Re: [PATCH] net: tap: check if the file descriptor is valid before using it

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 6/25/20 9:38 AM, Laurent Vivier wrote:
> On 25/06/2020 08:19, Philippe Mathieu-Daudé wrote:
>> On 6/24/20 9:00 PM, Laurent Vivier wrote:
>>> qemu_set_nonblock() checks that the file descriptor can be used and, if
>>> not, crashes QEMU. An assert() is used for that. The use of assert() is
>>> used to detect programming error and the coredump will allow to debug
>>> the problem.
>>>
>>> But in the case of the tap device, this assert() can be triggered by
>>> a misconfiguration by the user. At startup, it's not a real problem, but it
>>> can also happen during the hot-plug of a new device, and here it's a
>>> problem because we can crash a perfectly healthy system.
>>>
>>> For instance:
>>>  # ip link add link virbr0 name macvtap0 type macvtap mode bridge
>>>  # ip link set macvtap0 up
>>>  # TAP=/dev/tap$(ip -o link show macvtap0 | cut -d: -f1)
>>>  # qemu-system-x86_64 -machine q35 -device pcie-root-port,id=pcie-root-port-0 -monitor stdio 9<> $TAP
>>>  (qemu) netdev_add type=tap,id=hostnet0,vhost=on,fd=9
>>>  (qemu) device_add driver=virtio-net-pci,netdev=hostnet0,id=net0,bus=pcie-root-port-0
>>>  (qemu) device_del net0
>>>  (qemu) netdev_del hostnet0
>>>  (qemu) netdev_add type=tap,id=hostnet1,vhost=on,fd=9
>>>  qemu-system-x86_64: .../util/oslib-posix.c:247: qemu_set_nonblock: Assertion `f != -1' failed.
>>>  Aborted (core dumped)
>>>
>>> To avoid that, check the file descriptor is valid before passing it to qemu_set_non_block() for
>>> "fd=" and "fds=" parameters.
>>>
>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>> ---
>>>  include/qemu/sockets.h |  1 +
>>>  net/tap.c              | 13 +++++++++++++
>>>  util/oslib-posix.c     |  5 +++++
>>>  util/oslib-win32.c     |  6 ++++++
>>>  4 files changed, 25 insertions(+)
>>>
>>> diff --git a/include/qemu/sockets.h b/include/qemu/sockets.h
>>> index 57cd049d6edd..5b0c2d77ddad 100644
>>> --- a/include/qemu/sockets.h
>>> +++ b/include/qemu/sockets.h
>>> @@ -17,6 +17,7 @@ int qemu_socket(int domain, int type, int protocol);
>>>  int qemu_accept(int s, struct sockaddr *addr, socklen_t *addrlen);
>>>  int socket_set_cork(int fd, int v);
>>>  int socket_set_nodelay(int fd);
>>> +bool qemu_fd_is_valid(int fd);
>>>  void qemu_set_block(int fd);
>>>  void qemu_set_nonblock(int fd);
>>>  int socket_set_fast_reuse(int fd);
>>> diff --git a/net/tap.c b/net/tap.c
>>> index 6207f61f84ab..f65966aaccd8 100644
>>> --- a/net/tap.c
>>> +++ b/net/tap.c
>>> @@ -795,6 +795,12 @@ int net_init_tap(const Netdev *netdev, const char *name,
>>>              return -1;
>>>          }
>>>  
>>> +        /* Check if fd is valid */
>>> +        if (!qemu_fd_is_valid(fd)) {
>>> +            error_setg(errp, "Invalid file descriptor %d", fd);
>>> +            return -1;
>>> +        }
>>> +
>>>          qemu_set_nonblock(fd);
>>>  
>>>          vnet_hdr = tap_probe_vnet_hdr(fd);
>>> @@ -843,6 +849,13 @@ int net_init_tap(const Netdev *netdev, const char *name,
>>>                  goto free_fail;
>>>              }
>>>  
>>> +            /* Check if fd is valid */
>>> +            if (!qemu_fd_is_valid(fd)) {
>>> +                error_setg(errp, "Invalid file descriptor %d", fd);
>>> +                ret = -1;
>>> +                goto free_fail;
>>> +            }
>>> +
>>>              qemu_set_nonblock(fd);
>>>  
>>>              if (i == 0) {
>>> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
>>> index 916f1be2243a..8d5705f598d3 100644
>>> --- a/util/oslib-posix.c
>>> +++ b/util/oslib-posix.c
>>> @@ -244,6 +244,11 @@ void qemu_anon_ram_free(void *ptr, size_t size)
>>>      qemu_ram_munmap(-1, ptr, size);
>>>  }
>>>  
>>> +bool qemu_fd_is_valid(int fd)
>>> +{
>>> +    return fcntl(fd, F_GETFL) != -1;
>>> +}
>>> +
>>>  void qemu_set_block(int fd)
>>>  {
>>>      int f;
>>> diff --git a/util/oslib-win32.c b/util/oslib-win32.c
>>> index e9b14ab17847..a6be9445cfdb 100644
>>> --- a/util/oslib-win32.c
>>> +++ b/util/oslib-win32.c
>>> @@ -132,6 +132,12 @@ struct tm *localtime_r(const time_t *timep, struct tm *result)
>>>  }
>>>  #endif /* CONFIG_LOCALTIME_R */
>>>  
>>> +bool qemu_fd_is_valid(int fd)
>>> +{
>>> +    /* FIXME: how to check if fd is valid? */
>>> +    return true;
>>> +}
>>
>> Maybe:
>>
>>   bool qemu_fd_is_valid(int fd)
>>   {
>>       unsigned long res; /* ignored */
>>
>>       return ioctlsocket(fd, FIONREAD, &res) == NO_ERROR;
>>   }
> 
> I can do that, but I have no way to test the change doesn't break
> anything... whereas always returning true ensures me it continues to
> work as before.

I'm only suggesting in case someone has a clue and way to test ;)

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>