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

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

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;
  }

See:

https://docs.microsoft.com/en-us/windows/win32/winsock/winsock-ioctls

> +
>  void qemu_set_block(int fd)
>  {
>      unsigned long opt = 0;
>