
>>>> It's done by should_remove_suid() which is called upon write() and truncate().
>>
>> file_remove_privs() seems to be the right entry point.
>> __generic_file_write_iter in mm/filemap.c calls it, though. Are these
>> callbacks not used for mmap writes?
>
> They're certainly not used early enough -- we need to remove suid when
> the page becomes writable via mmap (wp_page_shared), not when
> writeback happens, or at least not only when writeback happens.

Well, I'm shy about the change there. For example, we don't strip in
on open(RDWR), just on write().

> But IIRC mmaped writes go through a different path -- they go through
> the address_space ops with names like writepages.

Ah-ha.

>>>>> Should the
>>>>> mmap MAP_SHARED-write trigger the loss of the set-gid bit too? While
>>>>> holding the file open with either open or mmap, I get a Text-in-use
>>>>> error, so I would kind of expect the same behavior between either
>>>>> close() and munmap(). I wonder if this is a bug, and if so, then your
>>>>> link patch is indeed useful again. :)
>>>>
>>>> I don't see how this could be done with mmap(). Maybe we have a way to know
>>>> when the first write is performed via this path, I have no idea.
>>>
>>> do_wp_page might be a decent bet.
>>
>> Or wp_page_shared? Can we get back to a file from the mm at that point?
>
> vma->vm_file, presumably (after checking whether it's null).
> wp_page_shared AFAIK only happens from process context, and the vma
> and its file should be valid.
>
> We could also get to an inode via page->address_space->mapping, but
> I'm guessing that vma->vm_file would be more appropriate here.

Yeah. Let me give it a try...

-Kees

-- 
Kees Cook
Chrome OS Security
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