Re: usb: use-after-free write in usb_hcd_link_urb_to_ep

[Dmitry Vyukov]

On Thu, Mar 23, 2017 at 4:04 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 23 Mar 2017, Dmitry Vyukov wrote:
>
>> > Putting these together:
>> >
>> >         The memory was allocated in usb_internal_control_msg() line 93.
>> >         The later events occurred within the call in line 100 to
>> >         usb_start_wait_urb().
>> >
>> >         The invalid access occurred within usb_start_wait_urb() line 56.
>> >
>> >         The memory was deallocated within usb_start_wait_urb() line 78.
>> >
>> > Since these routines don't involve any loops or backward jumps, this
>> > says that the invalid access occurred before the memory was
>> > deallocated!  So why is it reported as a problem?
>>
>>
>> My first guess would be that pid 3348 did 2 calls to open and the urb
>> was somehow referenced across these calls. Is it possible?
>
> I don't think so.  The URB gets allocated and deallocated separately
> for each call.  You can see this very plainly by reading the source
> code for usb_internal_control_msg() and usb_start_wait_urb().
>
> It's possible that the same memory location was allocated and
> deallocated for two different calls at different times.  That wouldn't
> fool syzkaller, would it?


Generally it does not fool KASAN because of heap memory quarantine.
I will take a closer look tomorrow.
Thanks for looking into this.