Re: AW: [PATCH] amd64: Fix csum_partial_copy_generic()

[Al Viro]

On Thu, Oct 19, 2023 at 06:02:50AM +0100, Al Viro wrote:
> On Thu, Oct 19, 2023 at 04:44:04AM +0000, gus Gusenleitner Klaus wrote:
> > > On Wed, Oct 18, 2023 at 06:18:05AM +0000, gus Gusenleitner Klaus wrote:
> > > > The checksum calculation is wrong in case of an source buffer
> > > > containing zero bytes only. The expected return value is 0, the
> > > > actual return value is 0xfffffff.
> > > 
> > > Expected where?  The actual checksum is defined modulo 0xffff, so
> > > 0 and 0xffffffff represent the same final value.
> > > 
> > > The only twist is that in some situations we internally use 0 for
> > > "not calculated yet".
> > > 
> > > > This problem occurs when a ICMP echo reply is sent that has set
> > > > zero identifier, sequence number and data.
> > > 
> > > What problem?  Could you please either point to specific RFC or
> > > show that packets are rejected by some existing system, or...?
> > 
> > Here's our situation:
> > Our device gets pinged by a third party manufacturer robot controller.
> > We have updated the kernel in our device to 5.15 from 4.9, the robot
> > controller is kept unchanged. At 4.9, our device's ping reply is accepted
> > by the robot controller, at 5.15 it's not.
> > 
> > Wireshark shows a bad checksum warning:
> >  'Checksum: 0x0000 incorrect, should be 0xffff' 
> > 
> 
> Lovely.  I think I see what's going on, give me a few to think about it...

The real source of trouble was switching csum_and_copy_{to,from}_user()
to reporting faults as 0.  And yes, it's broken.  Bugger...