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

[Laurent Vivier <lvivier@redhat.com>]

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.

Thanks,
Laurent