usb/xchi: avoid trigger assertion if guest write wrong epid

Commit Message

Longpeng(Mike) April 29, 2019, 7:39 a.m. UTC

From: Longpeng <[e-mail]>

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 (...)
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 (cpu=cpu@entry=0x1ab11b0)
12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
13 0x0000000000870631 in qemu_thread_start (args=args@entry=0x1acfb50)
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
(gdb) p /x tmp
$9 = 0x62481a00 <-- last byte 0x00 is @epid

xhci_doorbell_write() already check the upper bound of @slotid an @epid,
it also need to check the lower bound.

Cc: Gonglei <[e-mail]>
Signed-off-by: Longpeng <[e-mail]>
---
 hw/usb/hcd-xhci.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Philippe Mathieu-Daudé April 29, 2019, 11:16 a.m. UTC | #1

Hi Mike,

On 4/29/19 9:39 AM, Longpeng(Mike) wrote:
> From: Longpeng <[e-mail]>
> 
> 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 (cpu=cpu@entry=0x1ab11b0)
> 12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
> 13 0x0000000000870631 in qemu_thread_start (args=args@entry=0x1acfb50)
> 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().

> 
> xhci_doorbell_write() already check the upper bound of @slotid an @epid,
> it also need to check the lower bound.
> 
> Cc: Gonglei <[e-mail]>
> Signed-off-by: Longpeng <[e-mail]>
> ---
>  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...

>              DPRINTF("xhci: bad doorbell %d\n", (int)reg);
> -        } else if (epid > 31) {
> +        } else if (epid == 0 || epid > 31) {

Here neither.

>              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.

Longpeng(Mike) April 29, 2019, 11:42 a.m. UTC | #2

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 <[e-mail]>
>>
>> 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 (cpu=cpu@entry=0x1ab11b0)
>> 12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
>> 13 0x0000000000870631 in qemu_thread_start (args=args@entry=0x1acfb50)
>> 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
(mr=mr@entry=0x7fc6a0138df0, addr=addr@entry=156, data=1648892416,
size=size@entry=4, attrs=attrs@entry=...)
#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 (args=args@entry=0x1acfb50)
#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 <[e-mail]>
>> Signed-off-by: Longpeng <[e-mail]>
>> ---
>>  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.
> 
> 
>

Philippe Mathieu-Daudé April 29, 2019, 12:10 p.m. UTC | #3

On 4/29/19 1:42 PM, Longpeng (Mike) wrote:
> 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 <[e-mail]>
>>>
>>> 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 (cpu=cpu@entry=0x1ab11b0)
>>> 12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
>>> 13 0x0000000000870631 in qemu_thread_start (args=args@entry=0x1acfb50)
>>> 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
> (mr=mr@entry=0x7fc6a0138df0, addr=addr@entry=156, data=1648892416,
> size=size@entry=4, attrs=attrs@entry=...)

So this is a 32-bit access, to address 156 (which is the slotid) and
data=1648892416=0x62481a00 indeed.

But watch out access_with_adjusted_size() calls adjust_endianness()...

> #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 (args=args@entry=0x1acfb50)
> #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

... since memory_region_write_accessor() has the same argument, then I
can assume your guest is running in Little-Endian.

> 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 <[e-mail]>
>>> Signed-off-by: Longpeng <[e-mail]>
>>> ---
>>>  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.

If you use mainstream QEMU, we have:

static void qemu_xhci_instance_init(Object *obj)
{
    ...
    xhci->numslots = MAXSLOTS;
    ...
}

$ git grep define.*MAXSLOTS
hw/usb/hcd-xhci.c:52:#define LEN_DOORBELL    ((MAXSLOTS + 1) * 0x20)
hw/usb/hcd-xhci.h:33:#define MAXSLOTS 64
hw/usb/hcd-xhci.h:37:#define EV_QUEUE (((3 * 24) + 16) * MAXSLOTS)

> 
>>>              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); // <===

slotid >= 1 // true
slotid <= xhci->numslots // FALSE (156 > 64)

So this assertion won't pass.

>>     assert(epid >= 1 && epid <= 31);
>>
>> Regards,
>>
>> Phil.
>>

[e-mail] April 29, 2019, 3:05 p.m. UTC | #4

Patchew URL: https://patchew.org/QEMU/[e-mail]/



Hi,

This series failed the docker-mingw@fedora build test. Please find the testing commands and
their output below. If you have Docker installed, you can probably reproduce it
locally.

=== TEST SCRIPT BEGIN ===
#!/bin/bash
time make docker-test-mingw@fedora SHOW_ENV=1 J=14 NETWORK=1
=== TEST SCRIPT END ===




The full log is available at
http://patchew.org/logs/[e-mail]/testing.docker-mingw@fedora/?type=message.
---
Email generated automatically by Patchew [https://patchew.org/].
Please send your feedback to [e-mail]

Philippe Mathieu-Daudé April 29, 2019, 3:10 p.m. UTC | #5

Hi Paolo, Fam.

On 4/29/19 5:05 PM, [e-mail] wrote:
> Patchew URL: https://patchew.org/QEMU/[e-mail]/
[...]
> This series failed the docker-mingw@fedora build test. Please find the testing commands and
> their output below. If you have Docker installed, you can probably reproduce it
> locally.
[...]
> The full log is available at
> http://patchew.org/logs/[e-mail]/testing.docker-mingw@fedora/?type=message.

Weird patchew error:

Updating 3c8cf5a9c21ff8782164d1def7f44bd888713384
From https://github.com/patchew-project/qemu
 - [tag update]
patchew/[e-mail] ->
patchew/[e-mail]
 * [new tag]
patchew/[e-mail] ->
patchew/[e-mail]
[...]
error: rev-list died of signal 9
fatal: remote did not send all necessary objects
fatal: The remote end hung up unexpectedly

Traceback (most recent call last):
  File "patchew-tester/src/patchew-cli", line 521, in test_one
    git_clone_repo(clone, r["repo"], r["head"], logf, True)
  File "patchew-tester/src/patchew-cli", line 53, in git_clone_repo
    subprocess.check_call(clone_cmd, stderr=logf, stdout=logf)
  File "/usr/lib64/python3.4/subprocess.py", line 558, in check_call
    raise CalledProcessError(retcode, cmd)

[e-mail] April 29, 2019, 5:58 p.m. UTC | #6

Patchew URL: https://patchew.org/QEMU/[e-mail]/



Hi,

This series failed the asan build test. Please find the testing commands and
their output below. If you have Docker installed, you can probably reproduce it
locally.

=== TEST SCRIPT BEGIN ===
#!/bin/bash
time make docker-test-debug@fedora TARGET_LIST=x86_64-softmmu J=14 NETWORK=1
=== TEST SCRIPT END ===




The full log is available at
http://patchew.org/logs/[e-mail]/testing.asan/?type=message.
---
Email generated automatically by Patchew [https://patchew.org/].
Please send your feedback to [e-mail]

Longpeng(Mike) April 30, 2019, 2:02 a.m. UTC | #7

On 2019/4/29 20:10, Philippe Mathieu-Daudé wrote:

> On 4/29/19 1:42 PM, Longpeng (Mike) wrote:
>> 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 <[e-mail]>
>>>>
>>>> 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 (cpu=cpu@entry=0x1ab11b0)
>>>> 12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
>>>> 13 0x0000000000870631 in qemu_thread_start (args=args@entry=0x1acfb50)
>>>> 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
>> (mr=mr@entry=0x7fc6a0138df0, addr=addr@entry=156, data=1648892416,
>> size=size@entry=4, attrs=attrs@entry=...)
> 
> So this is a 32-bit access, to address 156 (which is the slotid) and
> data=1648892416=0x62481a00 indeed.
> 
> But watch out access_with_adjusted_size() calls adjust_endianness()...
> 
>> #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 (args=args@entry=0x1acfb50)
>> #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
> 
> ... since memory_region_write_accessor() has the same argument, then I
> can assume your guest is running in Little-Endian.
> 

Yes.

>> 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 <[e-mail]>
>>>> Signed-off-by: Longpeng <[e-mail]>
>>>> ---
>>>>  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.
> 
> If you use mainstream QEMU, we have:
> 
> static void qemu_xhci_instance_init(Object *obj)
> {
>     ...
>     xhci->numslots = MAXSLOTS;
>     ...
> }
> 
> $ git grep define.*MAXSLOTS
> hw/usb/hcd-xhci.c:52:#define LEN_DOORBELL    ((MAXSLOTS + 1) * 0x20)
> hw/usb/hcd-xhci.h:33:#define MAXSLOTS 64
> hw/usb/hcd-xhci.h:37:#define EV_QUEUE (((3 * 24) + 16) * MAXSLOTS)
> 
>>
>>>>              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); // <===
> 
> slotid >= 1 // true
> slotid <= xhci->numslots // FALSE (156 > 64)
> 
> So this assertion won't pass.
> 

Oh, you miss the following code in xhci_doorbell_write():
    reg >>= 2;

So the @slotid pass to xhci_kick_ep() is 156/4 = 39 < 64 and the
'assert(slotid >= 1 && slotid <= xhci->numslots)' assertion will pass.

Check the @epid in xhci_doorbell_write() is still needed.

>>>     assert(epid >= 1 && epid <= 31);
>>>
>>> Regards,
>>>
>>> Phil.
>>>
> 
> .
>

Philippe Mathieu-Daudé April 30, 2019, 5:06 a.m. UTC | #8

On 4/30/19 4:02 AM, Longpeng (Mike) wrote:
> On 2019/4/29 20:10, Philippe Mathieu-Daudé wrote:
>> On 4/29/19 1:42 PM, Longpeng (Mike) wrote:
>>> 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 <[e-mail]>
>>>>>
>>>>> 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 (cpu=cpu@entry=0x1ab11b0)
>>>>> 12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
>>>>> 13 0x0000000000870631 in qemu_thread_start (args=args@entry=0x1acfb50)
>>>>> 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
>>> (mr=mr@entry=0x7fc6a0138df0, addr=addr@entry=156, data=1648892416,
>>> size=size@entry=4, attrs=attrs@entry=...)
>>
>> So this is a 32-bit access, to address 156 (which is the slotid) and
>> data=1648892416=0x62481a00 indeed.
>>
>> But watch out access_with_adjusted_size() calls adjust_endianness()...
>>
>>> #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 (args=args@entry=0x1acfb50)
>>> #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
>>
>> ... since memory_region_write_accessor() has the same argument, then I
>> can assume your guest is running in Little-Endian.
>>
> 
> Yes.
> 
>>> 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 <[e-mail]>
>>>>> Signed-off-by: Longpeng <[e-mail]>
>>>>> ---
>>>>>  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.
>>
>> If you use mainstream QEMU, we have:
>>
>> static void qemu_xhci_instance_init(Object *obj)
>> {
>>     ...
>>     xhci->numslots = MAXSLOTS;
>>     ...
>> }
>>
>> $ git grep define.*MAXSLOTS
>> hw/usb/hcd-xhci.c:52:#define LEN_DOORBELL    ((MAXSLOTS + 1) * 0x20)
>> hw/usb/hcd-xhci.h:33:#define MAXSLOTS 64
>> hw/usb/hcd-xhci.h:37:#define EV_QUEUE (((3 * 24) + 16) * MAXSLOTS)
>>
>>>
>>>>>              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); // <===
>>
>> slotid >= 1 // true
>> slotid <= xhci->numslots // FALSE (156 > 64)
>>
>> So this assertion won't pass.
>>
> 
> Oh, you miss the following code in xhci_doorbell_write():
>     reg >>= 2;
> 
> So the @slotid pass to xhci_kick_ep() is 156/4 = 39 < 64 and the
> 'assert(slotid >= 1 && slotid <= xhci->numslots)' assertion will pass.
> 
> Check the @epid in xhci_doorbell_write() is still needed.

Oh! My bad, I missed that, I'm confused :S

So for your next patch with simply this change:

-        } else if (epid > 31) {
+        } else if (epid == 0 || epid > 31) {

You can include:
Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>

(Please include the full backtrace in the description).

Thanks for being patient with me with this patch :)

>>>>     assert(epid >= 1 && epid <= 31);
>>>>
>>>> Regards,
>>>>
>>>> Phil.
>>>>
>>
>> .
>>

Longpeng(Mike) April 30, 2019, 5:37 a.m. UTC | #9

On 2019/4/30 13:06, Philippe Mathieu-Daudé wrote:

> On 4/30/19 4:02 AM, Longpeng (Mike) wrote:
>> On 2019/4/29 20:10, Philippe Mathieu-Daudé wrote:
>>> On 4/29/19 1:42 PM, Longpeng (Mike) wrote:
>>>> 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 <[e-mail]>
>>>>>>
>>>>>> 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 (cpu=cpu@entry=0x1ab11b0)
>>>>>> 12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
>>>>>> 13 0x0000000000870631 in qemu_thread_start (args=args@entry=0x1acfb50)
>>>>>> 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
>>>> (mr=mr@entry=0x7fc6a0138df0, addr=addr@entry=156, data=1648892416,
>>>> size=size@entry=4, attrs=attrs@entry=...)
>>>
>>> So this is a 32-bit access, to address 156 (which is the slotid) and
>>> data=1648892416=0x62481a00 indeed.
>>>
>>> But watch out access_with_adjusted_size() calls adjust_endianness()...
>>>
>>>> #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 (args=args@entry=0x1acfb50)
>>>> #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
>>>
>>> ... since memory_region_write_accessor() has the same argument, then I
>>> can assume your guest is running in Little-Endian.
>>>
>>
>> Yes.
>>
>>>> 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 <[e-mail]>
>>>>>> Signed-off-by: Longpeng <[e-mail]>
>>>>>> ---
>>>>>>  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.
>>>
>>> If you use mainstream QEMU, we have:
>>>
>>> static void qemu_xhci_instance_init(Object *obj)
>>> {
>>>     ...
>>>     xhci->numslots = MAXSLOTS;
>>>     ...
>>> }
>>>
>>> $ git grep define.*MAXSLOTS
>>> hw/usb/hcd-xhci.c:52:#define LEN_DOORBELL    ((MAXSLOTS + 1) * 0x20)
>>> hw/usb/hcd-xhci.h:33:#define MAXSLOTS 64
>>> hw/usb/hcd-xhci.h:37:#define EV_QUEUE (((3 * 24) + 16) * MAXSLOTS)
>>>
>>>>
>>>>>>              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); // <===
>>>
>>> slotid >= 1 // true
>>> slotid <= xhci->numslots // FALSE (156 > 64)
>>>
>>> So this assertion won't pass.
>>>
>>
>> Oh, you miss the following code in xhci_doorbell_write():
>>     reg >>= 2;
>>
>> So the @slotid pass to xhci_kick_ep() is 156/4 = 39 < 64 and the
>> 'assert(slotid >= 1 && slotid <= xhci->numslots)' assertion will pass.
>>
>> Check the @epid in xhci_doorbell_write() is still needed.
> 
> Oh! My bad, I missed that, I'm confused :S
> 
> So for your next patch with simply this change:
> 
> -        } else if (epid > 31) {
> +        } else if (epid == 0 || epid > 31) {
> 
> You can include:
> Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>
> 
> (Please include the full backtrace in the description).
> 

OK.

> Thanks for being patient with me with this patch :)
> 

It is a good learning experience for me too :)

>>>>>     assert(epid >= 1 && epid <= 31);
>>>>>
>>>>> Regards,
>>>>>
>>>>> Phil.
>>>>>
>>>
>>> .
>>>
> 
> .
>

Patch

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,
     } else {
         epid = val & 0xff;
         streamid = (val >> 16) & 0xffff;
-        if (reg > xhci->numslots) {
+        if (reg == 0 || reg > xhci->numslots) {
             DPRINTF("xhci: bad doorbell %d\n", (int)reg);
-        } else if (epid > 31) {
+        } else if (epid == 0 || epid > 31) {
             DPRINTF("xhci: bad doorbell %d write: 0x%x\n",
                     (int)reg, (uint32_t)val);
         } else {