linux-kernel - Re: [Linux-kernel-mentees] [PATCH] drm/bufs: Prevent kernel-infoleak in copy_one

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Date:   Tue, 28 Jul 2020 10:11:09 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Peilin Ye <yepeilin.cs@...il.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Linux-kernel-mentees] [PATCH] drm/bufs: Prevent kernel-infoleak
 in copy_one_buf()

On Tue, Jul 28, 2020 at 3:45 AM Peilin Ye <yepeilin.cs@...il.com> wrote:
>
> copy_one_buf() is copying uninitialized stack memory to userspace due to
> the compiler not initializing holes in statically allocated structures.
> Fix it by initializing `v` with memset().

I would add 'potentially' somewhere in that description: it is architecture
dependent whether there are holes in this structure as 'enum' types
and 'long' are both dependent on the ABI, and even if there is a hole,
it is undefined behavior whether the hold gets initialized.

Other than that, the patch looks good.

> Cc: stable@...r.kernel.org
> Fixes: 5c7640ab6258 ("switch compat_drm_infobufs() to drm_ioctl_kernel()")
> Suggested-by: Dan Carpenter <dan.carpenter@...cle.com>
> Signed-off-by: Peilin Ye <yepeilin.cs@...il.com>

Acked-by: Arnd Bergmann <arnd@...db.de>

> ---
>  drivers/gpu/drm/drm_bufs.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_bufs.c b/drivers/gpu/drm/drm_bufs.c
> index a0735fbc144b..f99cd4a3f951 100644
> --- a/drivers/gpu/drm/drm_bufs.c
> +++ b/drivers/gpu/drm/drm_bufs.c
> @@ -1349,10 +1349,14 @@ static int copy_one_buf(void *data, int count, struct drm_buf_entry *from)
>  {
>         struct drm_buf_info *request = data;
>         struct drm_buf_desc __user *to = &request->list[count];
> -       struct drm_buf_desc v = {.count = from->buf_count,
> -                                .size = from->buf_size,
> -                                .low_mark = from->low_mark,
> -                                .high_mark = from->high_mark};
> +       struct drm_buf_desc v;
> +
> +       memset(&v, 0, sizeof(v));
> +
> +       v.count = from->buf_count;
> +       v.size = from->buf_size;
> +       v.low_mark = from->low_mark;
> +       v.high_mark = from->high_mark;
>
>         if (copy_to_user(to, &v, offsetof(struct drm_buf_desc, flags)))
>                 return -EFAULT;
> --
> 2.25.1
>