linux-kernel - Re: [PATCH v2 1/6] device-mapper: Check that target specs are sufficiently aligned

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Date:   Thu, 22 Jun 2023 12:28:28 -0400
From:   Mike Snitzer <snitzer@...nel.org>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>,
        mpatocka@...hat.com
Cc:     Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] device-mapper: Check that target specs are
 sufficiently aligned

On Sat, Jun 03 2023 at 10:52P -0400,
Demi Marie Obenour <demi@...isiblethingslab.com> wrote:

> Otherwise subsequent code will dereference a misaligned
> `struct dm_target_spec *`, which is undefined behavior.
> 
> Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.com>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@...r.kernel.org
> ---
>  drivers/md/dm-ioctl.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> index cc77cf3d410921432eb0c62cdede7d55b9aa674a..34fa74c6a70db8aa67aaba3f6a2fc4f38ef736bc 100644
> --- a/drivers/md/dm-ioctl.c
> +++ b/drivers/md/dm-ioctl.c
> @@ -1394,6 +1394,13 @@ static inline fmode_t get_mode(struct dm_ioctl *param)
>  static int next_target(struct dm_target_spec *last, uint32_t next, void *end,
>  		       struct dm_target_spec **spec, char **target_params)
>  {
> +	static_assert(_Alignof(struct dm_target_spec) <= 8,
> +		      "struct dm_target_spec has excessive alignment requirements");

Really not sure what you mean by "has excessive alignment requirements"...

> +	if (next % 8) {
> +		DMERR("Next target spec (offset %u) is not 8-byte aligned", next);
> +		return -EINVAL;
> +	}
> +
>  	*spec = (struct dm_target_spec *) ((unsigned char *) last + next);
>  	*target_params = (char *) (*spec + 1);
>  

But this patch and patches 2 and 3 need more review. I'd like Mikulas to review.

I did pick up patches 4-6 for the upcoming 6.5 merge window.

Note, please prefix with "dm ioctl" instead of "device-mapper".

(I just switched my "dm" prefix to "dm ioctl" and forced update on the
dm-6.5 branch, so the commit I referenced earlier for your version
copy patch is now here:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-6.5&id=a5a3de762b3ae8959347928843c12502b1b23163
)

Mike