On Wed 2009-12-16 12:31:43, Al Viro wrote: > On Mon, Nov 23, 2009 at 06:15:45PM -0500, Jeff Layton wrote: > > > The big question with all of this is: Should a task have the ability > > to follow a /proc/pid symlink to a path that it wouldn't ordinarily be > > able to resolve with a path lookup. The concensus that I got from the > > bugtraq discussion was that it should not, and this patch is an attempt > > to prevent that. > > > > I take it from you and Eric's comments that you disagree? If so, what's > > your rationale for allowing a task to resolve this symlink when it > > wouldn't ordinarily be able to do so if it were a "normal" symlink? > > WTF not? It's convenient and doesn't lose any real security. If your > code relies on inaccessibility of <path> since some component of that > path is inaccessible, you are *already* fscked. Consider e.g. fchdir() > and its implications - if you have an opened descriptor for parent, > having no exec permissions on grandparent won't stop you at all. Already. > On all Unices, regardless of openat(), etc. Consider FD passing over unix socket. Passing R/O file descriptor to the other task, then having the task write to the file is certainly bad. > I might buy the argument about restricting reopening with wider permissions, > but > a) we still are looking at possible userland breakage of the worst > kind - random scripts passing /dev/fd/42 as command line arguments to > random programs. Once in a while. With error checking being... not quite > sufficient. > b) it's not just open - we have at least chmod/chown/truncate to > deal with. That's indeed the sane way to solve that. > Prohibiting *all* access is a complete non-starter - things like > cmp foo /dev/stdin || .... > would bloody better work and nobody cares whether you have redirect > from something out of your reach at the moment. Ok. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
