linux-kernel - Re: [PATCH 04/22] firmware: arm_scmi: Validate BASE_DISCOVER_LIST

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Date:   Thu, 28 Apr 2022 14:55:04 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Cristian Marussi <cristian.marussi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        james.quinlan@...adcom.com, Jonathan.Cameron@...wei.com,
        f.fainelli@...il.com, etienne.carriere@...aro.org,
        vincent.guittot@...aro.org, souvik.chakravarty@....com
Subject: Re: [PATCH 04/22] firmware: arm_scmi: Validate
 BASE_DISCOVER_LIST_PROTOCOLS reply

On Thu, Apr 28, 2022 at 02:45:07PM +0100, Cristian Marussi wrote:
> On Thu, Apr 28, 2022 at 11:07:29AM +0100, Sudeep Holla wrote:
> > On Wed, Mar 30, 2022 at 04:05:33PM +0100, Cristian Marussi wrote:
> > > Do not blindly trust SCMI backend server reply about list of implemented
> > > protocols, instead validate the reported length of the list of protocols
> > > against the real payload size of the message reply.
> > >
> > > Fixes: b6f20ff8bd9 ("firmware: arm_scmi: add common infrastructure and support for base protocol")
> > > Signed-off-by: Cristian Marussi <cristian.marussi@....com>
> > > ---
> > >  drivers/firmware/arm_scmi/base.c | 21 +++++++++++++++++++++
> > >  1 file changed, 21 insertions(+)
> > >
> > > diff --git a/drivers/firmware/arm_scmi/base.c b/drivers/firmware/arm_scmi/base.c
> > > index f279146f8110..c1165d1282ef 100644
> > > --- a/drivers/firmware/arm_scmi/base.c
> > > +++ b/drivers/firmware/arm_scmi/base.c
> > > @@ -189,6 +189,9 @@ scmi_base_implementation_list_get(const struct scmi_protocol_handle *ph,
> > >  	list = t->rx.buf + sizeof(*num_ret);
> > >
> > >  	do {
> > > +		size_t real_list_sz;
> > > +		u32 calc_list_sz;
> > > +
> > >  		/* Set the number of protocols to be skipped/already read */
> > >  		*num_skip = cpu_to_le32(tot_num_ret);
> > >
> > > @@ -202,6 +205,24 @@ scmi_base_implementation_list_get(const struct scmi_protocol_handle *ph,
> > >  			break;
> > >  		}
> > >
> > > +		if (t->rx.len < (sizeof(u32) * 2)) {
> > > +			dev_err(dev, "Truncated reply - rx.len:%zd\n",
> > > +				t->rx.len);
> > > +			ret = -EPROTO;
> > > +			break;
> > > +		}
> > > +
> > > +		real_list_sz = t->rx.len - sizeof(u32);
> > > +		calc_list_sz = ((loop_num_ret / sizeof(u32)) +
> > > +				!!(loop_num_ret % sizeof(u32))) * sizeof(u32);
> > 
> > Any reason this can't be (loop_num_ret - 1) / sizeof(u32) + 1 ?
> > 
> 
> At first sight could be fine with your easier version BUT what if loop_num_ret
> is returned as zero ?
> 
> real_list_sz should be ZERO length and calc_list_sz
> 
> im my version:
> 
> calc_list_sz = ((0/4) +!!(0%4)) * 4   ===>> 0
> 
> while in the simplified one gets calculated wrong:
> 
> calc_list_sz = (0-1)/4 + 1 ====> 1
> 
> ...moreover being both loop_num_ret and calc_list_sz unsigned I am even
> not so sure about implicit casting messing things up evenm more :D
> 
> So I sticked to the more convoluted approach :D
> 
> ....Have I missed something else ?
>

Right, but shouldn't we break if it 0 much earlier. It must not happen with
your new logic and even if it does, wouldn't it be better to break earlier ?

-- 
Regards,
Sudeep