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

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Message-ID: <Pine.LNX.4.44L0.1705021014500.1758-100000@iolanthe.rowland.org>
Date:   Tue, 2 May 2017 10:19:20 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
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

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.

Alan Stern