ssh: fix source-address critical option bypass

Previously, CVE-2024-45337 fixed an authorization bypass
for misused ssh server configurations; if any other type
of callback is passed other than public key, then the
source-address validation would be skipped.

Fixes CVE-2026-46595
Fixes golang/go#79570

Change-Id: I08d86a961048a232c8672f23000e693ed5a0e2fd
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781642
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

ssh/server.go

@@ -837,6 +837,13 @@ userAuthLoop:
 					// considered verified and the callback must not run.
 					perms, authErr = config.VerifiedPublicKeyCallback(s, pubKey, perms, algo)
 				}
+				if authErr == nil && perms != nil && perms.CriticalOptions != nil {
+					if saco := perms.CriticalOptions[sourceAddressCriticalOption]; saco != "" {
+						if err := checkSourceAddress(s.RemoteAddr(), saco); err != nil {
+							authErr = err
+						}
+					}
+				}
 			}
 		case "gssapi-with-mic":
 			if authConfig.GSSAPIWithMICConfig == nil {