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Re: [PATCH 3/3] ima: use fs method to read integrity data (updated patch description)

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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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[PATCH 3/3] ima: use fs method to read integrity data Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-15 07:00 +0200
  Re: [PATCH 3/3] ima: use fs method to read integrity data Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-15 11:10 +0200
    Re: [PATCH 3/3] ima: use fs method to read integrity data Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-15 11:20 +0200
    Re: [PATCH 3/3] ima: use fs method to read integrity data Linus Torvalds <torvalds@linux-foundation.org> - 2017-09-15 20:10 +0200
  Re: [PATCH 3/3] ima: use fs method to read integrity data Christoph Hellwig <hch@infradead.org> - 2017-09-15 17:00 +0200
    Re: [PATCH 3/3] ima: use fs method to read integrity data Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-15 17:30 +0200
  Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-15 22:30 +0200
    Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Linus Torvalds <torvalds@linux-foundation.org> - 2017-09-16 20:30 +0200
      Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-17 07:50 +0200
      Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Christoph Hellwig <hch@infradead.org> - 2017-09-17 17:20 +0200
        Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Linus Torvalds <torvalds@linux-foundation.org> - 2017-09-17 17:30 +0200
          Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Christoph Hellwig <hch@infradead.org> - 2017-09-17 17:40 +0200
          Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-17 18:20 +0200
            Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Linus Torvalds <torvalds@linux-foundation.org> - 2017-09-17 18:40 +0200
              Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-17 18:40 +0200
                Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Steven Whitehouse <swhiteho@redhat.com> - 2017-09-18 11:20 +0200
                Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Jan Kara <jack@suse.cz> - 2017-09-18 12:20 +0200
                Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-18 17:00 +0200
                Re: [PATCH 3/3] ima: use fs method to read integrity data (updated  patch description) Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-25 01:00 +0200

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