[PATCH v2 0/2] net: tap: check file descriptor can be used

[PATCH v2 0/2] net: tap: check file descriptor can be used

[Laurent Vivier]

v2: Add patch from Daniel to check the fd can be used

    I have updated Daniel's patch not to check for EINVAL on TUNGETIFF
    as I think we can avoid this special case because TUNGETIFF
    is available since kernel v2.6.27 (October 2008)
    Moreover I think the code was wrong as it was checking with -EINVAL and
    not EINVAL.

Daniel P. Berrang�� (1):
  net: detect errors from probing vnet hdr flag for TAP devices

Laurent Vivier (1):
  net: tap: check if the file descriptor is valid before using it

 include/qemu/sockets.h |  1 +
 net/tap-bsd.c          |  2 +-
 net/tap-linux.c        |  8 +++++---
 net/tap-solaris.c      |  2 +-
 net/tap-stub.c         |  2 +-
 net/tap.c              | 38 +++++++++++++++++++++++++++++++++-----
 net/tap_int.h          |  2 +-
 util/oslib-posix.c     |  5 +++++
 util/oslib-win32.c     |  6 ++++++
 9 files changed, 54 insertions(+), 12 deletions(-)

-- 
2.26.2

[PATCH v2 1/2] net: tap: check if the file descriptor is valid before using it

[Laurent Vivier]

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;
+}
+
 void qemu_set_block(int fd)
 {
     unsigned long opt = 0;
-- 
2.26.2

Re: [PATCH v2 1/2] net: tap: check if the file descriptor is valid before using it

[Philippe Mathieu-Daudé]

On 6/30/20 4:57 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>

Reviewed-by: Philippe Mathieu-Daudé <philmd@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;
> +}
> +
>  void qemu_set_block(int fd)
>  {
>      unsigned long opt = 0;
> 

Re: [PATCH v2 1/2] net: tap: check if the file descriptor is valid before using it

[Greg Kurz]

On Tue, 30 Jun 2020 16:57:36 +0200
Laurent Vivier <lvivier@redhat.com> 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 the following ?

bool qemu_fd_is_valid(int fd)
{
    return _get_osfhandle(fd) != INVALID_HANDLE_VALUE;
}

https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=vs-2019

Anyway,

Reviewed-by: Greg Kurz <groug@kaod.org>

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

Re: [PATCH v2 1/2] net: tap: check if the file descriptor is valid before using it

[Laurent Vivier]

On 30/06/2020 18:06, Greg Kurz wrote:
> On Tue, 30 Jun 2020 16:57:36 +0200
> Laurent Vivier <lvivier@redhat.com> 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 the following ?
> 
> bool qemu_fd_is_valid(int fd)
> {
>     return _get_osfhandle(fd) != INVALID_HANDLE_VALUE;
> }
> 
> https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=vs-2019

For the v1, Philippe proposed:

  bool qemu_fd_is_valid(int fd)
  {
      unsigned long res; /* ignored */

      return ioctlsocket(fd, FIONREAD, &res) == NO_ERROR;
  }

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

The problem is I can't test. So I let the change to someone that will
need it, fix it and test it...

Thanks,
Laurent

Re: [PATCH v2 1/2] net: tap: check if the file descriptor is valid before using it

[Markus Armbruster]

Laurent Vivier <lvivier@redhat.com> writes:

> 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? */

Please explain the FIXME's impact in the commit message.

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

This patch is okay as it is.  But I'd solve the problem differently,
avoiding the FIXME: have qemu_try_set_block() return -errno on failure,
have qemu_set_block() wrap around it and assert it succeeds.  Then in
net_init_tap():

        ret = qemu_try_set_block(fd);
        if (ret < 0) {
            error_setg_errno(errp, -ret, "Can't use file descriptor %d",
                             fd);
        }

When @fd is bad (say -1), @ret is set to EBADF, and the error message
looks like

    Can't use file descriptor -1: Bad file descriptor

You should of course test the error message to see whether it makes
sense even with a complex command line.  If it doesn't, having
it mention @name could perhaps help.

Re: [PATCH v2 1/2] net: tap: check if the file descriptor is valid before using it

[Laurent Vivier]

On 01/07/2020 07:50, Markus Armbruster wrote:
> Laurent Vivier <lvivier@redhat.com> writes:
> 
>> 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? */
> 
> Please explain the FIXME's impact in the commit message.
> 
>> +    return true;
>> +}
>> +
>>  void qemu_set_block(int fd)
>>  {
>>      unsigned long opt = 0;
> 
> This patch is okay as it is.  But I'd solve the problem differently,
> avoiding the FIXME: have qemu_try_set_block() return -errno on failure,
> have qemu_set_block() wrap around it and assert it succeeds.  Then in
> net_init_tap():
> 
>         ret = qemu_try_set_block(fd);
>         if (ret < 0) {
>             error_setg_errno(errp, -ret, "Can't use file descriptor %d",
>                              fd);
>         }
> 
> When @fd is bad (say -1), @ret is set to EBADF, and the error message
> looks like
> 
>     Can't use file descriptor -1: Bad file descriptor
> 
> You should of course test the error message to see whether it makes
> sense even with a complex command line.  If it doesn't, having
> it mention @name could perhaps help.
> 

Yes, it's a good idea. I'm going to do that.

Thanks,
Laurent

[PATCH v2 2/2] net: detect errors from probing vnet hdr flag for TAP devices

[Laurent Vivier]

From: "Daniel P. Berrange" <berrange@redhat.com>

When QEMU sets up a tap based network device backend, it mostly ignores errors
reported from various ioctl() calls it makes, assuming the TAP file descriptor
is valid. This assumption can easily be violated when the user is passing in a
pre-opened file descriptor. At best, the ioctls may fail with a -EBADF, but if
the user passes in a bogus FD number that happens to clash with a FD number that
QEMU has opened internally for another reason, a wide variety of errnos may
result, as the TUNGETIFF ioctl number may map to a completely different command
on a different type of file.

By ignoring all these errors, QEMU sets up a zombie network backend that will
never pass any data. Even worse, when QEMU shuts down, or that network backend
is hot-removed, it will close this bogus file descriptor, which could belong to
another QEMU device backend.

There's no obvious guaranteed reliable way to detect that a FD genuinely is a
TAP device, as opposed to a UNIX socket, or pipe, or something else. Checking
the errno from probing vnet hdr flag though, does catch the big common cases.
ie calling TUNGETIFF will return EBADF for an invalid FD, and ENOTTY when FD is
a UNIX socket, or pipe which catches accidental collisions with FDs used for
stdio, or monitor socket.

Previously the example below where bogus fd 9 collides with the FD used for the
chardev saw:

$ ./x86_64-softmmu/qemu-system-x86_64 -netdev tap,id=hostnet0,fd=9 \
  -chardev socket,id=charchannel0,path=/tmp/qga,server,nowait \
  -monitor stdio -vnc :0
qemu-system-x86_64: -netdev tap,id=hostnet0,fd=9: TUNGETIFF ioctl() failed: Inappropriate ioctl for device
TUNSETOFFLOAD ioctl() failed: Bad address
QEMU 2.9.1 monitor - type 'help' for more information
(qemu) Warning: netdev hostnet0 has no peer

which gives a running QEMU with a zombie network backend.

With this change applied we get an error message and QEMU immediately exits
before carrying on and making a bigger disaster:

$ ./x86_64-softmmu/qemu-system-x86_64 -netdev tap,id=hostnet0,fd=9 \
  -chardev socket,id=charchannel0,path=/tmp/qga,server,nowait \
  -monitor stdio -vnc :0
qemu-system-x86_64: -netdev tap,id=hostnet0,vhost=on,fd=9: Unable to query TUNGETIFF on FD 9: Inappropriate ioctl for device

Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 20171027085548.3472-1-berrange@redhat.com
[lv: to simplify, don't check on EINVAL with TUNGETIFF as it exists since v2.6.27]
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 net/tap-bsd.c     |  2 +-
 net/tap-linux.c   |  8 +++++---
 net/tap-solaris.c |  2 +-
 net/tap-stub.c    |  2 +-
 net/tap.c         | 25 ++++++++++++++++++++-----
 net/tap_int.h     |  2 +-
 6 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/net/tap-bsd.c b/net/tap-bsd.c
index a5c3707f806d..77aaf674b19d 100644
--- a/net/tap-bsd.c
+++ b/net/tap-bsd.c
@@ -211,7 +211,7 @@ void tap_set_sndbuf(int fd, const NetdevTapOptions *tap, Error **errp)
 {
 }
 
-int tap_probe_vnet_hdr(int fd)
+int tap_probe_vnet_hdr(int fd, Error **errp)
 {
     return 0;
 }
diff --git a/net/tap-linux.c b/net/tap-linux.c
index e0dd442ee34f..b0635e9e32ce 100644
--- a/net/tap-linux.c
+++ b/net/tap-linux.c
@@ -147,13 +147,15 @@ void tap_set_sndbuf(int fd, const NetdevTapOptions *tap, Error **errp)
     }
 }
 
-int tap_probe_vnet_hdr(int fd)
+int tap_probe_vnet_hdr(int fd, Error **errp)
 {
     struct ifreq ifr;
 
     if (ioctl(fd, TUNGETIFF, &ifr) != 0) {
-        error_report("TUNGETIFF ioctl() failed: %s", strerror(errno));
-        return 0;
+        /* TUNGETIFF is available since kernel v2.6.27 */
+        error_setg_errno(errp, errno,
+                         "Unable to query TUNGETIFF on FD %d", fd);
+        return -1;
     }
 
     return ifr.ifr_flags & IFF_VNET_HDR;
diff --git a/net/tap-solaris.c b/net/tap-solaris.c
index 4725d2314eef..ae2ba6828415 100644
--- a/net/tap-solaris.c
+++ b/net/tap-solaris.c
@@ -206,7 +206,7 @@ void tap_set_sndbuf(int fd, const NetdevTapOptions *tap, Error **errp)
 {
 }
 
-int tap_probe_vnet_hdr(int fd)
+int tap_probe_vnet_hdr(int fd, Error **errp)
 {
     return 0;
 }
diff --git a/net/tap-stub.c b/net/tap-stub.c
index a9ab8f829362..de525a2e69d4 100644
--- a/net/tap-stub.c
+++ b/net/tap-stub.c
@@ -37,7 +37,7 @@ void tap_set_sndbuf(int fd, const NetdevTapOptions *tap, Error **errp)
 {
 }
 
-int tap_probe_vnet_hdr(int fd)
+int tap_probe_vnet_hdr(int fd, Error **errp)
 {
     return 0;
 }
diff --git a/net/tap.c b/net/tap.c
index f65966aaccd8..f0ba0ae069cb 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -597,7 +597,11 @@ int net_init_bridge(const Netdev *netdev, const char *name,
     }
 
     qemu_set_nonblock(fd);
-    vnet_hdr = tap_probe_vnet_hdr(fd);
+    vnet_hdr = tap_probe_vnet_hdr(fd, errp);
+    if (vnet_hdr < 0) {
+        close(fd);
+        return -1;
+    }
     s = net_tap_fd_init(peer, "bridge", name, fd, vnet_hdr);
 
     snprintf(s->nc.info_str, sizeof(s->nc.info_str), "helper=%s,br=%s", helper,
@@ -803,7 +807,11 @@ int net_init_tap(const Netdev *netdev, const char *name,
 
         qemu_set_nonblock(fd);
 
-        vnet_hdr = tap_probe_vnet_hdr(fd);
+        vnet_hdr = tap_probe_vnet_hdr(fd, errp);
+        if (vnet_hdr < 0) {
+            close(fd);
+            return -1;
+        }
 
         net_init_tap_one(tap, peer, "tap", name, NULL,
                          script, downscript,
@@ -859,8 +867,11 @@ int net_init_tap(const Netdev *netdev, const char *name,
             qemu_set_nonblock(fd);
 
             if (i == 0) {
-                vnet_hdr = tap_probe_vnet_hdr(fd);
-            } else if (vnet_hdr != tap_probe_vnet_hdr(fd)) {
+                vnet_hdr = tap_probe_vnet_hdr(fd, errp);
+                if (vnet_hdr < 0) {
+                    goto free_fail;
+                }
+            } else if (vnet_hdr != tap_probe_vnet_hdr(fd, NULL)) {
                 error_setg(errp,
                            "vnet_hdr not consistent across given tap fds");
                 ret = -1;
@@ -905,7 +916,11 @@ free_fail:
         }
 
         qemu_set_nonblock(fd);
-        vnet_hdr = tap_probe_vnet_hdr(fd);
+        vnet_hdr = tap_probe_vnet_hdr(fd, errp);
+        if (vnet_hdr < 0) {
+            close(fd);
+            return -1;
+        }
 
         net_init_tap_one(tap, peer, "bridge", name, ifname,
                          script, downscript, vhostfdname,
diff --git a/net/tap_int.h b/net/tap_int.h
index e3194b23f47d..225a49ea4843 100644
--- a/net/tap_int.h
+++ b/net/tap_int.h
@@ -34,7 +34,7 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
 ssize_t tap_read_packet(int tapfd, uint8_t *buf, int maxlen);
 
 void tap_set_sndbuf(int fd, const NetdevTapOptions *tap, Error **errp);
-int tap_probe_vnet_hdr(int fd);
+int tap_probe_vnet_hdr(int fd, Error **errp);
 int tap_probe_vnet_hdr_len(int fd, int len);
 int tap_probe_has_ufo(int fd);
 void tap_fd_set_offload(int fd, int csum, int tso4, int tso6, int ecn, int ufo);
-- 
2.26.2

Re: [PATCH v2 0/2] net: tap: check file descriptor can be used

[Daniel P. Berrangé]

On Tue, Jun 30, 2020 at 04:57:35PM +0200, Laurent Vivier wrote:
> v2: Add patch from Daniel to check the fd can be used
> 
>     I have updated Daniel's patch not to check for EINVAL on TUNGETIFF
>     as I think we can avoid this special case because TUNGETIFF
>     is available since kernel v2.6.27 (October 2008)
>     Moreover I think the code was wrong as it was checking with -EINVAL and
>     not EINVAL.

Agreed, given our documented OS platform targets, we don't need to
care about such old kernels.


Regards,
Daniel
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