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

[Willy Tarreau <w@1wt.eu>]

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().

> 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.

Willy