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

[Mikulas Patocka <mpatocka@redhat.com>]

On Sat, 3 Jun 2023, Demi Marie Obenour 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@invisiblethingslab.com>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@vger.kernel.org

Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>

> ---
>  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");
> +
> +	/*
> +	 * Number of bytes remaining, starting with last. This is always
> +	 * sizeof(struct dm_target_spec) or more, as otherwise *last was
> +	 * out of bounds already.
> +	 */
> +	size_t remaining = (char *)end - (char *)last;
> +
> +	/*
> +	 * There must be room for both the next target spec and the
> +	 * NUL-terminator of the target itself.
> +	 */
> +	if (remaining - sizeof(struct dm_target_spec) <= next) {
> +		DMERR("Target spec extends beyond end of parameters");
> +		return -EINVAL;
> +	}
> +
>  	if (next % 8) {
>  		DMERR("Next target spec (offset %u) is not 8-byte aligned", next);
>  		return -EINVAL;
> -- 
> Sincerely,
> Demi Marie Obenour (she/her/hers)
> Invisible Things Lab
> 
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