Messages in this thread | | | Subject | Re: [PATCH v2] nvme-tcp: Check if request has started before processing it | From | Sagi Grimberg <> | Date | Wed, 31 Mar 2021 15:37:22 -0700 |
| |
>>>> 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?
| |