Re: SECURITY PROBLEM: Any user can crash the kernel with TCP ZEROCOPY

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 22, 2024 at 05:30:18PM +0100, Eric Dumazet wrote:
> On Mon, Jan 22, 2024 at 5:04 PM Matthew Wilcox <willy@infradead.org> wrote:
> > I'm disappointed to have no reaction from netdev so far.  Let's see if a
> > more exciting subject line evinces some interest.
> 
> Hmm, perhaps some of us were enjoying their weekend ?

I am all in favour of people taking time off!  However the report came
in on Friday at 9am UTC so it had been more than a work day for anyone
anywhere in the world without response.

> I don't really know what changed recently, all I know is that TCP zero
> copy is for real network traffic.
> 
> Real trafic uses order-0 pages, 4K at a time.
> 
> If can_map_frag() needs to add another safety check, let's add it.

So it's your opinion that people don't actually use sendfile() from
a local file, and we can make this fail to zerocopy?  That's good
because I had a slew of questions about what expectations we had around
cache coherency between pages mapped this way and write()/mmap() of
the original file.  If we can just disallow this, we don't need to
have a discussion about it.

> syzbot is usually quite good at bisections, was a bug origin found ?

I have the impression that Huawei run syzkaller themselves without
syzbot.  I suspect this bug has been there for a good long time.
Wonder why nobody's found it before; it doesn't seem complicated for a
fuzzer to stumble into.