[PATCH][SMB3 client] allow us to get kerberos ticket when server advertises IAKerb

[Steve French <smfrench@xxxxxxxxx>]

There are now more servers which advertise support for IAKerb (passthrough
Kerberos authentication via proxy).  IAKerb is a public extension industry
standard Kerberos protocol that allows a client without line-of-sight
to a Domain Controller to authenticate. There can be cases where we
would fail to mount if the server only advertises the OID for IAKerb
in SPNEGO/GSSAPI.  Add code to allow us to still upcall to userspace
in these cases to obtain the Kerberos ticket.

See attached WIP patch

--

Thanks,

Steve

From 4772651ff18900098f7a3ee7088ffbfdc20c4c15 Mon Sep 17 00:00:00 2001
From: Steve French <stfrench@xxxxxxxxxxxxx>
Date: Tue, 28 Jan 2025 01:04:23 -0600
Subject: [PATCH] smb3: add support for IAKerb

There are now more servers which advertise support for IAKerb (passthrough
Kerberos authentication via proxy).  IAKerb is a public extension industry
standard Kerberos protocol that allows a client without line-of-sight
to a Domain Controller to authenticate. There can be cases where we
would fail to mount if the server only advertises the OID for IAKerb
in SPNEGO/GSSAPI.  Add code to allow us to still upcall to userspace
in these cases to obtain the Kerberos ticket.

Signed-off-by: Steve French <stfrench@xxxxxxxxxxxxx>
---
 fs/smb/client/asn1.c        | 2 ++
 fs/smb/client/cifs_spnego.c | 4 +++-
 fs/smb/client/cifsglob.h    | 4 ++++
 fs/smb/client/sess.c        | 3 ++-
 fs/smb/client/smb2pdu.c     | 2 +-
 5 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/fs/smb/client/asn1.c b/fs/smb/client/asn1.c
index b5724ef9f182..214a44509e7b 100644
--- a/fs/smb/client/asn1.c
+++ b/fs/smb/client/asn1.c
@@ -52,6 +52,8 @@ int cifs_neg_token_init_mech_type(void *context, size_t hdrlen,
 		server->sec_kerberos = true;
 	else if (oid == OID_ntlmssp)
 		server->sec_ntlmssp = true;
+	else if (oid == OID_IAKerb)
+		server->sec_iakerb = true;
 	else {
 		char buf[50];
 
diff --git a/fs/smb/client/cifs_spnego.c b/fs/smb/client/cifs_spnego.c
index 28f568b5fc27..bc1c1e9b288a 100644
--- a/fs/smb/client/cifs_spnego.c
+++ b/fs/smb/client/cifs_spnego.c
@@ -138,11 +138,13 @@ cifs_get_spnego_key(struct cifs_ses *sesInfo,
 
 	dp = description + strlen(description);
 
-	/* for now, only sec=krb5 and sec=mskrb5 are valid */
+	/* for now, only sec=krb5 and sec=mskrb5 and iakerb are valid */
 	if (server->sec_kerberos)
 		sprintf(dp, ";sec=krb5");
 	else if (server->sec_mskerberos)
 		sprintf(dp, ";sec=mskrb5");
+	else if (server->sec_iakerb)
+		sprintf(dp, ";sec=iakerb");
 	else {
 		cifs_dbg(VFS, "unknown or missing server auth type, use krb5\n");
 		sprintf(dp, ";sec=krb5");
diff --git a/fs/smb/client/cifsglob.h b/fs/smb/client/cifsglob.h
index 49ffc040f736..a765b91885fd 100644
--- a/fs/smb/client/cifsglob.h
+++ b/fs/smb/client/cifsglob.h
@@ -151,6 +151,7 @@ enum securityEnum {
 	NTLMv2,			/* Legacy NTLM auth with NTLMv2 hash */
 	RawNTLMSSP,		/* NTLMSSP without SPNEGO, NTLMv2 hash */
 	Kerberos,		/* Kerberos via SPNEGO */
+	IAKerb,			/* Kerberos proxy */
 };
 
 enum upcall_target_enum {
@@ -751,6 +752,7 @@ struct TCP_Server_Info {
 	bool	sec_kerberosu2u;	/* supports U2U Kerberos */
 	bool	sec_kerberos;		/* supports plain Kerberos */
 	bool	sec_mskerberos;		/* supports legacy MS Kerberos */
+	bool	sec_iakerb;		/* supports pass-through auth for Kerberos (krb5 proxy) */
 	bool	large_buf;		/* is current buffer large? */
 	/* use SMBD connection instead of socket */
 	bool	rdma;
@@ -2118,6 +2120,8 @@ static inline char *get_security_type_str(enum securityEnum sectype)
 		return "Kerberos";
 	case NTLMv2:
 		return "NTLMv2";
+	case IAKerb:
+		return "IAKerb";
 	default:
 		return "Unknown";
 	}
diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
index 91d4d409cb1d..faa80e7d54a6 100644
--- a/fs/smb/client/sess.c
+++ b/fs/smb/client/sess.c
@@ -1235,12 +1235,13 @@ cifs_select_sectype(struct TCP_Server_Info *server, enum securityEnum requested)
 		switch (requested) {
 		case Kerberos:
 		case RawNTLMSSP:
+		case IAKerb:
 			return requested;
 		case Unspecified:
 			if (server->sec_ntlmssp &&
 			    (global_secflags & CIFSSEC_MAY_NTLMSSP))
 				return RawNTLMSSP;
-			if ((server->sec_kerberos || server->sec_mskerberos) &&
+			if ((server->sec_kerberos || server->sec_mskerberos || server->sec_iakerb) &&
 			    (global_secflags & CIFSSEC_MAY_KRB5))
 				return Kerberos;
 			fallthrough;
diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
index 9f54596a6866..40ad9e79437a 100644
--- a/fs/smb/client/smb2pdu.c
+++ b/fs/smb/client/smb2pdu.c
@@ -1429,7 +1429,7 @@ smb2_select_sectype(struct TCP_Server_Info *server, enum securityEnum requested)
 		if (server->sec_ntlmssp &&
 			(global_secflags & CIFSSEC_MAY_NTLMSSP))
 			return RawNTLMSSP;
-		if ((server->sec_kerberos || server->sec_mskerberos) &&
+		if ((server->sec_kerberos || server->sec_mskerberos || server->sec_iakerb) &&
 			(global_secflags & CIFSSEC_MAY_KRB5))
 			return Kerberos;
 		fallthrough;
-- 
2.43.0