linux-kernel - Re: Bug report: unaligned access with ext4 encryption

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Message-ID: <20190110230114.GF149637@gmail.com>
Date:   Thu, 10 Jan 2019 15:01:14 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Aaro Koskinen <aaro.koskinen@....fi>
Cc:     David Howells <dhowells@...hat.com>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        linux-fscrypt@...r.kernel.org, linux-kernel@...r.kernel.org,
        keyrings@...r.kernel.org
Subject: Re: Bug report: unaligned access with ext4 encryption

On Fri, Jan 11, 2019 at 12:29:28AM +0200, Aaro Koskinen wrote:
> Hi,
> 
> On Fri, Jan 04, 2019 at 05:28:02PM +0000, David Howells wrote:
> > Eric Biggers <ebiggers@...nel.org> wrote:
> > > Hi Aaro, thanks for the bug report!  I think you're on the right track; it makes
> > > much more sense to have the keyrings subsystem store the payload with better
> > > alignment, than to work around the 2-byte alignment in fscrypt.
> > > 
> > > But how about '__aligned(__alignof__(u64))' instead?  4 bytes may not be enough.
> > > 
> > > David, what do you think?
> > 
> > Does that even work?
> 
> That should work.
> 
> > Might be better to just insert 6 bytes of padding with a comment, but yes I
> > agree that it's probably better to align it to at least machine word size.
> 
> Padding is fragile, e.g. if struct rcu_head changes. Using __aligned should
> make it always right automatically.
> 
> A.

I agree that __aligned is better.  It should work; see 'struct crypto_tfm' in
include/linux/crypto.h for another example of a struct that uses __aligned on a
flexible array at the end.

Aaro, can you send a formal patch?  If you don't I'll do so, but I figure I'll
ask first.

Thanks,

- Eric