Re: [PATCH] block/ssh: add support for sha256 host key fingerprints

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Jun 22, 2021 at 12:51:56PM +0100, Daniel P. Berrangé wrote:
> Currently the SSH block driver supports MD5 and SHA1 for host key
> fingerprints. This is a cryptographically sensitive operation and
> so these hash algorithms are inadequate by modern standards. This
> adds support for SHA256 which has been supported in libssh since
> the 0.8.1 release.
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

Patch looks sensible:

  Acked-by: Richard W.M. Jones <rjones@redhat.com>

Would you mind taking a look at nbdkit-ssh-plugin?  This is arguably
more important (for Red Hat) since we no longer use the SSH driver in
qemu for V2V and switched to using nbdkit some time ago.  The
nbdkit-ssh-plugin appears to only support SHA1 from a cursory look.

Rich.

> 
> Note I can't actually get iotest '207' to fully pass. It always
> complains that it can't validate the "known_hosts" file
> 
>   qemu-img: Could not open 'TEST_IMG': no host key was found in known_hosts
> 
> it seems to rely on some specific developer host setup that my
> laptop doesn't satisfy. It would be useful if any pre-requisite
> could be documented in the iotest.
> 
> At least the sha256 verification step I added to 207 does pass
> though.
> 
>  block/ssh.c                |  3 +++
>  qapi/block-core.json       |  3 ++-
>  tests/qemu-iotests/207     | 54 ++++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/207.out | 25 ++++++++++++++++++
>  4 files changed, 84 insertions(+), 1 deletion(-)
> 
> diff --git a/block/ssh.c b/block/ssh.c
> index b51a031620..d008caf059 100644
> --- a/block/ssh.c
> +++ b/block/ssh.c
> @@ -442,6 +442,9 @@ static int check_host_key(BDRVSSHState *s, SshHostKeyCheck *hkc, Error **errp)
>          } else if (hkc->u.hash.type == SSH_HOST_KEY_CHECK_HASH_TYPE_SHA1) {
>              return check_host_key_hash(s, hkc->u.hash.hash,
>                                         SSH_PUBLICKEY_HASH_SHA1, errp);
> +        } else if (hkc->u.hash.type == SSH_HOST_KEY_CHECK_HASH_TYPE_SHA256) {
> +            return check_host_key_hash(s, hkc->u.hash.hash,
> +                                       SSH_PUBLICKEY_HASH_SHA256, errp);
>          }
>          g_assert_not_reached();
>          break;
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 2ea294129e..4247dc46a5 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -3186,11 +3186,12 @@
>  #
>  # @md5: The given hash is an md5 hash
>  # @sha1: The given hash is an sha1 hash
> +# @sha256: The given hash is an sha256 hash
>  #
>  # Since: 2.12
>  ##
>  { 'enum': 'SshHostKeyCheckHashType',
> -  'data': [ 'md5', 'sha1' ] }
> +  'data': [ 'md5', 'sha1', 'sha256' ] }
>  
>  ##
>  # @SshHostKeyHash:
> diff --git a/tests/qemu-iotests/207 b/tests/qemu-iotests/207
> index f9f3fd7131..0f5c4bc8a0 100755
> --- a/tests/qemu-iotests/207
> +++ b/tests/qemu-iotests/207
> @@ -73,6 +73,9 @@ with iotests.FilePath('t.img') as disk_path, \
>      iotests.log("=== Test host-key-check options ===")
>      iotests.log("")
>  
> +    iotests.log("--- no host key checking --")
> +    iotests.log("")
> +
>      vm.launch()
>      blockdev_create(vm, { 'driver': 'ssh',
>                            'location': {
> @@ -90,6 +93,9 @@ with iotests.FilePath('t.img') as disk_path, \
>  
>      iotests.img_info_log(remote_path)
>  
> +    iotests.log("--- known_hosts key checking --")
> +    iotests.log("")
> +
>      vm.launch()
>      blockdev_create(vm, { 'driver': 'ssh',
>                            'location': {
> @@ -115,6 +121,7 @@ with iotests.FilePath('t.img') as disk_path, \
>      # Mappings of base64 representations to digests
>      md5_keys = {}
>      sha1_keys = {}
> +    sha256_keys = {}
>  
>      for key in keys:
>          md5_keys[key] = subprocess.check_output(
> @@ -125,6 +132,10 @@ with iotests.FilePath('t.img') as disk_path, \
>              'echo %s | base64 -d | sha1sum -b | cut -d" " -f1' % key,
>              shell=True).rstrip().decode('ascii')
>  
> +        sha256_keys[key] = subprocess.check_output(
> +            'echo %s | base64 -d | sha256sum -b | cut -d" " -f1' % key,
> +            shell=True).rstrip().decode('ascii')
> +
>      vm.launch()
>  
>      # Find correct key first
> @@ -150,6 +161,9 @@ with iotests.FilePath('t.img') as disk_path, \
>          vm.shutdown()
>          iotests.notrun('Did not find a key that fits 127.0.0.1')
>  
> +    iotests.log("--- explicit md5 key checking --")
> +    iotests.log("")
> +
>      blockdev_create(vm, { 'driver': 'ssh',
>                            'location': {
>                                'path': disk_path,
> @@ -164,6 +178,7 @@ with iotests.FilePath('t.img') as disk_path, \
>                                }
>                            },
>                            'size': 2097152 })
> +
>      blockdev_create(vm, { 'driver': 'ssh',
>                            'location': {
>                                'path': disk_path,
> @@ -182,6 +197,9 @@ with iotests.FilePath('t.img') as disk_path, \
>  
>      iotests.img_info_log(remote_path)
>  
> +    iotests.log("--- explicit sha1 key checking --")
> +    iotests.log("")
> +
>      vm.launch()
>      blockdev_create(vm, { 'driver': 'ssh',
>                            'location': {
> @@ -215,6 +233,42 @@ with iotests.FilePath('t.img') as disk_path, \
>  
>      iotests.img_info_log(remote_path)
>  
> +    iotests.log("--- explicit sha256 key checking --")
> +    iotests.log("")
> +
> +    vm.launch()
> +    blockdev_create(vm, { 'driver': 'ssh',
> +                          'location': {
> +                              'path': disk_path,
> +                              'server': {
> +                                  'host': '127.0.0.1',
> +                                  'port': '22'
> +                              },
> +                              'host-key-check': {
> +                                  'mode': 'hash',
> +                                  'type': 'sha256',
> +                                  'hash': 'wrong',
> +                              }
> +                          },
> +                          'size': 2097152 })
> +    blockdev_create(vm, { 'driver': 'ssh',
> +                          'location': {
> +                              'path': disk_path,
> +                              'server': {
> +                                  'host': '127.0.0.1',
> +                                  'port': '22'
> +                              },
> +                              'host-key-check': {
> +                                  'mode': 'hash',
> +                                  'type': 'sha256',
> +                                  'hash': sha256_keys[matching_key],
> +                              }
> +                          },
> +                          'size': 4194304 })
> +    vm.shutdown()
> +
> +    iotests.img_info_log(remote_path)
> +
>      #
>      # Invalid path and user
>      #
> diff --git a/tests/qemu-iotests/207.out b/tests/qemu-iotests/207.out
> index 1239d9d648..aeb8569d77 100644
> --- a/tests/qemu-iotests/207.out
> +++ b/tests/qemu-iotests/207.out
> @@ -16,6 +16,8 @@ virtual size: 4 MiB (4194304 bytes)
>  
>  === Test host-key-check options ===
>  
> +--- no host key checking --
> +
>  {"execute": "blockdev-create", "arguments": {"job-id": "job0", "options": {"driver": "ssh", "location": {"host-key-check": {"mode": "none"}, "path": "TEST_DIR/PID-t.img", "server": {"host": "127.0.0.1", "port": "22"}}, "size": 8388608}}}
>  {"return": {}}
>  {"execute": "job-dismiss", "arguments": {"id": "job0"}}
> @@ -25,6 +27,8 @@ image: TEST_IMG
>  file format: IMGFMT
>  virtual size: 8 MiB (8388608 bytes)
>  
> +--- known_hosts key checking --
> +
>  {"execute": "blockdev-create", "arguments": {"job-id": "job0", "options": {"driver": "ssh", "location": {"host-key-check": {"mode": "known_hosts"}, "path": "TEST_DIR/PID-t.img", "server": {"host": "127.0.0.1", "port": "22"}}, "size": 4194304}}}
>  {"return": {}}
>  {"execute": "job-dismiss", "arguments": {"id": "job0"}}
> @@ -34,6 +38,8 @@ image: TEST_IMG
>  file format: IMGFMT
>  virtual size: 4 MiB (4194304 bytes)
>  
> +--- explicit md5 key checking --
> +
>  {"execute": "blockdev-create", "arguments": {"job-id": "job0", "options": {"driver": "ssh", "location": {"host-key-check": {"hash": "wrong", "mode": "hash", "type": "md5"}, "path": "TEST_DIR/PID-t.img", "server": {"host": "127.0.0.1", "port": "22"}}, "size": 2097152}}}
>  {"return": {}}
>  Job failed: remote host key does not match host_key_check 'wrong'
> @@ -49,6 +55,8 @@ image: TEST_IMG
>  file format: IMGFMT
>  virtual size: 8 MiB (8388608 bytes)
>  
> +--- explicit sha1 key checking --
> +
>  {"execute": "blockdev-create", "arguments": {"job-id": "job0", "options": {"driver": "ssh", "location": {"host-key-check": {"hash": "wrong", "mode": "hash", "type": "sha1"}, "path": "TEST_DIR/PID-t.img", "server": {"host": "127.0.0.1", "port": "22"}}, "size": 2097152}}}
>  {"return": {}}
>  Job failed: remote host key does not match host_key_check 'wrong'
> @@ -64,6 +72,23 @@ image: TEST_IMG
>  file format: IMGFMT
>  virtual size: 4 MiB (4194304 bytes)
>  
> +--- explicit sha256 key checking --
> +
> +{"execute": "blockdev-create", "arguments": {"job-id": "job0", "options": {"driver": "ssh", "location": {"host-key-check": {"hash": "wrong", "mode": "hash", "type": "sha256"}, "path": "TEST_DIR/PID-t.img", "server": {"host": "127.0.0.1", "port": "22"}}, "size": 2097152}}}
> +{"return": {}}
> +Job failed: remote host key does not match host_key_check 'wrong'
> +{"execute": "job-dismiss", "arguments": {"id": "job0"}}
> +{"return": {}}
> +
> +{"execute": "blockdev-create", "arguments": {"job-id": "job0", "options": {"driver": "ssh", "location": {"host-key-check": {"hash": "HASH", "mode": "hash", "type": "sha256"}, "path": "TEST_DIR/PID-t.img", "server": {"host": "127.0.0.1", "port": "22"}}, "size": 4194304}}}
> +{"return": {}}
> +{"execute": "job-dismiss", "arguments": {"id": "job0"}}
> +{"return": {}}
> +
> +image: TEST_IMG
> +file format: IMGFMT
> +virtual size: 4 MiB (4194304 bytes)
> +
>  === Invalid path and user ===
>  
>  {"execute": "blockdev-create", "arguments": {"job-id": "job0", "options": {"driver": "ssh", "location": {"host-key-check": {"mode": "none"}, "path": "/this/is/not/an/existing/path", "server": {"host": "127.0.0.1", "port": "22"}}, "size": 4194304}}}
> -- 
> 2.31.1

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.
http://people.redhat.com/~rjones/virt-top