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Date: Fri, 6 Nov 2015 16:48:26 -0800 From: Andy Lutomirski <luto@...capital.net> To: Kees Cook <keescook@...omium.org> Cc: Theodore Tso <tytso@...gle.com>, Willy Tarreau <w@....eu>, Dirk Steinmetz <public@...tdrjgfuzkfg.com>, Michael Kerrisk-manpages <mtk.manpages@...il.com>, Serge Hallyn <serge.hallyn@...ntu.com>, Seth Forshee <seth.forshee@...onical.com>, Alexander Viro <viro@...iv.linux.org.uk>, Linux FS Devel <linux-fsdevel@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, "Eric W . Biederman" <ebiederm@...ssion.com>, Serge Hallyn <serge.hallyn@...onical.com>, "security@...nel.org" <security@...nel.org> Subject: Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids On Fri, Nov 6, 2015 at 4:16 PM, Kees Cook <keescook@...omium.org> wrote: > On Fri, Nov 6, 2015 at 4:11 PM, Kees Cook <keescook@...omium.org> wrote: >> On Fri, Nov 6, 2015 at 2:30 PM, Andy Lutomirski <luto@...capital.net> wrote: >>> On Fri, Nov 6, 2015 at 1:59 PM, Kees Cook <keescook@...omium.org> 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@...capital.net> wrote: >>>>> On Tue, Nov 3, 2015 at 10:58 PM, Willy Tarreau <w@....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? >>> >>> 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(). > > I take it back. Hooking wp_page_shared looks expensive. :) Maybe we do > need to hook the mmap? But file_update_time already pokes at the same (or nearby) cachelines, I think -- why would it be expensive? The whole thing could be guarded by if (unlikely(is setuid)), right? --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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