From: Phil Sutter <phil@nwl.cc>
Date: Fri, 22 Mar 2024 12:31:10 +0000 (+0100)
Subject: mergesort: Avoid accidental set element reordering
X-Git-Tag: v1.1.0~52
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=bf52af188b306acf5a30134d6a670f41f16a9459;p=thirdparty%2Fnftables.git

mergesort: Avoid accidental set element reordering

In corner cases, expr_msort_cmp() may return 0 for two non-identical
elements. An example are ORed tcp flags: 'syn' and 'syn | ack' are
considered the same value since expr_msort_value() reduces the latter to
its LHS.

Keeping the above in mind and looking at how list_expr_sort() works: The
list in 'head' is cut in half, the first half put into the temporary
list 'list' and finally 'list' is merged back into 'head' considering
each element's position. Shall expr_msort_cmp() return 0 for two
elements, the one from 'list' ends up after the one in 'head', thus
reverting their previous ordering.

The practical implication is that output never matches input for the
sample set '{ syn, syn | ack }' as the sorting after delinearization in
netlink_list_setelems() keeps swapping the elements. Out of coincidence,
the commit this fixes itself illustrates the use-case this breaks,
namely tracking a ruleset in git: Each ruleset reload will trigger an
update to the stored dump.

This change breaks interval set element deletion because __set_delete()
implicitly relies upon this reordering of duplicate entries by inserting
a clone of the one to delete into the start (via list_move()) and after
sorting assumes the clone will end up right behind the original. Fix
this by calling list_move_tail() instead.

Fixes: 14ee0a979b622 ("src: sort set elements in netlink_get_setelems()")
Signed-off-by: Phil Sutter <phil@nwl.cc>
---

diff --git a/src/intervals.c b/src/intervals.c
index 68728349..6c3f36fe 100644
--- a/src/intervals.c
+++ b/src/intervals.c
@@ -466,7 +466,7 @@ static int __set_delete(struct list_head *msgs, struct expr *i,	struct set *set,
 			unsigned int debug_mask)
 {
 	i->flags |= EXPR_F_REMOVE;
-	list_move(&i->list, &existing_set->init->expressions);
+	list_move_tail(&i->list, &existing_set->init->expressions);
 	list_expr_sort(&existing_set->init->expressions);
 
 	return setelem_delete(msgs, set, init, existing_set->init, debug_mask);
diff --git a/src/mergesort.c b/src/mergesort.c
index 4d0e280f..5e676be1 100644
--- a/src/mergesort.c
+++ b/src/mergesort.c
@@ -78,7 +78,7 @@ void list_splice_sorted(struct list_head *list, struct list_head *head)
 	while (l != list) {
 		if (h == head ||
 		    expr_msort_cmp(list_entry(l, typeof(struct expr), list),
-				   list_entry(h, typeof(struct expr), list)) < 0) {
+				   list_entry(h, typeof(struct expr), list)) <= 0) {
 			l = l->next;
 			list_add_tail(l->prev, h);
 			continue;
diff --git a/tests/shell/testcases/sets/dumps/0055tcpflags_0.json-nft b/tests/shell/testcases/sets/dumps/0055tcpflags_0.json-nft
index 6a351151..e37139f3 100644
--- a/tests/shell/testcases/sets/dumps/0055tcpflags_0.json-nft
+++ b/tests/shell/testcases/sets/dumps/0055tcpflags_0.json-nft
@@ -28,15 +28,6 @@
           {
             "|": [
               "fin",
-              "psh",
-              "ack",
-              "urg"
-            ]
-          },
-          {
-            "|": [
-              "fin",
-              "psh",
               "ack"
             ]
           },
@@ -50,21 +41,22 @@
           {
             "|": [
               "fin",
+              "psh",
               "ack"
             ]
           },
           {
             "|": [
-              "syn",
+              "fin",
               "psh",
               "ack",
               "urg"
             ]
           },
+          "syn",
           {
             "|": [
               "syn",
-              "psh",
               "ack"
             ]
           },
@@ -78,22 +70,22 @@
           {
             "|": [
               "syn",
+              "psh",
               "ack"
             ]
           },
-          "syn",
           {
             "|": [
-              "rst",
+              "syn",
               "psh",
               "ack",
               "urg"
             ]
           },
+          "rst",
           {
             "|": [
               "rst",
-              "psh",
               "ack"
             ]
           },
@@ -107,12 +99,13 @@
           {
             "|": [
               "rst",
+              "psh",
               "ack"
             ]
           },
-          "rst",
           {
             "|": [
+              "rst",
               "psh",
               "ack",
               "urg"
@@ -126,11 +119,18 @@
           },
           {
             "|": [
+              "psh",
               "ack",
               "urg"
             ]
           },
-          "ack"
+          "ack",
+          {
+            "|": [
+              "ack",
+              "urg"
+            ]
+          }
         ]
       }
     }
diff --git a/tests/shell/testcases/sets/dumps/0055tcpflags_0.nft b/tests/shell/testcases/sets/dumps/0055tcpflags_0.nft
index ffed5426..22bf5c46 100644
--- a/tests/shell/testcases/sets/dumps/0055tcpflags_0.nft
+++ b/tests/shell/testcases/sets/dumps/0055tcpflags_0.nft
@@ -2,9 +2,9 @@ table ip test {
 	set tcp_good_flags {
 		type tcp_flag
 		flags constant
-		elements = { fin | psh | ack | urg, fin | psh | ack, fin | ack | urg, fin | ack, syn | psh | ack | urg,
-			     syn | psh | ack, syn | ack | urg, syn | ack, syn, rst | psh | ack | urg,
-			     rst | psh | ack, rst | ack | urg, rst | ack, rst, psh | ack | urg,
-			     psh | ack, ack | urg, ack }
+		elements = { fin | ack, fin | ack | urg, fin | psh | ack, fin | psh | ack | urg, syn,
+			     syn | ack, syn | ack | urg, syn | psh | ack, syn | psh | ack | urg, rst,
+			     rst | ack, rst | ack | urg, rst | psh | ack, rst | psh | ack | urg, psh | ack,
+			     psh | ack | urg, ack, ack | urg }
 	}
 }