Subject: Re: [PATCH net 1/1] net: dsa: mv88e6xxx: Fix out-of-bound access

From: Simon Horman <[email-masked]>
Date: Wed, 21 Aug 2024 15:13:20 +0100

On Tue, Aug 20, 2024 at 03:21:57PM -0400, Joseph Huang wrote:
> On 8/20/2024 2:32 PM, Simon Horman wrote:
> > On Tue, Aug 20, 2024 at 12:58:05AM +0200, Andrew Lunn wrote:
> > > On Mon, Aug 19, 2024 at 06:26:40PM -0400, Joseph Huang wrote:
> > > > If an ATU violation was caused by a CPU Load operation, the SPID is 0xf,
> > > > which is larger than DSA_MAX_PORTS (the size of mv88e6xxx_chip.ports[]
> > > > array).
> > > 
> > > The 6390X datasheet says "IF SPID = 0x1f the source of the violation
> > > was the CPU's registers interface."
> > > 
> > > > +#define MV88E6XXX_G1_ATU_DATA_SPID_CPU				0x000f
> > > 
> > > So it seems to depend on the family.
> > > 
> > > >  >  /* Offset 0x0D: ATU MAC Address Register Bytes 0 & 1
> > > >   * Offset 0x0E: ATU MAC Address Register Bytes 2 & 3
> > > > diff --git a/drivers/net/dsa/mv88e6xxx/global1_atu.c b/drivers/net/dsa/mv88e6xxx/global1_atu.c
> > > > index ce3b3690c3c0..b6f15ae22c20 100644
> > > > --- a/drivers/net/dsa/mv88e6xxx/global1_atu.c
> > > > +++ b/drivers/net/dsa/mv88e6xxx/global1_atu.c
> > > > @@ -457,7 +457,8 @@ static irqreturn_t mv88e6xxx_g1_atu_prob_irq_thread_fn(int irq, void *dev_id)
> > > >  		trace_mv88e6xxx_atu_full_violation(chip->dev, spid,
> > > >  						   entry.portvec, entry.mac,
> > > >  						   fid);
> > > > -		chip->ports[spid].atu_full_violation++;
> > > > +		if (spid != MV88E6XXX_G1_ATU_DATA_SPID_CPU)
> > > > +			chip->ports[spid].atu_full_violation++;
> > > 
> > > So i think it would be better to do something like:
> > > 
> > > 		if (spid < ARRAY_SIZE(chip->ports))
> > > 			chip->ports[spid].atu_full_violation++;
> > 
> > Hi Joseph,
> > 
> > I am curious to know if bounds checking should also
> > be added to other accesses to chip->ports[spid] within this function.
> > 
> 
> Hi Simon,
> 
> From the spec it is unclear to me whether the Load operation could actually
> cause other exceptions. I was only able to reproduce and verify the full
> violation, and that's why I only included that one in the patch.
> 
> I guess we could proactively include the fix for other exceptions as well,
> but without a way to verify them, they could be just dead code and never be
> exercised. Perhaps people who are more familiar with the chip than me could
> chime in. I'm fine either way.

Thanks Joseph,

>From my PoV it would be nice to add the checks unless we can be sure they
are not needed. But I do not feel strongly about this.