linux-kernel - Re: [usb-host] question about pointer dereference before null check

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Message-ID: <20170502101358.Horde.vPk-vIgmhyJophIrWpgQP4k@gator4166.hostgator.com>
Date:   Tue, 02 May 2017 10:13:58 -0500
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Senna Tschudin <peter.senna@...il.com>
Subject: Re: [usb-host] question about pointer dereference before null check

Hi Alan,

Quoting Alan Stern <stern@...land.harvard.edu>:

> On Mon, 1 May 2017, Gustavo A. R. Silva wrote:
>
>> Hello everybody,
>>
>> While taking a look into Coverity ID 100828 I ran into the following
>> piece of code at drivers/usb/host/ehci-sched.c:1096:
>>
>> u32             addr;
>> int             think_time;
>> int             hs_transfers;
>>
>> addr = dev->ttport << 24;
>> if (!ehci_is_TDI(ehci)
>>                  || (dev->tt->hub !=
>>                          ehci_to_hcd(ehci)->self.root_hub))
>>          addr |= dev->tt->hub->devnum << 16;
>> addr |= epnum << 8;
>> addr |= dev->devnum;
>> stream->ps.usecs = HS_USECS_ISO(maxp);
>> think_time = dev->tt ? dev->tt->think_time : 0;
>>
>> The issue here is that dev->tt is being dereferenced before null check.
>>
>> I was thinking on placing think_time = dev->tt ? dev->tt->think_time :
>> 0; just before the _if_ statement. But this doesn't address the
>> problem of dev->tt actually being NULL.
>>
>> While looking into the callers of the function containing this piece
>> of code (iso_stream_init()) my impression is that dev->tt is not NULL
>> at the time this function is called and, a very simple patch like the
>> following can be applied in order to avoid the Coverity issue:
>>
>> -think_time = dev->tt ? dev->tt->think_time : 0;
>> +think_time = dev->tt->think_time;
>>
>> But I can't tell for sure, so in case dev->tt is NULL, a good strategy
>> to properly update the _addr_ variable would be needed.
>>
>> What do you think?
>>
>> I would really appreciate any comment on this,
>> Thank you!
>
> You are right; udev->tt cannot ever be NULL when this section of code
> runs.  The test should be removed.
>

Thanks for confirming, I'll send a patch shortly.
--
Gustavo A. R. Silva