On Mon, Aug 23, 2010 at 09:17:08AM +1000, Neil Brown wrote: > On Sat, 21 Aug 2010 18:30:24 +1000 > Nick Piggin <npiggin@xxxxxxxxx> wrote: > > > Thanks, I had both of the same concerns as Christoph with API > > change and exposing symlink fds last time I looked at the patces, > > actually. > > > > But they can probably be worked around or avoided. I think the more > > important thing is whether it is worth supporting. This is > > all restricted to root (or CAP_DAC_READ_SEARCH) only, right, and > > what exact semantics they want. I would like to see more discussion > > of what this enables and some results. > > They allow a credible user-space implementation of the server for some > network filesystem protocols such as NFS and apparently 9P. > > > > > For the case of avoiding expensive network revalidations in path name > > lookup, do we even need to open symlinks? Could the security issues be > > avoided by always having handle attached to an open fd? > > I don't see what you are getting at here... which particular security isses, > and what fd would you use? Well the issue that you need escalated privilges to use it. The other use case for it I understand is Andreas's file-handle-server which avoids a lot of path lookup costs on non-local filesystems. I'm wondering is that really useful if it's not availale to unprivileged users? > > As I understand it there are only two security issues that have been noted. > 1/ lookup-by-filehandle can bypass any 'search' permission tests on ancestor > directories. I cannot see any way to avoid this except require > CAP_DAC_READ_SEARCH > 2/ Creating a hardlink to an 'fd' allows a process that was given an 'fd' > that it could not have opened itself to prevent that file from being > removed (and space reclaimed) by creating a private hardlink. > This could be avoided by requiring CAP_DAC_READ_SEARCH for that particular > operation (and probably requiring i_nlink > 0 anyway) but that feels like > a very special-case restriction. Just so long as the process could have created a hardlink to the file otherwise via traditional operations, I think it's OK. > > Was it one of these that you were referring to? Just the general problem of security and inherent restrictions to using the syscalls. -- 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
