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* [Cake] [PATCH net] sched: sch_cake: add bounds checks to host bulk flow fairness counts
@ 2025-01-06 13:38 Toke Høiland-Jørgensen
  2025-01-07  3:14 ` kernel test robot
  0 siblings, 1 reply; 2+ messages in thread
From: Toke Høiland-Jørgensen @ 2025-01-06 13:38 UTC (permalink / raw)
  To: Toke Høiland-Jørgensen, Jamal Hadi Salim, Cong Wang,
	Jiri Pirko, Paolo Abeni
  Cc: Toke Høiland-Jørgensen, syzbot+f63600d288bfb7057424,
	David S. Miller, Eric Dumazet, Jakub Kicinski, Simon Horman,
	cake, netdev

Even though we fixed a logic error in the commit cited below, syzbot
still managed to trigger an underflow of the per-host bulk flow
counters, leading to an out of bounds memory access.

To avoid any such logic errors causing out of bounds memory accesses,
this commit factors out all accesses to the per-host bulk flow counters
to a series of helpers that perform bounds-checking before any
increments and decrements. This also has the benefit of improving
readability by moving the conditional checks for the flow mode into
these helpers, instead of having them spread out throughout the
code (which was the cause of the original logic error).

Fixes: 546ea84d07e3 ("sched: sch_cake: fix bulk flow accounting logic for host fairness")
Reported-by: syzbot+f63600d288bfb7057424@syzkaller.appspotmail.com
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 net/sched/sch_cake.c | 135 ++++++++++++++++++++++++-------------------
 1 file changed, 75 insertions(+), 60 deletions(-)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 8d8b2db4653c..8f61ecb78139 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -627,6 +627,63 @@ static bool cake_ddst(int flow_mode)
 	return (flow_mode & CAKE_FLOW_DUAL_DST) == CAKE_FLOW_DUAL_DST;
 }
 
+static void cake_dec_srchost_bulk_flow_count(struct cake_tin_data *q,
+					     struct cake_flow *flow,
+					     int flow_mode)
+{
+	if (likely(cake_dsrc(flow_mode) &&
+		   q->hosts[flow->srchost].srchost_bulk_flow_count))
+		q->hosts[flow->srchost].srchost_bulk_flow_count--;
+}
+
+static void cake_inc_srchost_bulk_flow_count(struct cake_tin_data *q,
+					     struct cake_flow *flow,
+					     int flow_mode)
+{
+	if (likely(cake_dsrc(flow_mode) &&
+		   q->hosts[flow->srchost].srchost_bulk_flow_count < CAKE_QUEUES))
+		q->hosts[flow->srchost].srchost_bulk_flow_count++;
+}
+
+static void cake_dec_dsthost_bulk_flow_count(struct cake_tin_data *q,
+					     struct cake_flow *flow,
+					     int flow_mode)
+{
+	if (likely(cake_ddst(flow_mode) &&
+		   q->hosts[flow->dsthost].dsthost_bulk_flow_count))
+		q->hosts[flow->dsthost].dsthost_bulk_flow_count--;
+}
+
+static void cake_inc_dsthost_bulk_flow_count(struct cake_tin_data *q,
+					     struct cake_flow *flow,
+					     int flow_mode)
+{
+	if (likely(cake_ddst(flow_mode) &&
+		   q->hosts[flow->dsthost].dsthost_bulk_flow_count < CAKE_QUEUES))
+		q->hosts[flow->dsthost].dsthost_bulk_flow_count++;
+}
+
+static u16 cake_get_flow_quantum(struct cake_tin_data *q,
+				 struct cake_flow *flow,
+				 int flow_mode)
+{
+	u16 host_load = 1;
+
+	if (cake_dsrc(flow_mode))
+		host_load = max(host_load,
+				q->hosts[flow->srchost].srchost_bulk_flow_count);
+
+	if (cake_ddst(flow_mode))
+		host_load = max(host_load,
+				q->hosts[flow->dsthost].dsthost_bulk_flow_count);
+
+	/* The get_random_u16() is a way to apply dithering to avoid
+	 * accumulating roundoff errors
+	 */
+	return (q->flow_quantum * quantum_div[host_load] +
+		get_random_u16()) >> 16;
+}
+
 static u32 cake_hash(struct cake_tin_data *q, const struct sk_buff *skb,
 		     int flow_mode, u16 flow_override, u16 host_override)
 {
@@ -773,10 +830,8 @@ static u32 cake_hash(struct cake_tin_data *q, const struct sk_buff *skb,
 		allocate_dst = cake_ddst(flow_mode);
 
 		if (q->flows[outer_hash + k].set == CAKE_SET_BULK) {
-			if (allocate_src)
-				q->hosts[q->flows[reduced_hash].srchost].srchost_bulk_flow_count--;
-			if (allocate_dst)
-				q->hosts[q->flows[reduced_hash].dsthost].dsthost_bulk_flow_count--;
+			cake_dec_srchost_bulk_flow_count(q, &q->flows[outer_hash + k], flow_mode);
+			cake_dec_dsthost_bulk_flow_count(q, &q->flows[outer_hash + k], flow_mode);
 		}
 found:
 		/* reserve queue for future packets in same flow */
@@ -801,9 +856,10 @@ static u32 cake_hash(struct cake_tin_data *q, const struct sk_buff *skb,
 			q->hosts[outer_hash + k].srchost_tag = srchost_hash;
 found_src:
 			srchost_idx = outer_hash + k;
-			if (q->flows[reduced_hash].set == CAKE_SET_BULK)
-				q->hosts[srchost_idx].srchost_bulk_flow_count++;
 			q->flows[reduced_hash].srchost = srchost_idx;
+
+			if (q->flows[reduced_hash].set == CAKE_SET_BULK)
+				cake_inc_srchost_bulk_flow_count(q, &q->flows[reduced_hash], flow_mode);
 		}
 
 		if (allocate_dst) {
@@ -824,9 +880,10 @@ static u32 cake_hash(struct cake_tin_data *q, const struct sk_buff *skb,
 			q->hosts[outer_hash + k].dsthost_tag = dsthost_hash;
 found_dst:
 			dsthost_idx = outer_hash + k;
-			if (q->flows[reduced_hash].set == CAKE_SET_BULK)
-				q->hosts[dsthost_idx].dsthost_bulk_flow_count++;
 			q->flows[reduced_hash].dsthost = dsthost_idx;
+
+			if (q->flows[reduced_hash].set == CAKE_SET_BULK)
+				cake_inc_dsthost_bulk_flow_count(q, &q->flows[reduced_hash], flow_mode);
 		}
 	}
 
@@ -1839,10 +1896,6 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 
 	/* flowchain */
 	if (!flow->set || flow->set == CAKE_SET_DECAYING) {
-		struct cake_host *srchost = &b->hosts[flow->srchost];
-		struct cake_host *dsthost = &b->hosts[flow->dsthost];
-		u16 host_load = 1;
-
 		if (!flow->set) {
 			list_add_tail(&flow->flowchain, &b->new_flows);
 		} else {
@@ -1852,18 +1905,8 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		flow->set = CAKE_SET_SPARSE;
 		b->sparse_flow_count++;
 
-		if (cake_dsrc(q->flow_mode))
-			host_load = max(host_load, srchost->srchost_bulk_flow_count);
-
-		if (cake_ddst(q->flow_mode))
-			host_load = max(host_load, dsthost->dsthost_bulk_flow_count);
-
-		flow->deficit = (b->flow_quantum *
-				 quantum_div[host_load]) >> 16;
+		flow->deficit = cake_get_flow_quantum(b, flow, q->flow_mode);
 	} else if (flow->set == CAKE_SET_SPARSE_WAIT) {
-		struct cake_host *srchost = &b->hosts[flow->srchost];
-		struct cake_host *dsthost = &b->hosts[flow->dsthost];
-
 		/* this flow was empty, accounted as a sparse flow, but actually
 		 * in the bulk rotation.
 		 */
@@ -1871,12 +1914,8 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		b->sparse_flow_count--;
 		b->bulk_flow_count++;
 
-		if (cake_dsrc(q->flow_mode))
-			srchost->srchost_bulk_flow_count++;
-
-		if (cake_ddst(q->flow_mode))
-			dsthost->dsthost_bulk_flow_count++;
-
+		cake_inc_srchost_bulk_flow_count(b, flow, q->flow_mode);
+		cake_inc_dsthost_bulk_flow_count(b, flow, q->flow_mode);
 	}
 
 	if (q->buffer_used > q->buffer_max_used)
@@ -1939,7 +1978,6 @@ static struct sk_buff *cake_dequeue(struct Qdisc *sch)
 	struct list_head *head;
 	bool first_flow = true;
 	struct sk_buff *skb;
-	u16 host_load;
 	u64 delay;
 	u32 len;
 
@@ -2042,7 +2080,6 @@ static struct sk_buff *cake_dequeue(struct Qdisc *sch)
 	/* triple isolation (modified DRR++) */
 	srchost = &b->hosts[flow->srchost];
 	dsthost = &b->hosts[flow->dsthost];
-	host_load = 1;
 
 	/* flow isolation (DRR++) */
 	if (flow->deficit <= 0) {
@@ -2055,11 +2092,8 @@ static struct sk_buff *cake_dequeue(struct Qdisc *sch)
 				b->sparse_flow_count--;
 				b->bulk_flow_count++;
 
-				if (cake_dsrc(q->flow_mode))
-					srchost->srchost_bulk_flow_count++;
-
-				if (cake_ddst(q->flow_mode))
-					dsthost->dsthost_bulk_flow_count++;
+				cake_inc_srchost_bulk_flow_count(b, flow, q->flow_mode);
+				cake_inc_dsthost_bulk_flow_count(b, flow, q->flow_mode);
 
 				flow->set = CAKE_SET_BULK;
 			} else {
@@ -2071,19 +2105,7 @@ static struct sk_buff *cake_dequeue(struct Qdisc *sch)
 			}
 		}
 
-		if (cake_dsrc(q->flow_mode))
-			host_load = max(host_load, srchost->srchost_bulk_flow_count);
-
-		if (cake_ddst(q->flow_mode))
-			host_load = max(host_load, dsthost->dsthost_bulk_flow_count);
-
-		WARN_ON(host_load > CAKE_QUEUES);
-
-		/* The get_random_u16() is a way to apply dithering to avoid
-		 * accumulating roundoff errors
-		 */
-		flow->deficit += (b->flow_quantum * quantum_div[host_load] +
-				  get_random_u16()) >> 16;
+		flow->deficit += cake_get_flow_quantum(b, flow, q->flow_mode);
 		list_move_tail(&flow->flowchain, &b->old_flows);
 
 		goto retry;
@@ -2107,11 +2129,8 @@ static struct sk_buff *cake_dequeue(struct Qdisc *sch)
 				if (flow->set == CAKE_SET_BULK) {
 					b->bulk_flow_count--;
 
-					if (cake_dsrc(q->flow_mode))
-						srchost->srchost_bulk_flow_count--;
-
-					if (cake_ddst(q->flow_mode))
-						dsthost->dsthost_bulk_flow_count--;
+					cake_dec_srchost_bulk_flow_count(b, flow, q->flow_mode);
+					cake_dec_dsthost_bulk_flow_count(b, flow, q->flow_mode);
 
 					b->decaying_flow_count++;
 				} else if (flow->set == CAKE_SET_SPARSE ||
@@ -2129,12 +2148,8 @@ static struct sk_buff *cake_dequeue(struct Qdisc *sch)
 				else if (flow->set == CAKE_SET_BULK) {
 					b->bulk_flow_count--;
 
-					if (cake_dsrc(q->flow_mode))
-						srchost->srchost_bulk_flow_count--;
-
-					if (cake_ddst(q->flow_mode))
-						dsthost->dsthost_bulk_flow_count--;
-
+					cake_dec_srchost_bulk_flow_count(b, flow, q->flow_mode);
+					cake_dec_dsthost_bulk_flow_count(b, flow, q->flow_mode);
 				} else
 					b->decaying_flow_count--;
 
-- 
2.47.1


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