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List:       linux-integrity
Subject:    Re: IMA/EVM writing xattrs during remount filesystem
From:       Sascha Hauer <s.hauer () pengutronix ! de>
Date:       2018-02-27 7:27:34
Message-ID: 20180227072734.resrlh6mpkkd5ko6 () pengutronix ! de
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On Mon, Feb 26, 2018 at 11:03:18AM -0500, Mimi Zohar wrote:
> >  * ubifs_remount_ro - re-mount in read-only mode.
> >  * @c: UBIFS file-system description object
> >  *
> >  * We assume VFS has stopped writing. Possibly the background thread could be
> >  * running a commit, however kthread_stop will wait in that case.
> >  */
> > 
> > >	sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (sb_flags & MS_RMT_MASK);
> > 
> > Here, *after* remount_fs has returned the MS_RDONLY sb flag is set which
> > EVM tests for before calling evm_update_evmxattr() and the race window
> > closes.
> 
> So the cause of the problem is not IMA, per se, but EVM converting the
> EVM signature to an HMAC.  There's no harm in not re-writing the xattr
> signature as an HMAC.  Feel free to add the additional
> "s_readonly_remount" test.

Ok, that should work. I'll give it some testing here before I send a
patch.

> 
> During this open window, we upstreamed support for EVM portable and
> immutable file signatures.  Please make sure you base the change on
> the linux-integrity #next-integrity branch.

sure, thanks

Sascha

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