Re: [PATCH v3 1/2] remoteproc: qcom_q6v5_mss: Don't reassign mpss region on shutdown

[Bjorn Andersson]

On Mon 10 Feb 15:05 PST 2020, Mathieu Poirier wrote:

> Hi Bjorn,
> 
> On Mon, Feb 03, 2020 at 10:26:40PM -0800, Bjorn Andersson wrote:
> > Trying to reclaim mpss memory while the mba is not running causes the
> > system to crash on devices with security fuses blown, so leave it
> > assigned to the remote on shutdown and recover it on a subsequent boot.
> > 
> > Fixes: 6c5a9dc2481b ("remoteproc: qcom: Make secure world call for mem ownership switch")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
> > ---
> > 
> > Changes since v2:
> > - The assignment of mpss memory back to Linux is rejected in the coredump case
> >   on production devices, so check the return value of q6v5_xfer_mem_ownership()
> >   before attempting to memcpy() the data.
> > 
> >  drivers/remoteproc/qcom_q6v5_mss.c | 23 +++++++++++++----------
> >  1 file changed, 13 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/remoteproc/qcom_q6v5_mss.c b/drivers/remoteproc/qcom_q6v5_mss.c
> > index 471128a2e723..25c03a26bf88 100644
> > --- a/drivers/remoteproc/qcom_q6v5_mss.c
> > +++ b/drivers/remoteproc/qcom_q6v5_mss.c
> > @@ -887,11 +887,6 @@ static void q6v5_mba_reclaim(struct q6v5 *qproc)
> >  		writel(val, qproc->reg_base + QDSP6SS_PWR_CTL_REG);
> >  	}
> >  
> > -	ret = q6v5_xfer_mem_ownership(qproc, &qproc->mpss_perm,
> > -				      false, qproc->mpss_phys,
> > -				      qproc->mpss_size);
> > -	WARN_ON(ret);
> > -
> >  	q6v5_reset_assert(qproc);
> >  
> >  	q6v5_clk_disable(qproc->dev, qproc->reset_clks,
> > @@ -981,6 +976,10 @@ static int q6v5_mpss_load(struct q6v5 *qproc)
> >  			max_addr = ALIGN(phdr->p_paddr + phdr->p_memsz, SZ_4K);
> >  	}
> >  
> > +	/* Try to reset ownership back to Linux */
> > +	q6v5_xfer_mem_ownership(qproc, &qproc->mpss_perm, false,
> > +				qproc->mpss_phys, qproc->mpss_size);
> 
> Would you mind adding more chatter here, something like what is mentioned in the
> changelog?  That way I anyone trying to review this code doesn't have to suffer
> through the same mental gymnastic. 
> 

Sure thing, as this patch shows this dynamic wasn't clear - and this
patch is based on my observations. With it we no longer crash the entire
system by hitting a security violation during a crash, but there's still
some details that I'm uncertain about.

> > +
> >  	mpss_reloc = relocate ? min_addr : qproc->mpss_phys;
> >  	qproc->mpss_reloc = mpss_reloc;
> >  	/* Load firmware segments */
> > @@ -1070,8 +1069,16 @@ static void qcom_q6v5_dump_segment(struct rproc *rproc,
> >  	void *ptr = rproc_da_to_va(rproc, segment->da, segment->size);
> >  
> >  	/* Unlock mba before copying segments */
> > -	if (!qproc->dump_mba_loaded)
> > +	if (!qproc->dump_mba_loaded) {
> >  		ret = q6v5_mba_load(qproc);
> > +		if (!ret) {
> > +			/* Try to reset ownership back to Linux */
> > +			ret = q6v5_xfer_mem_ownership(qproc, &qproc->mpss_perm,
> > +						      false,
> > +						      qproc->mpss_phys,
> > +						      qproc->mpss_size);
> > +		}
> 
> I'm a bit puzzled here as to why a different reclaim strategy is needed.  It is
> clear to me that if q6v5_mba_load() returns 0 then the MBA is running and we can
> safely reclaim ownership of the memory.  But is it absolutely needed when we
> know that 1) the MCU has crashed and 2) said memory will be reclaimed in
> q6v5_mpss_load()?
> 

The ownership transfer here is a jump into secure world, which somehow
together with the firmware running on the modem processor will update
the access permissions for the mpss memory region.

As we enter this function the recovery handling in the core has just
stopped the remote processor, so we know it's off. As such we must first
boot the remote processor again, in order to reclaim the access to the
mpss memory region.

New in this revision is the fact that this operation might actually be
rejected (e.g. on end-user hardware).

So we need to guard the memcpy below on either of these cases, as unless
we've successfully booted the modem processor and gotten permission to
read the mpss memory this operation will trigger a security violation
and the device will reboot.

> If so I think an explanation in the code is needed.
> 

Makes sense, I will formulate above explanation into a comment. As well
as review the other callers of q6v5_xfer_mem_ownership().

> I also assume there is no way to know if the mba is running, hence not taking
> any chance.  If that's the case it would be nice to add that to the comment in
> q6v5_mpss_load().
> 

We know that we enter q6v5_mpss_load() with the modem processor just
booted, but the memory assignment is there to handle the case where the
mpss memory region for some reason was left in the hands on the modem.
I will have to do some more digging to figure out if this is a valid
scenario or not.

Thanks for your review Mathieu!

Regards,
Bjorn

> Thanks,
> Mathieu
> 
> > +	}
> >  
> >  	if (!ptr || ret)
> >  		memset(dest, 0xff, segment->size);
> > @@ -1123,10 +1130,6 @@ static int q6v5_start(struct rproc *rproc)
> >  	return 0;
> >  
> >  reclaim_mpss:
> > -	xfermemop_ret = q6v5_xfer_mem_ownership(qproc, &qproc->mpss_perm,
> > -						false, qproc->mpss_phys,
> > -						qproc->mpss_size);
> > -	WARN_ON(xfermemop_ret);
> >  	q6v5_mba_reclaim(qproc);
> >  
> >  	return ret;
> > -- 
> > 2.23.0
> >