Re: [PATCH V3] USB: usbtmc: prevent kernel-usb-infoleak

[Edward Adam Davis]

On Sun, 8 Sep 2024 07:20:40 +0200, Greg KH wrote:
> On Sun, Sep 08, 2024 at 10:20:57AM +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
> > ...
> >
> > Note: #define USBTMC_HEADER_SIZE      12
> >
> > This results in the buffer[USBTMC_SEAD_SIZE+transfersize] and its
> > subsequent memory not being initialized.
> >
> > 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>
> > ---
> > V2 -> V3: Update condition and comments
> >
> >  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 (USBTMC_HEADER_SIZE + transfersize < aligned)
> > +		memset(&buffer[USBTMC_HEADER_SIZE + transfersize], 0,
> > +			aligned - USBTMC_HEADER_SIZE - transfersize);
> 
> As this is now a pain to read/understand, and there's no comment
> describing it so we'll not really understand it in a few months, let
> alone years, how about we just do the trivial thing and make the
> allocation with kzalloc() to start with?  And put a comment there saying
> why it's zeroed out.
Perhaps I wrote too much in my comments, but in essence, the logic behind
this version's fix is:
When aligned is greater than (USBTMC_HEADER_SIZE+transfersize), there are
(aligned - (USBTMC_HEADER_SIZE+transfersize) bytes after the header and data
that have not been initialized, and these bytes are then set to 0.
> 
> Sorry, I thought this was going to be a lot simpler based on your first
> patch than this type of logic.
As you mentioned in my first version patch, this approach is simple and
easy to understand, but it comes at the cost of losing the real issue,
and KMSAN will not find similar problems again in the future, which is
not conducive to making the program logic more robust.

BR,
Edward