linux-kernel - Re: [BUG] from x86: Support kmap

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Date:   Wed, 6 Jan 2021 23:44:54 -0500
From:   Willem de Bruijn <willemb@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        David Miller <davem@...emloft.net>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [BUG] from x86: Support kmap_local() forced debugging

On Wed, Jan 6, 2021 at 9:11 PM Willem de Bruijn <willemb@...gle.com> wrote:
>
> On Wed, Jan 6, 2021 at 8:49 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Wed, 6 Jan 2021 17:03:48 -0800 Linus Torvalds wrote:
> > > I wonder whether there is other code that "knows" about kmap() only
> > > affecting PageHighmem() pages thing that is no longer true.
> > >
> > > Looking at some other code, skb_gro_reset_offset() looks suspiciously
> > > like it also thinks highmem pages are special.
> > >
> > > Adding the networking people involved in this area to the cc too.

But there are three other kmap_atomic callers under net/ that do not
loop at all, so assume non-compound pages. In esp_output_head,
esp6_output_head and skb_seq_read. The first two directly use
skb_page_frag_refill, which can allocate compound (but not
__GFP_HIGHMEM) pages, and the third can be inserted with
netfilter xt_string in the path of tcp transmit skbs, which can also
have compound pages. I think that these could similarly access
data beyond the end of the kmap_atomic mapped page. I'll take
a closer look.