Re: [PATCH v2] nvme-tcp: Check if request has started before processing it

[Sagi Grimberg]

> > > > It is, but in this situation, the controller is sending a second
> > > > completion that results in a use-after-free, which makes the
> > > > transport irrelevant. Unless there is some other flow (which is
> > > > unclear
> > > > to me) that causes this which is a bug that needs to be fixed rather
> > > > than hidden with a safeguard.
> > >
> > > The kernel should not crash regardless of any network traffic that is
> > > sent to the system.  It should not be possible to either intentionally
> > > of mistakenly contruct packets that will deny service in this way.
> >
> > This is not specific to nvme-tcp. I can build an rdma or pci controller
> > that can trigger the same crash... I saw a similar patch from Hannes
> > implemented in the scsi level, and not the individual scsi transports..
>
> If scsi wants this too, this could be made generic at the blk-mq level.
> We just need to make something like blk_mq_tag_to_rq(), but return NULL
> if the request isn't started.

Makes sense...

> > I would also mention, that a crash is not even the scariest issue that
> > we can see here, because if the request happened to be reused we are
> > in the silent data corruption realm...
>
> If this does happen, I think we have to come up with some way to
> mitigate it. We're not utilizing the full 16 bits of the command_id, so
> maybe we can append something like a generation sequence number that can
> be checked for validity.

That's actually a great idea. scsi needs unique tags so it encodes the
hwq in the upper 16 bits giving the actual tag the lower 16 bits which
is more than enough for a single queue. We can do the same with
a gencnt that will increment in both submission and completion and we
can validate against it.

This will be useful for all transports, so maintaining it in
nvme_req(rq)->genctr and introducing a helper like:
rq = nvme_find_tag(tagset, cqe->command_id)
That will filter genctr, locate the request.

Also:
nvme_validate_request_gen(rq, cqe->command_id) that would
compare against it.


And then a helper to set the command_id like:
cmd->common.command_id = nvme_request_command_id(rq)
that will both increment the genctr and build a command_id
from it.

Thoughts?