6.6-stable review patch. If anyone has any objections, please let me know.
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From: Kuniyuki Iwashima kuniyu@google.com
[ Upstream commit 60e6489f8e3b086bd1130ad4450a2c112e863791 ]
Quang Le reported that the AF_UNIX GC could garbage-collect a receive queue of an alive in-flight socket, with a nice repro.
The repro consists of three stages.
1) 1-a. Create a single cyclic reference with many sockets 1-b. close() all sockets 1-c. Trigger GC
2) 2-a. Pass sk-A to an embryo sk-B 2-b. Pass sk-X to sk-X 2-c. Trigger GC
3) 3-a. accept() the embryo sk-B 3-b. Pass sk-B to sk-C 3-c. close() the in-flight sk-A 3-d. Trigger GC
As of 2-c, sk-A and sk-X are linked to unix_unvisited_vertices, and unix_walk_scc() groups them into two different SCCs:
unix_sk(sk-A)->vertex->scc_index = 2 (UNIX_VERTEX_INDEX_START) unix_sk(sk-X)->vertex->scc_index = 3
Once GC completes, unix_graph_grouped is set to true. Also, unix_graph_maybe_cyclic is set to true due to sk-X's cyclic self-reference, which makes close() trigger GC.
At 3-b, unix_add_edge() allocates unix_sk(sk-B)->vertex and links it to unix_unvisited_vertices.
unix_update_graph() is called at 3-a. and 3-b., but neither unix_graph_grouped nor unix_graph_maybe_cyclic is changed because both sk-B's listener and sk-C are not in-flight.
3-c decrements sk-A's file refcnt to 1.
Since unix_graph_grouped is true at 3-d, unix_walk_scc_fast() is finally called and iterates 3 sockets sk-A, sk-B, and sk-X:
sk-A -> sk-B (-> sk-C) sk-X -> sk-X
This is totally fine. All of them are not yet close()d and should be grouped into different SCCs.
However, unix_vertex_dead() misjudges that sk-A and sk-B are in the same SCC and sk-A is dead.
unix_sk(sk-A)->scc_index == unix_sk(sk-B)->scc_index <-- Wrong! && sk-A's file refcnt == unix_sk(sk-A)->vertex->out_degree ^-- 1 in-flight count for sk-B -> sk-A is dead !?
The problem is that unix_add_edge() does not initialise scc_index.
Stage 1) is used for heap spraying, making a newly allocated vertex have vertex->scc_index == 2 (UNIX_VERTEX_INDEX_START) set by unix_walk_scc() at 1-c.
Let's track the max SCC index from the previous unix_walk_scc() call and assign the max + 1 to a new vertex's scc_index.
This way, we can continue to avoid Tarjan's algorithm while preventing misjudgments.
Fixes: ad081928a8b0 ("af_unix: Avoid Tarjan's algorithm if unnecessary.") Reported-by: Quang Le quanglex97@gmail.com Signed-off-by: Kuniyuki Iwashima kuniyu@google.com Link: https://patch.msgid.link/20251109025233.3659187-1-kuniyu@google.com Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- net/unix/garbage.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)
diff --git a/net/unix/garbage.c b/net/unix/garbage.c index 0068e758be4dd..66fd606c43f45 100644 --- a/net/unix/garbage.c +++ b/net/unix/garbage.c @@ -136,6 +136,7 @@ enum unix_vertex_index { };
static unsigned long unix_vertex_unvisited_index = UNIX_VERTEX_INDEX_MARK1; +static unsigned long unix_vertex_max_scc_index = UNIX_VERTEX_INDEX_START;
static void unix_add_edge(struct scm_fp_list *fpl, struct unix_edge *edge) { @@ -144,6 +145,7 @@ static void unix_add_edge(struct scm_fp_list *fpl, struct unix_edge *edge) if (!vertex) { vertex = list_first_entry(&fpl->vertices, typeof(*vertex), entry); vertex->index = unix_vertex_unvisited_index; + vertex->scc_index = ++unix_vertex_max_scc_index; vertex->out_degree = 0; INIT_LIST_HEAD(&vertex->edges); INIT_LIST_HEAD(&vertex->scc_entry); @@ -480,10 +482,15 @@ static void __unix_walk_scc(struct unix_vertex *vertex, unsigned long *last_inde scc_dead = unix_vertex_dead(v); }
- if (scc_dead) + if (scc_dead) { unix_collect_skb(&scc, hitlist); - else if (!unix_graph_maybe_cyclic) - unix_graph_maybe_cyclic = unix_scc_cyclic(&scc); + } else { + if (unix_vertex_max_scc_index < vertex->scc_index) + unix_vertex_max_scc_index = vertex->scc_index; + + if (!unix_graph_maybe_cyclic) + unix_graph_maybe_cyclic = unix_scc_cyclic(&scc); + }
list_del(&scc); } @@ -498,6 +505,7 @@ static void unix_walk_scc(struct sk_buff_head *hitlist) unsigned long last_index = UNIX_VERTEX_INDEX_START;
unix_graph_maybe_cyclic = false; + unix_vertex_max_scc_index = UNIX_VERTEX_INDEX_START;
/* Visit every vertex exactly once. * __unix_walk_scc() moves visited vertices to unix_visited_vertices.