Hi Daniel,
On 2/22/19 1:24 PM, Daniel P. Berrangé wrote:
> On Fri, Feb 22, 2019 at 01:34:12AM +0100, Philippe Mathieu-Daudé wrote:
>> Hi Daniel,
>>
>> On 2/15/19 4:57 PM, Daniel P. Berrangé wrote:
>>> From: "Daniel P. Berrange" <berra...@redhat.com>
>>>
>>> Add an authorization backend that talks to PAM to check whether the user
>>> identity is allowed. This only uses the PAM account validation facility,
>>> which is essentially just a check to see if the provided username is
>>> permitted
>>> access. It doesn't use the authentication or session parts of PAM, since
>>> that's dealt with by the relevant part of QEMU (eg VNC server).
>>>
>>> Consider starting QEMU with a VNC server and telling it to use TLS with
>>> x509 client certificates and configuring it to use an PAM to validate
>>> the x509 distinguished name. In this example we're telling it to use PAM
>>> for the QAuthZ impl with a service name of "qemu-vnc"
>>>
>>> $ qemu-system-x86_64 \
>>> -object tls-creds-x509,id=tls0,dir=/home/berrange/security/qemutls,\
>>> endpoint=server,verify-peer=yes \
>>> -object authz-pam,id=authz0,service=qemu-vnc \
>>> -vnc :1,tls-creds=tls0,tls-authz=authz0
>>>
>>> This requires an /etc/pam/qemu-vnc file to be created with the auth
>>> rules. A very simple file based whitelist can be setup using
>>>
>>> $ cat > /etc/pam/qemu-vnc <<EOF
>>> account requisite pam_listfile.so item=user sense=allow
>>> file=/etc/qemu/vnc.allow
>>> EOF
>>>
>>> The /etc/qemu/vnc.allow file simply contains one username per line. Any
>>> username not in the file is denied. The usernames in this example are
>>> the x509 distinguished name from the client's x509 cert.
>>>
>>> $ cat > /etc/qemu/vnc.allow <<EOF
>>> CN=laptop.berrange.com,O=Berrange Home,L=London,ST=London,C=GB
>>> EOF
>>>
>>> More interesting would be to configure PAM to use an LDAP backend, so
>>> that the QEMU authorization check data can be centralized instead of
>>> requiring each compute host to have file maintained.
>>>
>>> The main limitation with this PAM module is that the rules apply to all
>>> QEMU instances on the host. Setting up different rules per VM, would
>>> require creating a separate PAM service name & config file for every
>>> guest. An alternative approach for the future might be to not pass in
>>> the plain username to PAM, but instead combine the VM name or UUID with
>>> the username. This requires further consideration though.
>>>
>>> Signed-off-by: Daniel P. Berrange <berra...@redhat.com>
>>> ---
>
>>> +static bool qauthz_pam_is_allowed(QAuthZ *authz,
>>> + const char *identity,
>>> + Error **errp)
>>> +{
>>> + QAuthZPAM *pauthz = QAUTHZ_PAM(authz);
>>> + const struct pam_conv pam_conversation = { 0 };
>>> + pam_handle_t *pamh = NULL;
>>> + int ret;
>>> +
>>> + trace_qauthz_pam_check(authz, identity, pauthz->service);
>>> + ret = pam_start(pauthz->service,
>>> + identity,
>>> + &pam_conversation,
>>> + &pamh);
>>> + if (ret != PAM_SUCCESS) {
>>> + error_setg(errp, "Unable to start PAM transaction: %s",
>>> + pam_strerror(NULL, ret));
"In an error case is the content of pamh undefined."
So it is safer to use NULL here indeed.
>>> + return false;
>>> + }
>>> +
>>> + ret = pam_acct_mgmt(pamh, PAM_SILENT);
>>> + if (ret != PAM_SUCCESS) {
>>> + error_setg(errp, "Unable to authorize user '%s': %s",
>>> + identity, pam_strerror(pamh, ret));
>>> + goto cleanup;
>>> + }
>>> +
>>> + cleanup:
>>> + pam_end(pamh, ret);
>>> + return ret == PAM_SUCCESS;
Hmm I find this fragile.
A 'cleanup' label means (to me) you expect someone to eventually add
more code around, and I'm worried someone add a pam_smth() call after
pam_acct_mgmt(), that sets ret to PAM_SUCCESS.
It looks safer to me to simply not use any label here (for the current
code, if it is extended, we'll see).
>>> +}
>>
>> I still need to digest this function (reading more about PAM).
>
> FWIW there's reasonably good manpages 'pam(3)' and 'pam(8)' are
> starting points.
Easier inverted, first 'pam(8)' then 'pam(3)' ;)
Here I realize last time I checked 'pam(3)' was 17 years ago...
>>> @@ -2864,6 +2870,33 @@ else
>>> fi
>>>
>>>
>>> +##########################################
>>> +# PAM probe
>>> +
>>> +if test "x$auth_pam" != "no"; then
>>
>> Either check "x$auth_pam" != "xno", or "$auth_pam" != "no" (the latter
>> seems to follow the style of this file).
>>
>> Currently this condition is always true, so the script always calls
>> compile_prog. And if an user has PAM locally installed, it is not
>> possible to not use it.
>
> Opps, yes, did I say I hate shell :-)
>
>>
>>> + cat > $TMPC <<EOF
>>> +#include <security/pam_appl.h>
>>> +#include <stdio.h>
>>> +int main(void) {
>>> + const char *service_name = "qemu";
>>> + const char *user = "frank";
>>> + const struct pam_conv *pam_conv = NULL;
>>> + pam_handle_t *pamh = NULL;
>>> + pam_start(service_name, user, pam_conv, &pamh);
>>> + return 0;
>>> +}
>>> +EOF
>>> + if compile_prog "" "-lpam" ; then
>>> + auth_pam=yes
>>> + else
>>> + if test "$auth_pam" = "yes"; then
>>> + feature_not_found "PAM" "Install PAM development package"
>>> + else
>>> + auth_pam=no
>>> + fi
>>> + fi
>
> I notice some indentation damage here now due to tabs that I'll also
> fix.
OK.
If you agree on removing the 'cleanup' label in qauthz_pam_is_allowed(),
for the whole patch:
Reviewed-by: Philippe Mathieu-Daudé <phi...@redhat.com>
Regards,
Phil.
>
> Regards,
> Daniel
>
