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, 17 Sep 2017 01:47:41 -0400 |
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On Sat, 2017-09-16 at 11:20 -0700, Linus Torvalds wrote: > On Fri, Sep 15, 2017 at 1:25 PM, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote: > > > > To resolve this locking problem, this patch defines a new > > ->integrity_read file operation method, which is equivalent to > > ->read_iter, except that it will not take the i_rwsem lock, but will > > be called with the i_rwsem held exclusively. > > > > Since taking the i_rwsem exclusively is not required for reading the > > file in order to calculate the file hash, the code only verifies > > that the lock has been taken. > > Ok, so I'm onboard with the commit message now, but realized that I'm > not actually convinced that i_rwsem is even meaningful. > > Sure, generic_file_write_iter() does take that lock exclusively, but > not everybody uses generic_file_write_iter() at all for writing.
> For example, xfs still uses that i_rwsem, but for block-aligned writes > it will only get it shared. And I'm not convinced some other > filesystem might not end up using some other lock entirely. > > So I'm basically not entirely convinced that these i_rwsem games make > any sense at all. > > The filesystem can do its own locking, and I'm starting to think that > it would be better to just pass this "this is an integrity read" down > to the filesystem, and expect the filesystem to do the locking based > on that.
IMA would still need to take the i_rwsem to write the xattr. Unless the i_rwsem was taken before calling the integrity_read, calculating the file hash would be serialized, but would not prevent the file hash from being calculated multiple times.
(Introducing a new lock would result in the locks being taken in reverse order for setxattr, chown, chmod syscalls.)
Mimi
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