fix(accounts): disallow reversing a reverse journal entry

check read permission on the source entry before the guards run, so the
reversal relationship is not disclosed to a user who cannot read it.

(cherry picked from commit 9dd37d5f32)

erpnext/accounts/doctype/journal_entry/journal_entry.js

@@ -95,7 +95,7 @@ frappe.ui.form.on("Journal Entry", {
 			);
 		}
 
-		if (frm.doc.docstatus == 1) {
+		if (frm.doc.docstatus == 1 && !frm.doc.reversal_of) {
 			frm.add_custom_button(
 				__("Reverse Journal Entry"),
 				function () {

erpnext/accounts/doctype/journal_entry/journal_entry.py

@@ -1777,6 +1777,20 @@ def make_inter_company_journal_entry(name, voucher_type, company):
 
 @frappe.whitelist()
 def make_reverse_journal_entry(source_name, target_doc=None):
+	# `get_mapped_doc` checks this as well, but the guards below disclose which entry
+	# reverses which, so read access has to be settled before they run
+	if not frappe.has_permission("Journal Entry", doc=source_name):
+		frappe.throw(_("Not permitted"), frappe.PermissionError)
+
+	reversal_of = frappe.db.get_value("Journal Entry", source_name, "reversal_of")
+	if reversal_of:
+		frappe.throw(
+			_("{0} is already a Reverse Journal Entry of {1}. Cancel it instead of reversing it.").format(
+				get_link_to_form("Journal Entry", source_name),
+				get_link_to_form("Journal Entry", reversal_of),
+			)
+		)
+
 	existing_reverse = frappe.db.exists("Journal Entry", {"reversal_of": source_name, "docstatus": 1})
 	if existing_reverse:
 		frappe.throw(

erpnext/accounts/doctype/journal_entry/test_journal_entry.py

@@ -249,7 +249,7 @@ class TestJournalEntry(ERPNextTestSuite):
 		self.check_gl_entries()
 
 	def test_disallow_reversal_of_a_reversal_journal_entry(self):
-		from erpnext.accounts.doctype.journal_entry.mapper import make_reverse_journal_entry
+		from erpnext.accounts.doctype.journal_entry.journal_entry import make_reverse_journal_entry
 
 		jv = make_journal_entry("_Test Bank - _TC", "Sales - _TC", 100, submit=True)