[SRU][M/F][PATCH 0/2] CVE-2024-26923

[Bethany Jamison]

[Impact]

af_unix: Fix garbage collector racing against connect()

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.

[Fix]

Noble:	fixed via stable
Mantic:	Clean cherry-pick from fix and prereq commit
Jammy:	fixed via stable
Focal:	Mantic patch applied cleanly
Bionic:	fix sent to esm ML
Xenial: fix sent to esm ML
Trusty:	not going to be fixed by us

[Test Case]

Compile and boot tested

[Where problems could occur]

This fix affects those who use the AF_UNIX socket family, an issue with
this fix would be visable to the user via decreased system performance.

Kuniyuki Iwashima (1):
  af_unix: Do not use atomic ops for unix_sk(sk)->inflight.

Michal Luczaj (1):
  af_unix: Fix garbage collector racing against connect()

 include/net/af_unix.h |  2 +-
 net/unix/af_unix.c    |  4 ++--
 net/unix/garbage.c    | 35 +++++++++++++++++++++++++----------
 net/unix/scm.c        |  8 +++++---
 4 files changed, 33 insertions(+), 16 deletions(-)

-- 
2.34.1