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));
> > + 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;
> > +}
>
> 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.
> > @@ -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.
Regards,
Daniel
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