linux-kernel - Re: [PATCH 5.4 047/215] iio:humidity:hdc100x Fix alignment and data leak issues

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Message-ID: <CAJCx=gmxHDm2Qhi8kXhSkpgrGc2-AKb7ymuU5oK+fcBqmQaROQ@mail.gmail.com>
Date:   Mon, 20 Jul 2020 11:55:43 -0700
From:   Matt Ranostay <matt.ranostay@...sulko.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        open list <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org, Lars-Peter Clausen <lars@...afoo.de>,
        Alison Schofield <amsfield22@...il.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH 5.4 047/215] iio:humidity:hdc100x Fix alignment and data
 leak issues

On Mon, Jul 20, 2020 at 9:50 AM Joe Perches <joe@...ches.com> wrote:
>
> On Mon, 2020-07-20 at 17:35 +0200, Greg Kroah-Hartman wrote:
> > From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> >
> > commit ea5e7a7bb6205d24371373cd80325db1bc15eded upstream.
> >
> > One of a class of bugs pointed out by Lars in a recent review.
> > iio_push_to_buffers_with_timestamp assumes the buffer used is aligned
> > to the size of the timestamp (8 bytes).  This is not guaranteed in
> > this driver which uses an array of smaller elements on the stack.
> > As Lars also noted this anti pattern can involve a leak of data to
> > userspace and that indeed can happen here.  We close both issues by
> > moving to a suitable structure in the iio_priv() data.
> > This data is allocated with kzalloc so no data can leak apart
> > from previous readings.
> []
> > +++ b/drivers/iio/humidity/hdc100x.c
> > @@ -38,6 +38,11 @@ struct hdc100x_data {
> >
> >       /* integration time of the sensor */
> >       int adc_int_us[2];
> > +     /* Ensure natural alignment of timestamp */
> > +     struct {
> > +             __be16 channels[2];
> > +             s64 ts __aligned(8);
>
> Why does an s64 need __aligned(8) ?

This is due to on 32-bit x86 it is aligned to 4 bytes by default.

- Matt

> This seems needlessly redundant.
>
> Isn't this naturally aligned by the compiler?
>
> The struct isn't packed.
>