linux-kernel - Re: [PATCH v2 2/6] device-mapper: Avoid pointer arithmetic overflow

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Date:   Thu, 22 Jun 2023 18:50:35 -0400
From:   Mike Snitzer <snitzer@...nel.org>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>
Cc:     Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/6] device-mapper: Avoid pointer arithmetic overflow

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

> Especially on 32-bit systems, it is possible for the pointer arithmetic
> to overflow and cause a userspace pointer to be dereferenced in the
> kernel.
> 
> 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 | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> index 34fa74c6a70db8aa67aaba3f6a2fc4f38ef736bc..64e8f16d344c47057de5e2d29e3d63202197dca0 100644
> --- a/drivers/md/dm-ioctl.c
> +++ b/drivers/md/dm-ioctl.c
> @@ -1396,6 +1396,25 @@ static int next_target(struct dm_target_spec *last, uint32_t next, void *end,
>  {
>  	static_assert(_Alignof(struct dm_target_spec) <= 8,
>  		      "struct dm_target_spec has excessive alignment requirements");
> +	static_assert(offsetof(struct dm_ioctl, data) >= sizeof(struct dm_target_spec),
> +		      "struct dm_target_spec too big");

I'm struggling to see the point for this compile-time check?
Especially when you consider (on x86_64):

sizeof(struct dm_target_spec) = 40
offsetof(struct dm_ioctl, data) = 305

Just feels like there is no utility offered by adding this check.

SO I've dropped it.  But if you feel there is some inherent value
please let me know.

Thanks,
Mike