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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Closes: https://syzkaller.appspot.com/bug?extid=9d34f80f841e948c3fdb
> > > Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> > > ---
> > > 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
