Re: [PATCH] media: dvb: fix a missing-check bug

[Wenwen Wang]

Hello,

Can anyone confirm this bug? Thanks!

Wenwen

On Fri, Oct 19, 2018 at 9:12 AM Wenwen Wang <wang6495@xxxxxxx> wrote:
>
> In dvb_audio_write(), the first byte of the user-space buffer 'buf' is
> firstly copied and checked to see whether this is a TS packet, which always
> starts with 0x47 for synchronization purposes. If yes, ts_play() will be
> called. Otherwise, dvb_aplay() will be called. In ts_play(), the content of
> 'buf', including the first byte, is copied again from the user space.
> However, after the copy, no check is re-enforced on the first byte of the
> copied data.  Given that 'buf' is in the user space, a malicious user can
> race to change the first byte after the check in dvb_audio_write() but
> before the copy in ts_play(). Through this way, the user can supply
> inconsistent code, which can cause undefined behavior of the kernel and
> introduce potential security risk.
>
> This patch adds a necessary check in ts_play() to make sure the first byte
> acquired in the second copy contains the expected value. Otherwise, an
> error code EINVAL will be returned.
>
> Signed-off-by: Wenwen Wang <wang6495@xxxxxxx>
> ---
>  drivers/media/pci/ttpci/av7110_av.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/media/pci/ttpci/av7110_av.c b/drivers/media/pci/ttpci/av7110_av.c
> index ef1bc17..1ff6062 100644
> --- a/drivers/media/pci/ttpci/av7110_av.c
> +++ b/drivers/media/pci/ttpci/av7110_av.c
> @@ -468,6 +468,8 @@ static ssize_t ts_play(struct av7110 *av7110, const char __user *buf,
>                 }
>                 if (copy_from_user(kb, buf, TS_SIZE))
>                         return -EFAULT;
> +               if (kb[0] != 0x47)
> +                       return -EINVAL;
>                 write_ts_to_decoder(av7110, type, kb, TS_SIZE);
>                 todo -= TS_SIZE;
>                 buf += TS_SIZE;
> --
> 2.7.4
>