Messages in this thread | | | Subject | Re: [PATCH 3/3] ima: use fs method to read integrity data (updated patch description) | From | Mimi Zohar <> | Date | Sun, 24 Sep 2017 18:55:06 -0400 |
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On Mon, 2017-09-18 at 10:55 -0400, Mimi Zohar wrote: > On Mon, 2017-09-18 at 12:13 +0200, Jan Kara wrote: > > On Mon 18-09-17 10:19:25, Steven Whitehouse wrote: > > > On 17/09/17 17:38, Al Viro wrote: > > > >On Sun, Sep 17, 2017 at 09:34:01AM -0700, Linus Torvalds wrote: > > > >>Now, I suspect most (all?) do, but that's a historical artifact rather > > > >>than "design". In particular, the VFS layer used to do the locking for > > > >>the filesystems, to guarantee the POSIX requirements (POSIX requires > > > >>that writes be seen atomically). > > > >> > > > >>But that lock was pushed down into the filesystems, since some > > > >>filesystems really wanted to have parallel writes (particularly for > > > >>direct IO, where that POSIX serialization requirement doesn't exist). > > > >> > > > >>That's all many years ago, though. New filesystems are likely to have > > > >>copied the pattern from old ones, but even then.. > > > >> > > > >>Also, it's worth noting that "inode->i_rwlock" isn't even well-defined > > > >>as a lock. You can have the question of *which* inode gets talked > > > >>about when you have things like eoverlayfs etc. Normally it would be > > > >>obvious, but sometimes you'd use "file->f_mapping->host" (which is the > > > >>same thing in the simple cases), and sometimes it really wouldn't be > > > >>obvious at all.. > > > >> > > > >>So... I'm really not at all convinced that i_rwsem is sensible. It's > > > >>one of those things that are "mostly right for the simple cases", > > > >>but... > > > >The thing pretty much common to all of them is that write() might need > > > >to modify permissions (suid removal), which brings ->i_rwsem in one > > > >way or another - notify_change() needs that held...
> > > For GFS2, if we are to hold the inode info constant while it is checked, we > > > would need to take a glock (read lock in this case) across the relevant > > > operations. The glock will be happy under i_rwlock, since we have a lock > > > ordering that takes local locks ahead of cluster locks. I've not dug into > > > this enough to figure out whether the current proposal will allow this to > > > work with GFS2 though. Does IMA cache the results from the > > > ->read_integrity() operation? > > Up to now, the hash calculation was stored in the iint structure, > which is then used to extend the TPM, verify the file's integrity > compared to the value stored in the xattr, and included in an audit > message. > > A new patch set by Thiago Bauermann will add appended signature > support, re-using the kernel module signature appended method, which > might require re-calculating the file hash based on a different hash > algorithm. > > > So I have asked Mimi about clustered filesystems before. And for now the > > answer was that IMA for clustered filesystems is not supported (it will > > return some error since ->integrity_read is NULL). If we would ever want to > > support those it would require larger overhaul of the IMA architecture to > > give filesystem more control over the locking (which is essentially what > > Linus wants). > > For performance reasons, IMA is not on a write hook, but detects file > change on the last __fput() opened for write. At that point, the > cached info is reset. The file hash is re-calculated and written out > as an xattr. On the next file access (in policy), the file hash is > re-calculated and stored in the iint. > > In terms of remote/clustered/fuse filesystems, we wouldn't be on the > __fput() path. Support for remote/clustered/fuse filesystems, would > be similar to filesystems that do not support i_version. Meaning only > the first file access (in policy) would be measured/appraised, but not > subsequent ones. Even if we could detect file change, we would be > dependent on the remote/clustered/fuse filesystem to inform us of the > change. What type of integrity guarantees would that provide?
After thinking this over a bit, perhaps we shouldn't cache the file integrity results for these filesystems, since we can't rely on them to notify us of a change (eg. malicious fs), but simply re-measure/re- validate files each time.
Mimi
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