Subject: Re: [PATCH] net: chelsio: cxgb4: Avoid potential negative array offset

From: Kees Cook <[email-masked]>
Date: Thu, 5 May 2022 16:21:46 -0700

On Wed, May 04, 2022 at 08:13:58PM -0700, Jakub Kicinski wrote:
> On Tue,  3 May 2022 07:44:25 -0700 Kees Cook wrote:
> > diff --git a/drivers/net/ethernet/chelsio/cxgb4/t4_hw.c b/drivers/net/ethernet/chelsio/cxgb4/t4_hw.c
> > index e7b4e3ed056c..f119ec7323e5 100644
> > --- a/drivers/net/ethernet/chelsio/cxgb4/t4_hw.c
> > +++ b/drivers/net/ethernet/chelsio/cxgb4/t4_hw.c
> > @@ -2793,14 +2793,14 @@ int t4_get_raw_vpd_params(struct adapter *adapter, struct vpd_params *p)
> >  		goto out;
> >  	na = ret;
> >  
> > -	memcpy(p->id, vpd + id, min_t(int, id_len, ID_LEN));
> > +	memcpy(p->id, vpd + id, clamp_t(int, id_len, 0, ID_LEN));
> 
> The typing is needed because of the enum, right? The variable is
> unsigned, seems a little strange to use clamp(int, ..., 0, constant)
> min(unsigned int, ..., constant) will be equivalent with fewer branches.
> Is it just me?

I just chased down the origin of "unsigned int", but it's actually a
u16 out of the VPD:

static u16 pci_vpd_lrdt_size(const u8 *lrdt)
{
        return get_unaligned_le16(lrdt + 1);
}

static int pci_vpd_find_tag(const u8 *buf, unsigned int len, u8 rdt, unsigned int *size)
{
	...
                unsigned int lrdt_len = pci_vpd_lrdt_size(buf + i);
	...
                                *size = lrdt_len;

I'm not sure why it was expanded to unsigned int size, maybe in other
call sites it was easier to deal with for possible math, etc?

Anyway, doesn't need changing. I'll send the int/unsigned int shortly...

-- 
Kees Cook