From mboxrd@z Thu Jan 1 00:00:00 1970 From: zohar@linux.vnet.ibm.com (Mimi Zohar) Date: Sun, 17 Sep 2017 01:47:41 -0400 Subject: [PATCH 3/3] ima: use fs method to read integrity data (updated patch description) In-Reply-To: References: <1505451494-30228-1-git-send-email-zohar@linux.vnet.ibm.com> <1505451494-30228-4-git-send-email-zohar@linux.vnet.ibm.com> <1505507142.4200.103.camel@linux.vnet.ibm.com> Message-ID: <1505627261.4200.161.camel@linux.vnet.ibm.com> To: linux-security-module@vger.kernel.org List-Id: linux-security-module.vger.kernel.org On Sat, 2017-09-16 at 11:20 -0700, Linus Torvalds wrote: > On Fri, Sep 15, 2017 at 1:25 PM, Mimi Zohar 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 -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo at vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html