Re: WARNING in csum_and_copy_to_iter

[Al Viro]

On Sat, Nov 24, 2018 at 11:40:03AM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    edeca3a769ad Merge tag 'sound-4.20-rc4' of git://git.kerne..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=12bee26d400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=73e2bc0cb6463446
> dashboard link: https://syzkaller.appspot.com/bug?extid=ce18da013d76d837144d
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ccd1f5400000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+ce18da013d76d837144d@xxxxxxxxxxxxxxxxxxxxxxxxx

Caused by commit 95506588d2c1d72ca29adef8ae9bf771bcfb4ced
Author: Slavomir Kaslev <kaslevs@xxxxxxxxxx>
Date:   Fri Nov 16 11:27:53 2018 +0200

    socket: do a generic_file_splice_read when proto_ops has no splice_read

exposing all ->recvmsg() instances to pipe-backed iov_iter as possible destination.
It's not all that hard to fix (I'll probably have a candidate patch by tonight,
it's just a matter of adding the only missing primitive), but... shouldn't that
patch have sat in -next for at least some testing first?  Because it's very
easy to reproduce - splice from e.g. UDP socket will step into it.  Sure, the
sky is not falling (unless you set panic-on-WARN, that is); the damn thing
would've failed anyway, but...