linux-kernel - Re: [RFC PATCH v10 11/17] dm-verity: consume root hash digest and signature data via LSM hook

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Date:   Wed, 9 Aug 2023 11:02:42 -0700
From:   Fan Wu <wufan@...ux.microsoft.com>
To:     Paul Moore <paul@...l-moore.com>,
        Mike Snitzer <snitzer@...nel.org>, corbet@....net,
        zohar@...ux.ibm.com, jmorris@...ei.org, serge@...lyn.com,
        tytso@....edu, ebiggers@...nel.org, axboe@...nel.dk,
        agk@...hat.com, eparis@...hat.com, linux-doc@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fscrypt@...r.kernel.org, linux-block@...r.kernel.org,
        dm-devel@...hat.com, audit@...r.kernel.org,
        roberto.sassu@...wei.com, linux-kernel@...r.kernel.org,
        Deven Bowers <deven.desai@...ux.microsoft.com>
Subject: Re: [RFC PATCH v10 11/17] dm-verity: consume root hash digest and
 signature data via LSM hook

On Wed, Aug 09, 2023 at 12:40:23AM +0100, Alasdair G Kergon wrote:
> On Tue, Aug 08, 2023 at 03:45:03PM -0700, Fan Wu wrote:
> > On Tue, Jul 25, 2023 at 04:43:48PM -0400, Paul Moore wrote:
> > > Where would the finalize() hook be called?
> > 
> > It is in the __bind function in drivers/md/dm.c, calling just before 
> > rcu_assign_pointer(md->map, (void *)t) which activates the inactive table.
>  
> That would be after the existing commit point, meaning the table swap
> cannot be cancelled there, so is the finalize() you are proposing void()
> i.e. designed so it always succeeds?
> 
> Alasdair

Thanks for the input.

Actually, no, the hook can be failed. I noticed the existing call before rcu_assign_pointer(md->map, (void *)t);
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/md/dm.c#n2255)
can also be failed so I was following the same pattern.

Could you explain a bit more about the "commit point"? It sounds like it might be better to move
the hook call just before the commit point instead.

-Fan