Re: [PATCH] iscsi-target: Reject immediate data underflow larger than SCSI transfer length

[Nicholas A. Bellinger]

Hi Bart,

On Thu, 2017-06-08 at 23:55 -0700, Nicholas A. Bellinger wrote:
> On Thu, 2017-06-08 at 15:37 +0000, Bart Van Assche wrote:
> > On Thu, 2017-06-08 at 04:21 +0000, Nicholas A. Bellinger wrote:
> > > +	/*
> > > +	 * Check for underflow case where both EDTL and immediate data payload
> > > +	 * exceeds what is presented by CDB's TRANSFER LENGTH, and what has
> > > +	 * already been set in target_cmd_size_check() as se_cmd->data_length.
> > > +	 *
> > > +	 * For this special case, fail the command and dump the immediate data
> > > +	 * payload.
> > > +	 */
> > > +	if (cmd->first_burst_len > cmd->se_cmd.data_length) {
> > > +		cmd->sense_reason = TCM_INVALID_CDB_FIELD;
> > > +		goto after_immediate_data;
> > > +	}
> > 
> > A quote from the iSCSI RFC (https://tools.ietf.org/html/rfc5048):
> > 
> >    If SPDTL < EDTL for a task, iSCSI Underflow MUST be signaled in the
> >    SCSI Response PDU as specified in [RFC3720].  The Residual Count MUST
> >    be set to the numerical value of (EDTL - SPDTL).
> > 
> > Sorry but I don't think that sending TCM_INVALID_CDB_FIELD back to the
> > initiator is compliant with the iSCSI RFC.
> 
> Alas, the nuance of what this patch actually does was missed when you
> cut the context.
> 
> First, a bit of history.  LIO has rejected underflow for all WRITEs for
> the first ~12.5 years of RFC-3720, and in the context of iscsi-target
> mode there has never been a single host environment that ever once
> cared.
> 
> Since Roland's patch to allow underflow for control CDBs in v4.3+ opened
> this discussion for control CDBs with a WRITE payload in order to make
> MSFT/FCP cert for PERSISTENT_RESERVE_OUT happy, the question has become
> what control CDB WRITE underflow cases should we allow..?
> 
> The point with this patch is when a host is sending a underflow with a
> iscsi immediate data payload that exceeds SCSI transfer length, it's a
> bogus request with a garbage payload.  It's a garbage payload because
> the SCSI CDB itself obviously doesn't want anything to do it.
> 
> I'm very dubious of any host environment who's trying to do this for any
> CDB, and expects achieve expected results.
> 
> Of course, since v4.3+ normal overflow where SCSI transfer length
> matches the iscsi immediate data payload just works with or without this
> patch.
> 
> So to that extent, I'm going to push this patch as a defensive fix for
> v4.3+, to let those imaginary iscsi host environments know they being
> very, very naughty.
> 
> >  Please note that a fix that is
> > compliant with the iSCSI RFC is present in the following patch series: [PATCH
> > 00/33] SCSI target driver patches for kernel v4.13, 23 May 2017
> > (https://www.spinics.net/lists/target-devel/msg15370.html).
> 
> So I might still consider this as a v4.13-rc item for control CDB
> underflow, but no way as stable material.
> 
> Also, there is certainly no way I'm going to allow a patch to randomly
> enable underflow/overflow for all WRITE non control CDBs tree-wide
> across all fabric drivers, because 1) no host environments actually care
> about this, and 2) it's still dangerous to do for all fabrics without
> some serious auditing.

After further consideration, I've decided against allowing iscsi-target
underflow with a immediate data payload larger than SCSI transfer
length.

Any host environment that attempts to send an underflow with a immediate
data payload larger than SCSI transfer length, expects the target to
automatically truncate immediate data, and still return GOOD status is
completely bogus.  Any host that attempts this is buggy, and needs to be
fixed.

This is because for the last ~12 years of RFC-3720:

  - There has never been a host environment in the wild that exhibits 
    this behavior.
  - There has never been a conformance suite which expects this 
    behavior.

So rejecting this case as already done in commit abb85a9b51 is the
correct approach for >= v4.3.y.

Of course, the typical underflow scenario which Roland's v4.3.y commit
enabled, underflow where immediate data matches the SCSI transfer length
is supported for control CDBs.

That said, thanks for high-lighting this particular corner case, so it
could be fixed in >= v4.3.y.