linux-kernel - Re: [PATCH V2] USB: usbtmc: prevent kernel-usb-infoleak

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Message-ID: <608621b0-6a6b-46d3-bfa8-ff907fb83148@rowland.harvard.edu>
Date: Sat, 7 Sep 2024 10:45:52 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Edward Adam Davis <eadavis@...com>
Cc: gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org,
	syzbot+9d34f80f841e948c3fdb@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH V2] USB: usbtmc: prevent kernel-usb-infoleak

On Sat, Sep 07, 2024 at 10:08:57AM +0800, Edward Adam Davis wrote:
> On Fri, 6 Sep 2024 10:28:11 -0400, Alan Stern wrote:
> > On Fri, Sep 06, 2024 at 10:11:03PM +0800, Edward Adam Davis wrote:
> > > The syzbot reported a kernel-usb-infoleak in usbtmc_write.
> > > 
> > > The expression "aligned = (transfersize + (USBTMC_HEADER_SIZE + 3)) & ~3;"
> > > in usbtmcw_write() follows the following pattern:
> > > 
> > > aligned = (1 + 12 + 3) & ~3 = 16   // 3 bytes have not been initialized
> > > aligned = (2 + 12 + 3) & ~3 = 16   // 2 bytes have not been initialized
> > > aligned = (3 + 12 + 3) & ~3 = 16   // 1 byte has not been initialized
> > > aligned = (4 + 12 + 3) & ~3 = 16   // All bytes have been initialized
> > > aligned = (5 + 12 + 3) & ~3 = 20   // 3 bytes have not been initialized
> > > aligned = (6 + 12 + 3) & ~3 = 20   // 2 bytes have not been initialized
> > > aligned = (7 + 12 + 3) & ~3 = 20   // 1 byte has not been initialized
> > > aligned = (8 + 12 + 3) & ~3 = 20   // All bytes have been initialized
> > > aligned = (9 + 12 + 3) & ~3 = 24
> > > ...
> > 
> > What is the purpose of aligned?  Why doesn't the driver simply use
> > USBTMC_HEADER_SIZE + transfersize instead of rounding it up to a 
> > multiple of 4?
> I just found out that the logic of aligned calculation is like this. 
> As for why it is calculated like this, perhaps Guido Kiener can provide
> a clearer explanation.
> It was introduced by commit 4d5e18d9ed93 ("usb: usbtmc: Optimize usbtmc_write").
> > 
> > > Note: #define USBTMC_HEADER_SIZE      12
> > > 
> > > This results in the buffer[USBTMC_SEAD_SIZE+transfersize] and its
> > > subsequent memory not being initialized.
> > > 
> > > The condition aligned < buflen is used to avoid out of bounds access to
> > > the buffer[USBTMC_HEADER_SIZE + transfersize] when "transfersize = 
> > > buflen - USBTMC_HEADER_SIZE".
> > > 
> > > Fixes: 4ddc645f40e9 ("usb: usbtmc: Add ioctl for vendor specific write")
> > > Reported-and-tested-by: syzbot+9d34f80f841e948c3fdb@...kaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?extid=9d34f80f841e948c3fdb
> > > Signed-off-by: Edward Adam Davis <eadavis@...com>
> > > ---
> > >  drivers/usb/class/usbtmc.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > > 
> > > diff --git a/drivers/usb/class/usbtmc.c b/drivers/usb/class/usbtmc.c
> > > index 6bd9fe565385..faf8c5508997 100644
> > > --- a/drivers/usb/class/usbtmc.c
> > > +++ b/drivers/usb/class/usbtmc.c
> > > @@ -1591,6 +1591,10 @@ static ssize_t usbtmc_write(struct file *filp, const char __user *buf,
> > >  		goto exit;
> > >  	}
> > >  
> > > +	if (aligned < buflen && (transfersize % 4))
> > 
> > Shouldn't this be
> > 
> > 	if (USBTMC_HEADER_SIZE + transfersize < aligned)
> Logically, it seems possible to write it this way.

In fact, what you wrote is wrong.  Consider the case where buflen is 32 
and transfersize is 17.  Then aligned = (12 + 17 + 3) & ~3 = 32, so your 
condition would fail to initialize the extra 3 bytes.

Alan Stern