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

[Andy Lutomirski]

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.

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

>
>>>
>>>> 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.

--Andy
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