sfq: fix sfq class stats handling

sfq_walk() runs without qdisc lock. By the time it selects a non empty
hash slot and sfq_dump_class_stats() is run (with lock held), slot might
have been freed : We then access q->slots[SFQ_EMPTY_SLOT], out of
bounds, and crash in slot_queue_walk()

On previous kernels, bug is here but out of bounds qs[SFQ_DEPTH] and
allot[SFQ_DEPTH] are located in struct sfq_sched_data, so no illegal
memory access happens, only possibly wrong data reported to user.

Also, slot_dequeue_tail() should make sure slot skb chain is correctly
terminated, or sfq_dump_class_stats() can access freed skbs.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 11 insertions, 5 deletions

diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 13322e8a0456..6a2f88fea6d8 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -281,6 +281,7 @@ static inline struct sk_buff *slot_dequeue_tail(struct sfq_slot *slot)
         struct sk_buff *skb = slot->skblist_prev;
 
         slot->skblist_prev = skb->prev;
+        skb->prev->next = (struct sk_buff *)slot;
         skb->next = skb->prev = NULL;
         return skb;
 }
@@ -608,14 +609,19 @@ static int sfq_dump_class_stats(struct Qdisc *sch, unsigned long cl,
                                 struct gnet_dump *d)
 {
         struct sfq_sched_data *q = qdisc_priv(sch);
-        const struct sfq_slot *slot = &q->slots[q->ht[cl - 1]];
-        struct gnet_stats_queue qs = { .qlen = slot->qlen };
-        struct tc_sfq_xstats xstats = { .allot = slot->allot };
+        sfq_index idx = q->ht[cl - 1];
+        struct gnet_stats_queue qs = { 0 };
+        struct tc_sfq_xstats xstats = { 0 };
         struct sk_buff *skb;
 
-        slot_queue_walk(slot, skb)
-                qs.backlog += qdisc_pkt_len(skb);
+        if (idx != SFQ_EMPTY_SLOT) {
+                const struct sfq_slot *slot = &q->slots[idx];
 
+                xstats.allot = slot->allot;
+                qs.qlen = slot->qlen;
+                slot_queue_walk(slot, skb)
+                        qs.backlog += qdisc_pkt_len(skb);
+        }
         if (gnet_stats_copy_queue(d, &qs) < 0)
                 return -1;
         return gnet_stats_copy_app(d, &xstats, sizeof(xstats));