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

[Kees Cook <keescook@chromium.org>]

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@amacapital.net> wrote:
> On Tue, Nov 3, 2015 at 10:58 PM, Willy Tarreau <w@1wt.eu> 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?

>>
>>> 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?

-Kees

-- 
Kees Cook
Chrome OS Security