From: Andrea Parri <parri.andrea@xxxxxxxxx> Sent: Thursday, April 14, 2022 11:42 PM > > On Fri, Apr 15, 2022 at 03:33:23AM +0000, Michael Kelley (LINUX) wrote: > > From: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx> Sent: Wednesday, April 13, > 2022 1:48 PM > > > > > > The function returns NULL if the ring buffer has no enough space > > > available for a packet descriptor. The ring buffer's write_index > > > > The first sentence wording is a bit scrambled. I think you mean the > > ring buffer doesn't contain enough readable bytes to constitute a > > packet descriptor. > > Indeed, replaced with your working. > > > > > is in memory which is shared with the Hyper-V host, its value is > > > thus subject to being changed at any time. > > > > This second sentence is true, but I'm not making the connection > > with the code change below. Evidently, there is some previous > > check made to ensure that enough bytes are available to be > > received when hvs_stream_dequeue() is called, so we assumed that > > NULL could never be returned? I looked but didn't find such a check, > > so maybe I didn't look carefully enough. But now we are assuming > > that Hyper-V might have invalidated that previous check by > > subsequently changing the write_index in a bogus way? So now, NULL > > could be returned when previously we assumed it couldn't. > > I think you're looking for hvs_stream_has_data(). (Previous checks > apart, hvs_stream_dequeue() will "dereference" the pointer so...) Agreed. I didn't say this explicitly, but I was wondering about the risk in the current code (without these hardening patches) of getting a NULL pointer from hv_pkt_iter_first_raw(), and then dereferencing it. Michael
