Miklos Szeredi <miklos@xxxxxxxxxx> writes: > 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. Good point as truncate is as bad as opening read-write. Shrug. >> > 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? Because as I recall O_NODE skips all permission checks. Which would mean you can limit who holds a reference to your inode 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. Devices nodes specifically were the case I was thinking of. Changing the mode bits to all-zero at the final unlink would be a lot more reliable and certain in the kernel. >> 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. Eric -- 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
