On 20.06.19 22:03, Pino Toscano wrote:
> On Thursday, 20 June 2019 14:58:40 CEST Max Reitz wrote:
>> On 20.06.19 11:49, Pino Toscano wrote:
>>> On Tuesday, 18 June 2019 15:14:30 CEST Max Reitz wrote:
>>>> On 18.06.19 11:24, Pino Toscano wrote:
>>>>> Rewrite the implementation of the ssh block driver to use libssh instead
>>>>> of libssh2. The libssh library has various advantages over libssh2:
>>>>> - easier API for authentication (for example for using ssh-agent)
>>>>> - easier API for known_hosts handling
>>>>> - supports newer types of keys in known_hosts
>>>>>
>>>>> Use APIs/features available in libssh 0.8 conditionally, to support
>>>>> older versions (which are not recommended though).
>>>>>
>>>>> Adjust the tests according to the different error message, and to the
>>>>> newer host keys (ed25519) that are used by default with OpenSSH >= 6.7
>>>>> and libssh >= 0.7.0.
>>>>>
>>>>> Adjust the various Docker/Travis scripts to use libssh when available
>>>>> instead of libssh2. The mingw/mxe testing is dropped for now, as there
>>>>> are no packages for it.
>>>>>
>>>>> Signed-off-by: Pino Toscano <ptosc...@redhat.com>
>>>>> Tested-by: Philippe Mathieu-Daudé <phi...@redhat.com>
>>>>> Acked-by: Alex Bennée <alex.ben...@linaro.org>
>>>>> ---
>>>>>
>>>>> Changes from v9:
>>>>> - restored "default" case in the server status switch for libssh < 0.8.0
>>>>> - print the host key type & fingerprint on mismatch with known_hosts
>>>>> - improve/fix message for failed socket_set_nodelay()
>>>>> - reset s->sock properly
>>>>>
>>>>> Changes from v8:
>>>>> - use a newer key type in iotest 207
>>>>> - improve the commit message
>>>>>
>>>>> Changes from v7:
>>>>> - #if HAVE_LIBSSH_0_8 -> #ifdef HAVE_LIBSSH_0_8
>>>>> - ptrdiff_t -> size_t
>>>>>
>>>>> Changes from v6:
>>>>> - fixed few checkpatch style issues
>>>>> - detect libssh 0.8 via symbol detection
>>>>> - adjust travis/docker test material
>>>>> - remove dead "default" case in a switch
>>>>> - use variables for storing MIN() results
>>>>> - adapt a documentation bit
>>>>>
>>>>> Changes from v5:
>>>>> - adapt to newer tracing APIs
>>>>> - disable ssh compression (mimic what libssh2 does by default)
>>>>> - use build time checks for libssh 0.8, and use newer APIs directly
>>>>>
>>>>> Changes from v4:
>>>>> - fix wrong usages of error_setg/session_error_setg/sftp_error_setg
>>>>> - fix few return code checks
>>>>> - remove now-unused parameters in few internal functions
>>>>> - allow authentication with "none" method
>>>>> - switch to unsigned int for the port number
>>>>> - enable TCP_NODELAY on the socket
>>>>> - fix one reference error message in iotest 207
>>>>>
>>>>> Changes from v3:
>>>>> - fix socket cleanup in connect_to_ssh()
>>>>> - add comments about the socket cleanup
>>>>> - improve the error reporting (closer to what was with libssh2)
>>>>> - improve EOF detection on sftp_read()
>>>>>
>>>>> Changes from v2:
>>>>> - used again an own fd
>>>>> - fixed co_yield() implementation
>>>>>
>>>>> Changes from v1:
>>>>> - fixed jumbo packets writing
>>>>> - fixed missing 'err' assignment
>>>>> - fixed commit message
>>>>>
>>>>> .travis.yml | 4 +-
>>>>> block/Makefile.objs | 6 +-
>>>>> block/ssh.c | 665 ++++++++++--------
>>>>> block/trace-events | 14 +-
>>>>> configure | 65 +-
>>>>> docs/qemu-block-drivers.texi | 2 +-
>>>>> .../dockerfiles/debian-win32-cross.docker | 1 -
>>>>> .../dockerfiles/debian-win64-cross.docker | 1 -
>>>>> tests/docker/dockerfiles/fedora.docker | 4 +-
>>>>> tests/docker/dockerfiles/ubuntu.docker | 2 +-
>>>>> tests/docker/dockerfiles/ubuntu1804.docker | 2 +-
>>>>> tests/qemu-iotests/207 | 4 +-
>>>>> tests/qemu-iotests/207.out | 2 +-
>>>>> 13 files changed, 423 insertions(+), 349 deletions(-)
>>>>
>>>> [...]
>>>>
>>>>> diff --git a/block/ssh.c b/block/ssh.c
>>>>> index 6da7b9cbfe..644ae8b82c 100644
>>>>> --- a/block/ssh.c
>>>>> +++ b/block/ssh.c
>>>>
>>>> [...]
>>>>
>>>>> + case SSH_SERVER_KNOWN_CHANGED:
>>>>> + ret = -EINVAL;
>>>>> + r = ssh_get_publickey(s->session, &pubkey);
>>>>> + if (r == 0) {
>>>>> + r = ssh_get_publickey_hash(pubkey, SSH_PUBLICKEY_HASH_SHA1,
>>>>> + &server_hash, &server_hash_len);
>>>>> + pubkey_type = ssh_key_type(pubkey);
>>>>> + ssh_key_free(pubkey);
>>>>> + }
>>>>> + if (r == 0) {
>>>>> + fingerprint =
>>>>> ssh_get_fingerprint_hash(SSH_PUBLICKEY_HASH_SHA1,
>>>>> + server_hash,
>>>>> + server_hash_len);
>>>>> + ssh_clean_pubkey_hash(&server_hash);
>>>>> + }
>>>>> + if (fingerprint) {
>>>>> + error_setg(errp,
>>>>> + "host key (%s key with fingerprint %s) does not
>>>>> match "
>>>>> + "the one in known_hosts",
>>>>> + ssh_key_type_to_char(pubkey_type), fingerprint);
>>>>> + ssh_string_free_char(fingerprint);
>>>>> + } else {
>>>>> + error_setg(errp, "host key does not match the one in
>>>>> known_hosts");
>>>>> + }
>>>>
>>>> Usually I’d say that more information can’t be bad. But here I don’t
>>>> see how this additional information is useful. known_hosts contains
>>>> base64-encoded full public keys. This only prints the SHA-1 digest.
>>>
>>> Note that SHA-1 is printed with libssh < 0.8; with libssh >= 0.8 SHA-256
>>> is used instead.
>>>
>>>> The user cannot add that to known_hosts, or easily scan known_hosts to
>>>> see whether it perhaps belongs to some other hosts (maybe because DHCP
>>>> scrambled something).
>>>>
>>>> Actually, even if this printed the base64 representation of the full
>>>> key, the user probably wouldn’t just add that to known_hosts or do
>>>> anything with it. They’ll debug the problem with other tools.
>>>>
>>>> So I don’t think this new information is really useful...?
>>>
>>> When this situation happens with openssh, the big scary message prints
>>> three things:
>>> 1) the fingerprint of the server
>>> 2) the line of the server in the known_hosts file
>>> 3) how to remove the keys of the server from known_hosts, i.e. a
>>> copy-paste'able `ssh-keygen -R host`
>>>
>>> Here I'm doing only (1). Also, the current libssh2 driver does
>>> something else, i.e. print the base64/printable representation of the
>>> server key in known_hosts.
>>
>> Well, I don’t know whether it’s necessary to reproduce the exact message
>> with all the data it contains. As I said, I think users can and will
>> investigate the exact root of the problem with tools outside of qemu.
>> (Such as openssh’s ssh itself.)
>>
>>>> (Hmm, I don’t know if this is a response to my “But the specification
>>>> requires a warning!!1!”, but if so, I was actually not referring to more
>>>> information but to this:
>>>
>>> You mentioned this few times: can you please point me to this bit?
>>> I read few RFCs related to ssh, and I did not find this information...
>>
>> For example:
>> http://api.libssh.org/master/group__libssh__session.html#gacbc5d04fe66beee863a0c61a93fdf765
>>> SSH_KNOWN_HOSTS_CHANGED: The server key has changed. Either you are under
>>> attack or the administrator changed the key. You HAVE to warn the user
>>> about a possible attack.
>
> Ah, now I see what you mean! I thought that "libssh specification" was
> any of the SSH RFCs, and that's why I did not find what you meant.
> I see you were talking about the libssh API documentation :-)
> (Never heard the API docs of a library called as "specification" before,
> TBH.)Ah, sorry. I have no excuse. I just call everything a “spec” that’s telling me what to do. (I should probably stop doing that.) >> This doesn’t require any specific formatting or data to be given to the >> user. All it requires is “to warn the user about a possible attack”. >> That can be as simple as appending “This may be due to a >> man-in-the-middle attack” to the error message. > > Makes sense -- I just asked to the libssh people, and appending > "this may be a possible attack" should be enough, especially that this > is not a UI message like the one written by the ssh client. OK, great! >>>> $ ssh 192.168.0.12 >>>> @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ >>>> @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ >>>> @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ >>>> IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! >>>> Someone could be eavesdropping on you right now (man-in-the-middle attack)! >>>> It is also possible that a host key has just been changed. >>>> >>>> >>>> I mean, I was also only half-serious. I should be serious because the >>>> libssh specification requires us to print some warning like that, but, >>>> well. Yes, I’ll stop mumbling about this stuff now.) >>> >>> To be on the explic side: are you asking to basically put the first 6 >>> lines of the openssh error message (as you quoted them above) as error >>> message in the ssh driver? >> >> God forbid no. I was just making an example of “Here is a warning about >> a possible attack”. I fully agree with Dan (and probably you) that this >> is completely unsuitable to qemu’s interface. >> >> Sorry if that came across in another way. > > Not a problem. I preferred to ask explicitly to make sure to get it > right -- any amount of information shown is fine for me, I want to make > sure to follow QEMU best practices (if any). I mean, as you’ve seen yourself, there currently is no warning. So it’s not like there is any practice, not to mention a best one... Max
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