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

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Date:   Thu, 3 Jan 2019 09:17:00 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Aaro Koskinen <aaro.koskinen@....fi>,
        David Howells <dhowells@...hat.com>
Cc:     "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 Sun, Dec 30, 2018 at 06:29:06PM +0200, Aaro Koskinen wrote:
> Hi,
> 
> When using ext4 encryption on SPARC, there's plenty of dmesg noise about
> unaligned access:
> 
> [  167.269526] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
> [  167.270152] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
> [  181.087451] log_unaligned: 5 callbacks suppressed
> [  181.087511] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
> [  181.092435] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
> [  181.095816] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
> 
> And also seen on an ARM machine:
> 
> $ cat /proc/cpu/alignment
> User:           0
> System:         1028193 (find_and_lock_process_key+0x84/0x10c)
> Skipped:        0
> Half:           0
> Word:           1028193
> DWord:          0
> Multi:          0
> User faults:    0 (ignored)
> 
> Looks like user_key_payload layout is not optimal when data address
> is used for fscrypt_key... I tried the below change and got rid of the
> messages. Not sure what the proper fix should be?
> 
> A.
> 
> diff --git a/include/keys/user-type.h b/include/keys/user-type.h
> index e098cbe27db5..6495ffcfe510 100644
> --- a/include/keys/user-type.h
> +++ b/include/keys/user-type.h
> @@ -31,7 +31,7 @@
>  struct user_key_payload {
>  	struct rcu_head	rcu;		/* RCU destructor */
>  	unsigned short	datalen;	/* length of this data */
> -	char		data[0];	/* actual data */
> +	char data[0] __aligned(4);	/* actual data */
>  };
>  
>  extern struct key_type key_type_user;
> 

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?

- Eric