Re: Is the possible cross-talking between unrelated file-descriptors on bsg-device by design?

[Benjamin Block <bblock@linux.vnet.ibm.com>]

On Tue, Sep 19, 2017 at 02:16:26PM -0400, Douglas Gilbert wrote:
> On 2017-09-19 10:56 AM, Benjamin Block wrote:
> > Hello linux-block,
> > 
> > I wrote some tests recently to test patches against bsg.c and bsg-lib.c,
> > and while writing those I noticed something strange:
> > 
> > When you use the write() and read() call on multiple file-descriptors
> > for a single bsg-device (FC or SCSI), it is possible that you get
> > cross-talk between those different file-descriptors.
> > 
> > E.g.: You have two independent processes open a FD on /dev/bsg/fc_host0
> > and you send commands via write() in both processes, when they both do
> > read() later on - to read the response for the commands they send before
> > -, it is possible that process A gets the response (Sense-Data,
> > FC-Protocol-Data, ...) for a command that process B wrote and vis-a-vis.
> > 
> > I noticed this because in my tests I 'tag' each command I send with
> > write() via a value I write into the usr_ptr field of sg_io_v4. When I
> > later user read() to receive responses I check for this tag in a
> > hash-table and so can look up the original command. When I used this and
> > spawned multiple processes with the same target bsg-device, I got
> > responses for commands I don't have tags for in my hash-table, so they
> > were written by an other process. This never happend when I only spawn
> > one test-process.
> > 
> > This seems awfully contra-productive.. so much that I don't see any
> > point in allowing users to open more than one FD per bsg-device, because
> > that would inevitably lead to very hard to debug 'bugs'. And I wonder if
> > this is really by design or some regression that happend over time.
> > 
> > I looked at the code in bsg.c and yes, it seems this is what is coded
> > right now. We have a hash-table which manages all 'struct bsg_device's
> > who are indexed by device-minors, so one hash-table entry per
> > device-node.
> > 
> > So eh, long talk short question, is that intended?
> 
> Hi,
> About once a year I point out that major misfeature in the bsg driver
> and no-one seems to care. Most recently I pointed it out in a
> discussion about SCSI SG CVE-2017-0794 6 days ago:
>   " ... Last time I looked at the bsg driver, a SCSI command
>     could be issued on one file descriptor and its data-in block
>     and SCSI status delivered to another file descriptor to the
>     same block device (potentially in another process). IOW chaos"
> 
> It is hard to imagine any sane application relying on this bizarre
> behaviour so fixing it should not break any applications. My guess
> is that it would require a non-trivial patch set to fix. Would you
> like to volunteer?
> 

Interesting. So this is not a regression then.

Well, personally I am intrigued to try to fix this, but professionally
it is not really up to me to choose to work on this. Especially because
as you say, this looks not trivial - at least if you want to let
multiple applications open a FD for a single BSG device node. Lets see.

But thank you for elaborating on this!


                                                    Beste Gr��e / Best regards,
                                                      - Benjamin Block
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