[RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively

[Mimi Zohar]

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 <zohar at 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.
 Now, even more recently, I received a patch that claims that
i_version is just a performance improvement.  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.

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().
 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).

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.

There should be no open writers in ima_check_last_writer(), so the
file shouldn't be changing.

Mimi

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