Messages in this thread | | | Date | Fri, 6 Nov 2015 13:59:50 -0800 | Subject | Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids | From | Kees Cook <> |
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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
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