On Fri, 2010-07-09 at 08:48 +1000, James Morris wrote: > On Thu, 8 Jul 2010, David P. Quigley wrote: > > > > The maximum security label size on Linux is: > > > > > > #define XATTR_SIZE_MAX 65536 > > > > > > Why arbitrarily limit this over the network? > > > > Because there is no easy way not to. The specification doesn't specify a > > limit to label size in the IETF draft. However there is no way to do > > allocation of the memory needed to store the label where we first get > > access to its size. We tried this before and it failed. When I asked > > trond about it he said doing memory allocation in the rpc context isn't > > allowed. > > In the NFSv3 code, the workaround I've been using is to always allocate > 64k, but the correct way of doing this apparently is to use the page > cache, as is used for ACLs and symlinks. > > > For the most part what would make this label size inadequate would be > > the MLS component. There are some cases where people want every other > > compartment or something crazy like that. In terms of a normal label > > though 4096 should be more than enough. > > Yes, but we should not unnecessarily limit the network protocol when > something is valid and possible in the local implementation (which is ~64k > under Linux). > > > Just to put this in perspective the string below is 4096 a's. > > A security label include just about anything, e.g. an x509 certificate, or > a base64 encoded image. > > In the Linux implementation, if we can store a local label up to 64k, then > we should try and ensure that it can be conveyed via NFS. You can't store a local label up to 64k on Linux; that is just what the xattr API permits, not the underlying filesystem implementations (at least ext[234]). # touch foobar # setfattr -n user.foo -v `perl -e 'print "a" x 4096'` foobar setfattr: foobar: No space left on device Also the /proc/self/attr and selinuxfs APIs are presently limited to page size. -- Stephen Smalley National Security Agency -- This message was distributed to subscribers of the selinux mailing list. If you no longer wish to subscribe, send mail to majordomo@xxxxxxxxxxxxx with the words "unsubscribe selinux" without quotes as the message.
