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

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Date:   Thu, 22 Jun 2023 19:29:52 +0200 (CEST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>
cc:     Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [dm-devel] [PATCH v2 1/6] device-mapper: Check that target specs
 are sufficiently aligned



On Sat, 3 Jun 2023, Demi Marie Obenour 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");
> +	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);
>  
> -- 
> Sincerely,
> Demi Marie Obenour (she/her/hers)
> Invisible Things Lab

Hi

Some architectures (such as 32-bit x86) specify that the alignment of 
64-bit integers is only 4-byte. This could in theory break old userspace 
code that only uses 4-byte alignment. I would change "next % 8" to "next % 
__alignof__(struct dm_target_spec)".

I think that there is no need to backport this patch series to the stable 
kernels because the bugs that it fixes may only be exploited by the user 
with CAP_SYS_ADMIN privilege. So, there is no security or reliability 
problem being fixed.

Mikulas