Re: [Qemu-devel] [PATCH] usb/xchi: avoid trigger assertion if guest write wrong epid

[Longpeng (Mike)]

Hi Philippe,

On 2019/4/29 19:16, Philippe Mathieu-Daudé wrote:

> Hi Mike,
> 
> On 4/29/19 9:39 AM, Longpeng(Mike) wrote:
>> From: Longpeng <address@hidden>
>>
>> we found the following core in our environment:
>> 0  0x00007fc6b06c2237 in raise ()
>> 1  0x00007fc6b06c3928 in abort ()
>> 2  0x00007fc6b06bb056 in __assert_fail_base ()
>> 3  0x00007fc6b06bb102 in __assert_fail ()
>> 4  0x0000000000702e36 in xhci_kick_ep (...)
> 
>   5 xhci_doorbell_write?
> 
>> 6  0x000000000047767f in access_with_adjusted_size (...)
>> 7  0x000000000047944d in memory_region_dispatch_write (...)
>> 8  0x000000000042df17 in address_space_write_continue (...)
>> 10 0x000000000043084d in address_space_rw (...)
>> 11 0x000000000047451b in kvm_cpu_exec (address@hidden)
>> 12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
>> 13 0x0000000000870631 in qemu_thread_start (address@hidden)
>> 14 0x00000000008959a7 in thread_entry_for_hotfix (pthread_cb=<optimized out>)
>> 15 0x00007fc6b0a60dd5 in start_thread ()
>> 16 0x00007fc6b078a59d in clone ()
>> (gdb) bt
>> (gdb) f 5
> 
> This is the frame you removed...
> 
>> (gdb) p /x tmp
>> $9 = 0x62481a00 <-- last byte 0x00 is @epid
> 
> I don't see 'tmp' in xhci_doorbell_write().
> 
> Can you use trace events?
> 
> There we have trace_usb_xhci_doorbell_write().
> 

Sorry , I'm careless to remove the important information.


This is our whole frame:

(gdb) bt
#0  0x00007fc6b06c2237 in raise () from /usr/lib64/libc.so.6
#1  0x00007fc6b06c3928 in abort () from /usr/lib64/libc.so.6
#2  0x00007fc6b06bb056 in __assert_fail_base () from /usr/lib64/libc.so.6
#3  0x00007fc6b06bb102 in __assert_fail () from /usr/lib64/libc.so.6
#4  0x0000000000702e36 in xhci_kick_ep (...)
#5  0x000000000047897a in memory_region_write_accessor (...)
#6  0x000000000047767f in access_with_adjusted_size (...)
#7  0x000000000047944d in memory_region_dispatch_write
(address@hidden, address@hidden, data=1648892416,
address@hidden, address@hidden)
#8  0x000000000042df17 in address_space_write_continue (...)
#9  0x00000000004302d5 in address_space_write (...)
#10 0x000000000043084d in address_space_rw (...)
#11 0x000000000047451b in kvm_cpu_exec (...)
#12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
#13 0x0000000000870631 in qemu_thread_start (address@hidden)
#14 0x00000000008959a7 in thread_entry_for_hotfix (pthread_cb=<optimized out>)
#15 0x00007fc6b0a60dd5 in start_thread () from /usr/lib64/libpthread.so.0
#16 0x00007fc6b078a59d in clone () from /usr/lib64/libc.so.6

(gdb) f 5
#5  0x000000000047897a in memory_region_write_accessor (...)
529         mr->ops->write(mr->opaque, addr, tmp, size);
(gdb) p /x tmp
$9 = 0x62481a00


static void xhci_doorbell_write(void *ptr, hwaddr reg,
                                uint64_t val, unsigned size)
So, the @val is 0x62481a00, and the last byte is epid, right?

>>
>> xhci_doorbell_write() already check the upper bound of @slotid an @epid,
>> it also need to check the lower bound.
>>
>> Cc: Gonglei <address@hidden>
>> Signed-off-by: Longpeng <address@hidden>
>> ---
>>  hw/usb/hcd-xhci.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/hw/usb/hcd-xhci.c b/hw/usb/hcd-xhci.c
>> index ec28bee..b4e6bfc 100644
>> --- a/hw/usb/hcd-xhci.c
>> +++ b/hw/usb/hcd-xhci.c
>> @@ -3135,9 +3135,9 @@ static void xhci_doorbell_write(void *ptr, hwaddr reg,
> 
> Expanding the diff:
> 
>        if (reg == 0) {
>            if (val == 0) {
>                xhci_process_commands(xhci);
>            } else {
>                DPRINTF("xhci: bad doorbell 0 write: 0x%x\n",
>                        (uint32_t)val);
>            }
>>      } else {
>>          epid = val & 0xff;
>>          streamid = (val >> 16) & 0xffff;
>> -        if (reg > xhci->numslots) {
>> +        if (reg == 0 || reg > xhci->numslots) {
> 
> So 'reg' can not be zero here...
> 

Oh, you're right.

>>              DPRINTF("xhci: bad doorbell %d\n", (int)reg);
>> -        } else if (epid > 31) {
>> +        } else if (epid == 0 || epid > 31) {
> 
> Here neither.
> 

In our frame, the epid is zero. The @val is from guest which is untrusted, when
this problem happened, I saw it wrote many invalid value, not only usb but also
other devices.

>>              DPRINTF("xhci: bad doorbell %d write: 0x%x\n",
>>                      (int)reg, (uint32_t)val);
>>          } else {
>>
> 
> Isn't it the other line that triggered the assertion?
> 
> static void xhci_kick_ep(XHCIState *xhci, unsigned int slotid,
>                          unsigned int epid, unsigned int streamid)
> {
>     XHCIEPContext *epctx;
> 
>     assert(slotid >= 1 && slotid <= xhci->numslots); // <===
>     assert(epid >= 1 && epid <= 31);
> 
> Regards,
> 
> Phil.
> 
> 
> 


-- 
Regards,
Longpeng(Mike)