[PATCH] ksmbd: defer destroy_previous_session() until after NTLM authentication

[James Montgomery <james_montgomery@disroot.org>]

In ntlm_authenticate(), destroy_previous_session() is called using a
user pointer resolved from the client-supplied NTLM blob username field
before the NTLMv2 response is validated. An authenticated attacker can
set the NTLM blob username to match a victim account and set
PreviousSessionId to the victim's session ID; destroy_previous_session()
destroys the victim's session while ksmbd_decode_ntlmssp_auth_blob()
subsequently rejects the request with -EPERM.

Move destroy_previous_session() to after ksmbd_decode_ntlmssp_auth_blob()
returns success and use sess->user rather than the pre-authentication
lookup result. This matches the ordering already used by
krb5_authenticate(), where destroy_previous_session() is called only
after ksmbd_krb5_authenticate() returns success.

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: James Montgomery <james_montgomery@disroot.org>
---
Note: portions of this patch were suggested by an AI coding assistant
(Claude Code / Anthropic). The ordering issue was identified via static
analysis of ntlm_authenticate() and validated empirically using a PoC
against ksmbd running Linux 7.1.0 in QEMU. The fix was verified by:
checkpatch.pl --strict (0 errors, 0 warnings), sparse (HEAD, 0 warnings),
and build test (make fs/smb/server/, clean). The assistant was prompted
to identify and fix the destroy_previous_session() ordering issue relative
to NTLMv2 validation, matching the pattern already used in krb5_authenticate().

 fs/smb/server/smb2pdu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c
index 5859fa68bb84..1ce13b23cf6c 100644
--- a/fs/smb/server/smb2pdu.c
+++ b/fs/smb/server/smb2pdu.c
@@ -1670,10 +1670,7 @@ static int ntlm_authenticate(struct ksmbd_work *work,
 		return -EPERM;
 	}
 
-	/* Check for previous session */
 	prev_id = le64_to_cpu(req->PreviousSessionId);
-	if (prev_id && prev_id != sess->id)
-		destroy_previous_session(conn, user, prev_id);
 
 	if (sess->state == SMB2_SESSION_VALID) {
 		/*
@@ -1712,6 +1709,9 @@ static int ntlm_authenticate(struct ksmbd_work *work,
 		}
 	}
 
+	if (prev_id && prev_id != sess->id)
+		destroy_previous_session(conn, sess->user, prev_id);
+
 	/*
 	 * If session state is SMB2_SESSION_VALID, We can assume
 	 * that it is reauthentication. And the user/password
-- 
2.47.3