On Sat, 07 Nov 2009, ebiederm@xxxxxxxxxxxx (Eric W. Biederman wrote: > Miklos Szeredi <miklos@xxxxxxxxxx> writes: > > > On Fri, 06 Nov 2009, ebiederm@xxxxxxxxxxxx (Eric W. Biederman wrote: > >> So far no one who believes this to be a security hole has found it > >> worth their while to look at nd->intent.open in proc_pid_follow_link > >> and write a patch. > > > > A rather disgusting patch that would be. The fact is, checking > > permissions on follow_link makes little to no sense. Consider > > truncate(2), for example. Will we add another intent for that? I > > really hope not > > No. I was just thinking we have the open intent that is there for > combining lookup and open. We can look test for LOOKUP_OPEN and do > exactly what we need. No, because you just covered half the cases. truncate(2) will still work fine on the /proc/PID/fd/FD belonging to a O_RDONLY file descriptor. > > I'm more and more convinced, that the current behavior is the right > > one. > > I think the 15 or so years we have had the current behavior without > problems is persuasive. > > I think it is an interesting puzzle on how to get dup instead of > reopen as there are cases where that could be useful behavior as well. Probably doable with ptrace(). > The usefulness of an O_NONE flag increases significantly if you can > open the reference file later with more permissions. Essentially > making a hardlink into a running program. Hmm. Weird cases do seem > to show up when the last dir entry is removed. Why are they more weird than files opened without O_NODE? The only really weird case Alan spotted is device nodes, where the actual device registered to a major/minor pair changes over time, possibly allowing a re-open to access a device it otherwise was not meant to. BTW if the device number reuse happens really quickly, this could even be a race for a plain open. Real solution might actually be in udev: when deregistering a device, change mode bits to all-zero before removing the device node. > I wonder if we want a rule that you can't open a file with link count > of 0. Reasoning may get truly strange otherwise. Again, don't see why this would be different for O_NODE as for non-O_NODE files descriptors. Thanks, Miklos -- 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
