[PATCH] netem: fix return value if duplicate enqueue fails

[PATCH] netem: fix return value if duplicate enqueue fails

[Stephen Hemminger]

There is a bug in netem_enqueue() introduced by
commit 5845f706388a ("net: netem: fix skb length BUG_ON in __skb_to_sgvec")
that can lead to a use-after-free.

This commit made netem_enqueue() always return NET_XMIT_SUCCESS
when a packet is duplicated, which can cause the parent qdisc's q.qlen to be
mistakenly incremented. When this happens qlen_notify() may be skipped on the
parent during destruction, leaving a dangling pointer for some classful qdiscs
like DRR.

There are two ways for the bug happen:

- If the duplicated packet is dropped by rootq->enqueue() and then the original
  packet is also dropped.
- If rootq->enqueue() sends the duplicated packet to a different qdisc and the
  original packet is dropped.

In both cases NET_XMIT_SUCCESS is returned even though no packets are enqueued
at the netem qdisc.

The fix is to defer the enqueue of the duplicate packet until after the
original packet has been guaranteed to return NET_XMIT_SUCCESS.

Fixes: 5845f706388a ("net: netem: fix skb length BUG_ON in __skb_to_sgvec")
Reported-by: Budimir Markovic <markovicbudimir@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 net/sched/sch_netem.c | 47 ++++++++++++++++++++++++++-----------------
 1 file changed, 29 insertions(+), 18 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index edc72962ae63..0f8d581438c3 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -446,12 +446,10 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	struct netem_sched_data *q = qdisc_priv(sch);
 	/* We don't fill cb now as skb_unshare() may invalidate it */
 	struct netem_skb_cb *cb;
-	struct sk_buff *skb2;
+	struct sk_buff *skb2 = NULL;
 	struct sk_buff *segs = NULL;
 	unsigned int prev_len = qdisc_pkt_len(skb);
 	int count = 1;
-	int rc = NET_XMIT_SUCCESS;
-	int rc_drop = NET_XMIT_DROP;
 
 	/* Do not fool qdisc_drop_all() */
 	skb->prev = NULL;
@@ -480,19 +478,11 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		skb_orphan_partial(skb);
 
 	/*
-	 * If we need to duplicate packet, then re-insert at top of the
-	 * qdisc tree, since parent queuer expects that only one
-	 * skb will be queued.
+	 * If we need to duplicate packet, then clone it before
+	 * original is modified.
 	 */
-	if (count > 1 && (skb2 = skb_clone(skb, GFP_ATOMIC)) != NULL) {
-		struct Qdisc *rootq = qdisc_root_bh(sch);
-		u32 dupsave = q->duplicate; /* prevent duplicating a dup... */
-
-		q->duplicate = 0;
-		rootq->enqueue(skb2, rootq, to_free);
-		q->duplicate = dupsave;
-		rc_drop = NET_XMIT_SUCCESS;
-	}
+	if (count > 1)
+		skb2 = skb_clone(skb, GFP_ATOMIC);
 
 	/*
 	 * Randomized packet corruption.
@@ -504,7 +494,8 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		if (skb_is_gso(skb)) {
 			skb = netem_segment(skb, sch, to_free);
 			if (!skb)
-				return rc_drop;
+				goto finish_segs;
+
 			segs = skb->next;
 			skb_mark_not_on_list(skb);
 			qdisc_skb_cb(skb)->pkt_len = skb->len;
@@ -530,7 +521,24 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		/* re-link segs, so that qdisc_drop_all() frees them all */
 		skb->next = segs;
 		qdisc_drop_all(skb, sch, to_free);
-		return rc_drop;
+		if (skb2)
+			__qdisc_drop(skb2, to_free);
+		return NET_XMIT_DROP;
+	}
+
+	/*
+	 * If doing duplication then re-insert at top of the
+	 * qdisc tree, since parent queuer expects that only one
+	 * skb will be queued.
+	 */
+	if (skb2) {
+		struct Qdisc *rootq = qdisc_root_bh(sch);
+		u32 dupsave = q->duplicate; /* prevent duplicating a dup... */
+
+		q->duplicate = 0;
+		rootq->enqueue(skb2, rootq, to_free);
+		q->duplicate = dupsave;
+		skb2 = NULL;
 	}
 
 	qdisc_qstats_backlog_inc(sch, skb);
@@ -601,9 +609,12 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	}
 
 finish_segs:
+	if (skb2)
+		__qdisc_drop(skb2, to_free);
+
 	if (segs) {
 		unsigned int len, last_len;
-		int nb;
+		int rc, nb;
 
 		len = skb ? skb->len : 0;
 		nb = skb ? 1 : 0;
-- 
2.43.0

Re: [PATCH] netem: fix return value if duplicate enqueue fails

[Simon Horman]

On Mon, Aug 19, 2024 at 10:56:45AM -0700, Stephen Hemminger wrote:
> There is a bug in netem_enqueue() introduced by
> commit 5845f706388a ("net: netem: fix skb length BUG_ON in __skb_to_sgvec")
> that can lead to a use-after-free.
> 
> This commit made netem_enqueue() always return NET_XMIT_SUCCESS
> when a packet is duplicated, which can cause the parent qdisc's q.qlen to be
> mistakenly incremented. When this happens qlen_notify() may be skipped on the
> parent during destruction, leaving a dangling pointer for some classful qdiscs
> like DRR.
> 
> There are two ways for the bug happen:
> 
> - If the duplicated packet is dropped by rootq->enqueue() and then the original
>   packet is also dropped.
> - If rootq->enqueue() sends the duplicated packet to a different qdisc and the
>   original packet is dropped.
> 
> In both cases NET_XMIT_SUCCESS is returned even though no packets are enqueued
> at the netem qdisc.
> 
> The fix is to defer the enqueue of the duplicate packet until after the
> original packet has been guaranteed to return NET_XMIT_SUCCESS.
> 
> Fixes: 5845f706388a ("net: netem: fix skb length BUG_ON in __skb_to_sgvec")
> Reported-by: Budimir Markovic <markovicbudimir@gmail.com>
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

Thanks Stephen,

The code changes all make sense to me.

Reviewed-by: Simon Horman <horms@kernel.org>

Re: [PATCH] netem: fix return value if duplicate enqueue fails

[Stephen Hemminger]

On Tue, 20 Aug 2024 16:42:13 +0100
Simon Horman <horms@kernel.org> wrote:

> On Mon, Aug 19, 2024 at 10:56:45AM -0700, Stephen Hemminger wrote:
> > There is a bug in netem_enqueue() introduced by
> > commit 5845f706388a ("net: netem: fix skb length BUG_ON in __skb_to_sgvec")
> > that can lead to a use-after-free.
> > 
> > This commit made netem_enqueue() always return NET_XMIT_SUCCESS
> > when a packet is duplicated, which can cause the parent qdisc's q.qlen to be
> > mistakenly incremented. When this happens qlen_notify() may be skipped on the
> > parent during destruction, leaving a dangling pointer for some classful qdiscs
> > like DRR.
> > 
> > There are two ways for the bug happen:
> > 
> > - If the duplicated packet is dropped by rootq->enqueue() and then the original
> >   packet is also dropped.
> > - If rootq->enqueue() sends the duplicated packet to a different qdisc and the
> >   original packet is dropped.
> > 
> > In both cases NET_XMIT_SUCCESS is returned even though no packets are enqueued
> > at the netem qdisc.
> > 
> > The fix is to defer the enqueue of the duplicate packet until after the
> > original packet has been guaranteed to return NET_XMIT_SUCCESS.
> > 
> > Fixes: 5845f706388a ("net: netem: fix skb length BUG_ON in __skb_to_sgvec")
> > Reported-by: Budimir Markovic <markovicbudimir@gmail.com>
> > Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>  
> 
> Thanks Stephen,
> 
> The code changes all make sense to me.
> 
> Reviewed-by: Simon Horman <horms@kernel.org>

Reported by doesn't really do enough credit here. The commit log is from
the original bug report which had more detail. Fixing it was the easy part.

Re: [PATCH] netem: fix return value if duplicate enqueue fails

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon, 19 Aug 2024 10:56:45 -0700 you wrote:
> There is a bug in netem_enqueue() introduced by
> commit 5845f706388a ("net: netem: fix skb length BUG_ON in __skb_to_sgvec")
> that can lead to a use-after-free.
> 
> This commit made netem_enqueue() always return NET_XMIT_SUCCESS
> when a packet is duplicated, which can cause the parent qdisc's q.qlen to be
> mistakenly incremented. When this happens qlen_notify() may be skipped on the
> parent during destruction, leaving a dangling pointer for some classful qdiscs
> like DRR.
> 
> [...]

Here is the summary with links:
  - netem: fix return value if duplicate enqueue fails
    https://git.kernel.org/netdev/net/c/c07ff8592d57

You are awesome, thank you!
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