On Mon, 6 Aug 2007 09:54:03 -0400 Jeff Layton <jlayton@xxxxxxxxxx> wrote: > Apologies for the resend, but the original sending had the date in the > email header and it caused some of these to bounce... > > ( Please consider trimming the Cc list if discussing some aspect of this > that doesn't concern everyone.) > > When an unprivileged process attempts to modify a file that has the > setuid or setgid bits set, the VFS will attempt to clear these bits. The > VFS will set the ATTR_KILL_SUID or ATTR_KILL_SGID bits in the ia_valid > mask, and then call notify_change to clear these bits and set the mode > accordingly. > > With a networked filesystem (NFS in particular but most likely others), > the client machine may not have credentials that allow for the clearing > of these bits. In some situations, this can lead to file corruption, or > to an operation failing outright because the setattr fails. > > In this situation, we'd like to just leave the handling of this to > the server and ignore these bits. The problem is that by the time > nfs_setattr is called, the VFS has already reinterpreted the ATTR_KILL_* > bits into a mode change. We can't fix this in the filesystems where > this is a problem, as doing so would leave us having to second-guess > what the VFS wants us to do. So we need to change it so that filesystems > have more flexibility in how to interpret the ATTR_KILL_* bits. > > The first patch in the following patchset moves this logic into a helper > function, and then only calls this helper function for inodes that do > not have a setattr operation defined. The subsequent patches fix up > individual filesystem setattr functions to call this helper function. > > The upshot of this is that with this change, filesystems that define > a setattr inode operation are now responsible for handling the ATTR_KILL > bits as well. They can trivially do so by calling the helper, but they > must do so. > > Some of the follow-on patches may not be strictly necessary, but I > decided that it was better to take the conservative approach and call > the helper when I wasn't sure. I've tried to CC the maintainers > for the individual filesystems as well where I could find them, > please let me know if there are others who should be informed. > > Comments and suggestions appreciated... > >From a purely practical standpoint: it's a concern that all filesytems need patching to continue to correctly function after this change. There might be filesystems which you missed, and there are out-of-tree filesystems which won't be updated. And I think the impact upon the out-of-tree filesystems would be fairly severe: they quietly and subtly get their secutiry guarantees broken (I think?) Is there any way in which we can prevent these problems? Say - rename something so that unconverted filesystems will reliably fail to compile? - leave existing filesystems alone, but add a new inode_operations.setattr_jeff, which the networked filesytems can implement, and teach core vfs to call setattr_jeff in preference to setattr? Something else? - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
