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

[Dmitry Vyukov]

On Thu, Mar 23, 2017 at 3:34 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 23 Mar 2017, Dmitry Vyukov wrote:
>
>> Hello,
>>
>> I've got the following report while running syzkaller fuzzer on
>> 093b995e3b55a0ae0670226ddfcb05bfbf0099ae. Not the preceding injected
>> kmalloc failure, most likely it's the root cause.
>
> I find this bug report puzzling.  Maybe I don't understand it
> correctly -- it appears that the so-called use-after-free actually
> occurs _before_ the memory is deallocated!
>
>> FAULT_INJECTION: forcing a failure.
> Skipping this part.  Is it relevant?  It seems to refer to a different
> memory buffer.
>
>> ==================================================================
>> BUG: KASAN: use-after-free in __list_add_valid+0xc6/0xd0
>> lib/list_debug.c:26 at addr ffff88003c377a20
>> Read of size 8 by task syz-executor7/3348
>> CPU: 3 PID: 3348 Comm: syz-executor7 Not tainted 4.11.0-rc3+ #364
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>> Call Trace:
>
> Here are the revelant pieces of the stack traces.  Everything below
> these parts is the same, and everything above them is unimportant.
> (And everything happened in the same process.)  The use-after-free
> access occurred within this call:
>
>>  usb_start_wait_urb+0x135/0x320 drivers/usb/core/message.c:56
>>  usb_internal_control_msg drivers/usb/core/message.c:100 [inline]
>
>
> Here's where the allocation call occurred:
>
>> Allocated:
>> PID = 3348
> ...
>>  usb_internal_control_msg drivers/usb/core/message.c:93 [inline]
>
>
> And here's where the buffer was deallocated:
>
>> Freed:
>> PID = 3348
> ...
>>  usb_start_wait_urb+0x234/0x320 drivers/usb/core/message.c:78
>>  usb_internal_control_msg drivers/usb/core/message.c:100 [inline]
>
> 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?