Re: [PATCH] isdn/i4l: fetch the ppp_write buffer in one shot

[David Miller]

From: Meng Xu <mengxu.gatech@xxxxxxxxx>
Date: Tue, 19 Sep 2017 21:49:55 -0400

> In isdn_ppp_write(), the header (i.e., protobuf) of the buffer is
> fetched twice from userspace. The first fetch is used to peek at the
> protocol of the message and reset the huptimer if necessary; while the
> second fetch copies in the whole buffer. However, given that buf resides
> in userspace memory, a user process can race to change its memory content
> across fetches. By doing so, we can either avoid resetting the huptimer
> for any type of packets (by first setting proto to PPP_LCP and later
> change to the actual type) or force resetting the huptimer for LCP
> packets.
> 
> This patch changes this double-fetch behavior into two single fetches
> decided by condition (lp->isdn_device < 0 || lp->isdn_channel <0).
> A more detailed discussion can be found at
> https://marc.info/?l=linux-kernel&m=150586376926123&w=2
> 
> Signed-off-by: Meng Xu <mengxu.gatech@xxxxxxxxx>

Applied, thank you.