af_unix: Fix garbage collector racing against connect()

jira LE-2015
cve CVE-2024-26923
Rebuild_History Non-Buildable kernel-5.14.0-427.42.1.el9_4
commit-author Michal Luczaj <mhal@rbox.co>
commit 47d8ac0
Empty-Commit: Cherry-Pick Conflicts during history rebuild.
Will be included in final tarball splat. Ref for failed cherry-pick at:
ciq/ciq_backports/kernel-5.14.0-427.42.1.el9_4/47d8ac01.failed

Garbage collector does not take into account the risk of embryo getting
enqueued during the garbage collection. If such embryo has a peer that
carries SCM_RIGHTS, two consecutive passes of scan_children() may see a
different set of children. Leading to an incorrectly elevated inflight
count, and then a dangling pointer within the gc_inflight_list.

sockets are AF_UNIX/SOCK_STREAM
S is an unconnected socket
L is a listening in-flight socket bound to addr, not in fdtable
V's fd will be passed via sendmsg(), gets inflight count bumped

connect(S, addr)	sendmsg(S, [V]); close(V)	__unix_gc()
----------------	-------------------------	-----------

NS = unix_create1()
skb1 = sock_wmalloc(NS)
L = unix_find_other(addr)
unix_state_lock(L)
unix_peer(S) = NS
			// V count=1 inflight=0

 			NS = unix_peer(S)
 			skb2 = sock_alloc()
			skb_queue_tail(NS, skb2[V])

			// V became in-flight
			// V count=2 inflight=1

			close(V)

			// V count=1 inflight=1
			// GC candidate condition met

						for u in gc_inflight_list:
						  if (total_refs == inflight_refs)
						    add u to gc_candidates

						// gc_candidates={L, V}

						for u in gc_candidates:
						  scan_children(u, dec_inflight)

						// embryo (skb1) was not
						// reachable from L yet, so V's
						// inflight remains unchanged
__skb_queue_tail(L, skb1)
unix_state_unlock(L)
						for u in gc_candidates:
						  if (u.inflight)
						    scan_children(u, inc_inflight_move_tail)

						// V count=1 inflight=2 (!)

If there is a GC-candidate listening socket, lock/unlock its state. This
makes GC wait until the end of any ongoing connect() to that socket. After
flipping the lock, a possibly SCM-laden embryo is already enqueued. And if
there is another embryo coming, it can not possibly carry SCM_RIGHTS. At
this point, unix_inflight() can not happen because unix_gc_lock is already
taken. Inflight graph remains unaffected.

Fixes: 1fd05ba ("[AF_UNIX]: Rewrite garbage collector, fixes race.")
	Signed-off-by: Michal Luczaj <mhal@rbox.co>
	Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240409201047.1032217-1-mhal@rbox.co
	Signed-off-by: Paolo Abeni <pabeni@redhat.com>
(cherry picked from commit 47d8ac0)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

# Conflicts:
#	net/unix/garbage.c

Author: PlaidCat

ciq/ciq_backports/kernel-5.14.0-427.42.1.el9_4/47d8ac01.failed

@@ -0,0 +1,122 @@
+af_unix: Fix garbage collector racing against connect()
+
+jira LE-2015
+cve CVE-2024-26923
+Rebuild_History Non-Buildable kernel-5.14.0-427.42.1.el9_4
+commit-author Michal Luczaj <mhal@rbox.co>
+commit 47d8ac011fe1c9251070e1bd64cb10b48193ec51
+Empty-Commit: Cherry-Pick Conflicts during history rebuild.
+Will be included in final tarball splat. Ref for failed cherry-pick at:
+ciq/ciq_backports/kernel-5.14.0-427.42.1.el9_4/47d8ac01.failed
+
+Garbage collector does not take into account the risk of embryo getting
+enqueued during the garbage collection. If such embryo has a peer that
+carries SCM_RIGHTS, two consecutive passes of scan_children() may see a
+different set of children. Leading to an incorrectly elevated inflight
+count, and then a dangling pointer within the gc_inflight_list.
+
+sockets are AF_UNIX/SOCK_STREAM
+S is an unconnected socket
+L is a listening in-flight socket bound to addr, not in fdtable
+V's fd will be passed via sendmsg(), gets inflight count bumped
+
+connect(S, addr)	sendmsg(S, [V]); close(V)	__unix_gc()
+----------------	-------------------------	-----------
+
+NS = unix_create1()
+skb1 = sock_wmalloc(NS)
+L = unix_find_other(addr)
+unix_state_lock(L)
+unix_peer(S) = NS
+			// V count=1 inflight=0
+
+ 			NS = unix_peer(S)
+ 			skb2 = sock_alloc()
+			skb_queue_tail(NS, skb2[V])
+
+			// V became in-flight
+			// V count=2 inflight=1
+
+			close(V)
+
+			// V count=1 inflight=1
+			// GC candidate condition met
+
+						for u in gc_inflight_list:
+						  if (total_refs == inflight_refs)
+						    add u to gc_candidates
+
+						// gc_candidates={L, V}
+
+						for u in gc_candidates:
+						  scan_children(u, dec_inflight)
+
+						// embryo (skb1) was not
+						// reachable from L yet, so V's
+						// inflight remains unchanged
+__skb_queue_tail(L, skb1)
+unix_state_unlock(L)
+						for u in gc_candidates:
+						  if (u.inflight)
+						    scan_children(u, inc_inflight_move_tail)
+
+						// V count=1 inflight=2 (!)
+
+If there is a GC-candidate listening socket, lock/unlock its state. This
+makes GC wait until the end of any ongoing connect() to that socket. After
+flipping the lock, a possibly SCM-laden embryo is already enqueued. And if
+there is another embryo coming, it can not possibly carry SCM_RIGHTS. At
+this point, unix_inflight() can not happen because unix_gc_lock is already
+taken. Inflight graph remains unaffected.
+
+Fixes: 1fd05ba5a2f2 ("[AF_UNIX]: Rewrite garbage collector, fixes race.")
+	Signed-off-by: Michal Luczaj <mhal@rbox.co>
+	Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
+Link: https://lore.kernel.org/r/20240409201047.1032217-1-mhal@rbox.co
+	Signed-off-by: Paolo Abeni <pabeni@redhat.com>
+(cherry picked from commit 47d8ac011fe1c9251070e1bd64cb10b48193ec51)
+	Signed-off-by: Jonathan Maple <jmaple@ciq.com>
+
+# Conflicts:
+#	net/unix/garbage.c
+diff --cc net/unix/garbage.c
+index 2405f0f9af31,6433a414acf8..000000000000
+--- a/net/unix/garbage.c
++++ b/net/unix/garbage.c
+@@@ -234,17 -274,26 +234,32 @@@ void unix_gc(void
+  	 * receive queues.  Other, non candidate sockets _can_ be
+  	 * added to queue, so we must make sure only to touch
+  	 * candidates.
++ 	 *
++ 	 * Embryos, though never candidates themselves, affect which
++ 	 * candidates are reachable by the garbage collector.  Before
++ 	 * being added to a listener's queue, an embryo may already
++ 	 * receive data carrying SCM_RIGHTS, potentially making the
++ 	 * passed socket a candidate that is not yet reachable by the
++ 	 * collector.  It becomes reachable once the embryo is
++ 	 * enqueued.  Therefore, we must ensure that no SCM-laden
++ 	 * embryo appears in a (candidate) listener's queue between
++ 	 * consecutive scan_children() calls.
+  	 */
+  	list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
++ 		struct sock *sk = &u->sk;
+  		long total_refs;
+ +		long inflight_refs;
+  
+++<<<<<<< HEAD
+ +		total_refs = file_count(u->sk.sk_socket->file);
+ +		inflight_refs = atomic_long_read(&u->inflight);
+++=======
++ 		total_refs = file_count(sk->sk_socket->file);
+++>>>>>>> 47d8ac011fe1 (af_unix: Fix garbage collector racing against connect())
+  
+ -		WARN_ON_ONCE(!u->inflight);
+ -		WARN_ON_ONCE(total_refs < u->inflight);
+ -		if (total_refs == u->inflight) {
+ +		BUG_ON(inflight_refs < 1);
+ +		BUG_ON(total_refs < inflight_refs);
+ +		if (total_refs == inflight_refs) {
+  			list_move_tail(&u->link, &gc_candidates);
+  			__set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
+  			__set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
+* Unmerged path net/unix/garbage.c