On Mon, 2017-10-02 at 15:35 +1100, Dave Chinner wrote: > On Sun, Oct 01, 2017 at 07:42:42PM -0400, Mimi Zohar wrote: > > On Mon, 2017-10-02 at 09:34 +1100, Dave Chinner wrote: > > > On Sun, Oct 01, 2017 at 11:41:48AM -0700, Linus Torvalds wrote: > > > > On Sun, Oct 1, 2017 at 5:08 AM, Mimi Zohar <zo...@linux.vnet.ibm.com> > > > > wrote: > > > > > > > > > > Right, re-introducing the iint->mutex and a new i_generation field in > > > > > the iint struct with a separate set of locks should work. It will be > > > > > reset if the file metadata changes (eg. setxattr, chown, chmod). > > > > > > > > Note that the "inner lock" could possibly be omitted if the > > > > invalidation can be just a single atomic instruction. > > > > > > > > So particularly if invalidation could be just an atomic_inc() on the > > > > generation count, there might not need to be any inner lock at all. > > > > > > > > You'd have to serialize the actual measurement with the "read > > > > generation count", but that should be as simple as just doing a > > > > smp_rmb() between the "read generation count" and "do measurement on > > > > file contents". > > > > > > We already have a change counter on the inode, which is modified on > > > any data or metadata write (i_version) under filesystem locks. The > > > i_version counter has well defined semantics - it's required by > > > NFSv4 to increment on any metadata or data change - so we should be > > > able to rely on it's behaviour to implement IMA as well. Filesystems > > > that support i_version are marked with [SB|MS]_I_VERSION in the > > > superblock (IS_I_VERSION(inode)) so it should be easy to tell if IMA > > > can be supported on a specific filesystem (btrfs, ext4, fuse and xfs > > > ATM). > > > > Recently I received a patch to replace i_version with mtime/atime. > > mtime is not guaranteed to change on data writes - the resolution of > the filesystem timestamps may mean mtime only changes once a second > regardless of the number of writes performed to that file. That's > why NFS can't use it as a change attribute, and hence we have > i_version.... > > > Now, even more recently, I received a patch that claims that > > i_version is just a performance improvement. > > Did you ask them to explain/quantify the performance improvement?
Using i_version is a performance improvement as opposed to always calculating the file hash and writing the xattr. The patch is intended for filesystems that don't support i_version (eg. ubifs). > e.g. Using i_version on XFS slows down performance on small > writes by 2-3% because i_version because all data writes log a > version change rather than only logging a change when mtime updates. > We take that penalty because NFS requires specific change attribute > behaviour, otherwise we wouldn't have implemented it at all in > XFS... > > > For file systems that > > don't support i_version, assume that the file has changed. > > > > For file systems that don't support i_version, instead of assuming > > that the file has changed, we can at least use i_generation. > > I'm not sure what you mean here - the struct inode already has a > i_generation variable. It's a lifecycle indicator used to > discriminate between alloc/free cycles on the same inode number. > i.e. It only changes at inode allocation time, not whenever the data > in the inode changes... Sigh, my error. > > > With Linus' suggested changes, I think this will work nicely. > > > > > The IMA code should be able to sample that at measurement time and > > > either fail or be retried if i_version changes during measurement. > > > We can then simply make the IMA xattr write conditional on the > > > i_version value being unchanged from the sample the IMA code passes > > > into the filesystem once the filesystem holds all the locks it needs > > > to write the xattr... > > > > > I note that IMA already grabs the i_version in > > > ima_collect_measurement(), so this shouldn't be too hard to do. > > > Perhaps we don't need any new locks or counterst all, maybe just > > > the ability to feed a version cookie to the set_xattr method? > > > > The security.ima xattr is normally written out in > > ima_check_last_writer(), not in ima_collect_measurement(). > > Which, if IIUC, does this to measure and update the xattr: > > ima_check_last_writer > -> ima_update_xattr > -> ima_collect_measurement > -> ima_fix_xattr > > > ima_collect_measurement() calculates the file hash for storing in the > > measurement list (IMA-measurement), verifying the hash/signature (IMA- > > appraisal) already stored in the xattr, and auditing (IMA-audit). > > Yup, and it samples the i_version before it calculates the hash and > stores it in the iint, which then gets passed to ima_fix_xattr(). > Looks like all that is needed is to pass the i_version back to the > filesystem through the xattr call.... > > IOWs, sample the i_version early while we hold the inode lock and > check the writer count, then if it is the last writer drop the inode > lock and call ima_update_xattr(). The sampled i_version then tells > us if the file has changed before we write the updated xattr... > > > The only time that ima_collect_measurement() writes the file xattr is > > in "fix" mode. Writing the xattr will need to be deferred until after > > the iint->mutex is released. > > ima_collect_measurement() doesn't write an xattr at all - it just > reads the file data and calculates the hash. There's another call to ima_fix_xattr() from ima_appraise_measurement(). > > There should be no open writers in ima_check_last_writer(), so the > > file shouldn't be changing. > > If that code is not holding the inode i_rwsem across > ima_update_xattr(), then the writer check is racy as hell. We're > trying to get rid of the need for this code to hold the inode lock > to stabilise the writer count for the entire operation, and it looks > to me like everything is there to use the i_version to ensure the > the IMA code doesn't need to hold the inode lock across > ima_collect_measurement() and ima_fix_xattr()... Ok Mimi
