linux-kernel - Re: IMA + O_DIRECT (Re: [PATCH 0/1] fix IMA + Apparmor kernel panic)

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Date:	Fri, 9 May 2014 18:56:02 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"J. R. Okajima" <hooanon05g@...il.com>
Cc:	Dmitry Kasatkin <d.kasatkin@...sung.com>, ebiederm@...ssion.com,
	linux-security-module@...r.kernel.org, eparis@...hat.com,
	zohar@...ux.vnet.ibm.com, dmitry.kasatkin@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: IMA + O_DIRECT (Re: [PATCH 0/1] fix IMA + Apparmor kernel panic)

On Fri, May 09, 2014 at 12:10:03PM +0900, J. R. Okajima wrote:
> 
> Dmitry Kasatkin:
> > Following patch replaces IMA usage of kernel_read() with special
> > version which skips security check that triggers kernel panic
> > when Apparmor and IMA appraisal are enabled together.
> 
> I know this is related to exit(2), but this behaviour of IMA is related
> to open(2) too.
> When O_DIRECT is specified, some filesystems (for example, ext2) call
> do_blockdev_direct_IO() which acquires i_mutex. But
> IMA:process_measurement() already acquires i_mutex before kernel_read().
> It causes a deadlock even if you replace kernel_read() by a simpler one.
> How can we stop reading the file from IMA?

Disabling it would be a good start...  And no, I'm not kidding - the mess
you are proposing is such that it's not at all obvious that this stuff
is worth the trouble.

Why the devil is it playing with ->i_mutex?  IMA, that is.  Its use for
data is absolutely fs-dependent.  Again, filesystem is *NOT* required
to take ->i_mutex anywhere on the write path.  At all.
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