[pve-devel] [PATCH pve-storage] cifs: fix connection check for kerberos authenticated shares

[Hannes Laimer <h.laimer@proxmox.com>]

With smbclient 4.22 (shipped with trixie) `-U Guest -N` does not fall
back to not using a username at all, but just failed for shares where
kerberos was used. smbclient 4.17 (shipped with bookworm) did fall back
to anonymouse, which then succeeded if kbr was used.

Since -U is never correct with kerberos and just worked because
smbclient fell back to no user if `-U Guest` didn't work, we drop
`-U` here.

This fixes the connection check for cifs shares with kerberos. The most
recent changes to when and how a fallback to no user is done in the
smbclient was from 2016 (samba, 35051a860c75bc119e0ac7755bd69a9ea06695a1[1]).
The handling of `-U` also didn't change between the two versions, also a
change in default smb verion didn't seem to have happened in that
timeframe either cause the last I could find was in 2019 (samba,
3264b1f317d6c603cc72eb2a150fe244c47aa3ac[2]). I did not find a
conclusive answer for why exactly this stopped working, but given that
we shouldn't use `-U Guest` with kerberos at all, this change would also
make sense even if it would have continued working.

We use the presence of `sec=krb5...` in the options to determine if
kerberos is used.

[1] https://gitlab.com/samba-team/samba/-/commit/35051a860c75bc119e0ac7755bd69a9ea06695a1
[2] https://gitlab.com/samba-team/samba/-/commit/3264b1f317d6c603cc72eb2a150fe244c47aa3ac

Signed-off-by: Hannes Laimer <h.laimer@proxmox.com>
---
this came up in support.

I could also reproduce this locally, this patch fixes it.

As to the concrete change between 4.17 and 4.22 that could have caused
this I could not find something, so maybe someone who is more familiar with
cifs, sepcifically in combination with kerberos, could take a look.

 src/PVE/Storage/CIFSPlugin.pm | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/PVE/Storage/CIFSPlugin.pm b/src/PVE/Storage/CIFSPlugin.pm
index 5b35daf..5b61e67 100644
--- a/src/PVE/Storage/CIFSPlugin.pm
+++ b/src/PVE/Storage/CIFSPlugin.pm
@@ -66,6 +66,17 @@ sub get_cred_file {
     return undef;
 }
 
+sub cifs_uses_kerberos : prototype($) {
+    my ($scfg) = @_;
+
+    my $options = $scfg->{options};
+    return 0 if !defined($options) || $options eq '';
+
+    $options =~ s/\s+//g;
+
+    return $options =~ m/(?:^|,)sec=krb5(?:i|p)?(?:,|$)/i;
+}
+
 sub cifs_mount : prototype($$$$$) {
     my ($scfg, $storeid, $smbver, $user, $domain) = @_;
 
@@ -77,7 +88,10 @@ sub cifs_mount : prototype($$$$$) {
 
     my $cmd = ['/bin/mount', '-t', 'cifs', $source, $mountpoint, '-o', 'soft', '-o'];
 
-    if (my $cred_file = get_cred_file($storeid)) {
+    if (cifs_uses_kerberos($scfg)) {
+        # no options needed for kerberos, adding username= or domain= would only be informal
+        # adding the if-branch here to have it explicit, and not just by not adding guest
+    } elsif (my $cred_file = get_cred_file($storeid)) {
         push @$cmd, "username=$user", '-o', "credentials=$cred_file";
         push @$cmd, '-o', "domain=$domain" if defined($domain);
     } else {
@@ -280,7 +294,9 @@ sub check_connection {
         push @$cmd, '-m', "smb" . int($scfg->{smbversion});
     }
 
-    if (my $cred_file = get_cred_file($storeid)) {
+    if (cifs_uses_kerberos($scfg)) {
+        push @$cmd, '--use-kerberos=required';
+    } elsif (my $cred_file = get_cred_file($storeid)) {
         push @$cmd, '-U', $scfg->{username}, '-A', $cred_file;
         push @$cmd, '-W', $scfg->{domain} if $scfg->{domain};
     } else {
-- 
2.47.3



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