Subject: Re: KASAN: slab-use-after-free Read in ip_finish_output

From: Florian Westphal <[email-masked]>
Date: Fri, 15 Mar 2024 01:07:58 +0100

Florian Westphal <[email-masked]> wrote:
> Eric Dumazet <[email-masked]> wrote:
> > Thanks for taking a look Florian.
> > 
> > Perhaps not messing with truesize at all would help ?
> >
> > Something based on this POC :
> > 
> >                 spin_lock(&qp->q.lock);
> > +               if (!qp->q.sk) {
> > +                       struct sock *sk = skb->sk;
> > 
> > +                       if (sk && refcount_inc_not_zero(&sk->sk_refcnt))
> > +                               qp->q.sk = sk;
> 
> Unfortunetely I did not get this to work.
> 
> sk_refcnt is 0.  sk is kept alive by sock_wfree destructor.
> 
> I don't know how to recover from this, refcnt cannot be "repaired"
> anymore.
> 
> I could artificially inflate sk_wmem counter by 1, to prevent release,
> but that needs yet another sock_wfree-like destructor.
> 
> I'm already not sure how this existing scheme works, there are
> multiple places that check for skb->destructor == sock_wfree,
> yet we have is_skb_wmem helper that checks __sock_wfree and tcp_wfree.
> 
> Removing defrag from output seems like best option, but it will
> surely break some scenarios.
> 
> Or, I could just fail reasm if sk refcount is already 0, that would
> likely work too?

I am going to test (and formally submit if no issues found):

inet: inet_defrag: prevent sk release while still in use

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 3023bc2be6a1..b1aeed510f24 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -753,8 +753,6 @@ typedef unsigned char *sk_buff_data_t;
  *	@list: queue head
  *	@ll_node: anchor in an llist (eg socket defer_list)
  *	@sk: Socket we are owned by
- *	@ip_defrag_offset: (aka @sk) alternate use of @sk, used in
- *		fragmentation management
  *	@dev: Device we arrived on/are leaving by
  *	@dev_scratch: (aka @dev) alternate use of @dev when @dev would be %NULL
  *	@cb: Control buffer. Free for use by every layer. Put private vars here
@@ -875,10 +873,7 @@ struct sk_buff {
 		struct llist_node	ll_node;
 	};
 
-	union {
-		struct sock		*sk;
-		int			ip_defrag_offset;
-	};
+	struct sock		*sk;
 
 	union {
 		ktime_t		tstamp;
diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 7072fc0783ef..fab1ea5df089 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -39,6 +39,7 @@ struct ipfrag_skb_cb {
 	};
 	struct sk_buff		*next_frag;
 	int			frag_run_len;
+	int			ip_defrag_offset;
 };
 
 #define FRAG_CB(skb)		((struct ipfrag_skb_cb *)((skb)->cb))
@@ -396,12 +397,12 @@ int inet_frag_queue_insert(struct inet_frag_queue *q, struct sk_buff *skb,
 	 */
 	if (!last)
 		fragrun_create(q, skb);  /* First fragment. */
-	else if (last->ip_defrag_offset + last->len < end) {
+	else if (FRAG_CB(last)->ip_defrag_offset + last->len < end) {
 		/* This is the common case: skb goes to the end. */
 		/* Detect and discard overlaps. */
-		if (offset < last->ip_defrag_offset + last->len)
+		if (offset < FRAG_CB(last)->ip_defrag_offset + last->len)
 			return IPFRAG_OVERLAP;
-		if (offset == last->ip_defrag_offset + last->len)
+		if (offset == FRAG_CB(last)->ip_defrag_offset + last->len)
 			fragrun_append_to_last(q, skb);
 		else
 			fragrun_create(q, skb);
@@ -418,13 +419,13 @@ int inet_frag_queue_insert(struct inet_frag_queue *q, struct sk_buff *skb,
 
 			parent = *rbn;
 			curr = rb_to_skb(parent);
-			curr_run_end = curr->ip_defrag_offset +
+			curr_run_end = FRAG_CB(curr)->ip_defrag_offset +
 					FRAG_CB(curr)->frag_run_len;
-			if (end <= curr->ip_defrag_offset)
+			if (end <= FRAG_CB(curr)->ip_defrag_offset)
 				rbn = &parent->rb_left;
 			else if (offset >= curr_run_end)
 				rbn = &parent->rb_right;
-			else if (offset >= curr->ip_defrag_offset &&
+			else if (offset >= FRAG_CB(curr)->ip_defrag_offset &&
 				 end <= curr_run_end)
 				return IPFRAG_DUP;
 			else
@@ -438,7 +439,7 @@ int inet_frag_queue_insert(struct inet_frag_queue *q, struct sk_buff *skb,
 		rb_insert_color(&skb->rbnode, &q->rb_fragments);
 	}
 
-	skb->ip_defrag_offset = offset;
+	FRAG_CB(skb)->ip_defrag_offset = offset;
 
 	return IPFRAG_OK;
 }
@@ -448,13 +449,29 @@ void *inet_frag_reasm_prepare(struct inet_frag_queue *q, struct sk_buff *skb,
 			      struct sk_buff *parent)
 {
 	struct sk_buff *fp, *head = skb_rb_first(&q->rb_fragments);
-	struct sk_buff **nextp;
+	struct sk_buff **nextp = NULL;
+	struct sock *sk = skb->sk;
 	int delta;
 
+	if (sk) {
+		if (!refcount_inc_not_zero(&sk->sk_refcnt))
+			return NULL;
+
+		/* need to orphan now, both this function and
+		 * inet_frag_reasm_finish alter skb->truesize, which
+		 * messes up sk->sk_wmem accounting.
+		 * skb->sk might have been passed as argument to dst->output
+		 * and must remain valid until tx completes.
+		 */
+		skb_orphan(skb);
+	}
+
 	if (head != skb) {
 		fp = skb_clone(skb, GFP_ATOMIC);
-		if (!fp)
-			return NULL;
+		if (!fp) {
+			head = skb;
+			goto out_restore_sk;
+		}
 		FRAG_CB(fp)->next_frag = FRAG_CB(skb)->next_frag;
 		if (RB_EMPTY_NODE(&skb->rbnode))
 			FRAG_CB(parent)->next_frag = fp;
@@ -470,13 +487,14 @@ void *inet_frag_reasm_prepare(struct inet_frag_queue *q, struct sk_buff *skb,
 		consume_skb(head);
 		head = skb;
 	}
-	WARN_ON(head->ip_defrag_offset != 0);
+	WARN_ON(FRAG_CB(head)->ip_defrag_offset != 0);
+	DEBUG_NET_WARN_ON_ONCE(head->sk);
 
 	delta = -head->truesize;
 
 	/* Head of list must not be cloned. */
 	if (skb_unclone(head, GFP_ATOMIC))
-		return NULL;
+		goto out_restore_sk;
 
 	delta += head->truesize;
 	if (delta)
@@ -492,7 +510,7 @@ void *inet_frag_reasm_prepare(struct inet_frag_queue *q, struct sk_buff *skb,
 
 		clone = alloc_skb(0, GFP_ATOMIC);
 		if (!clone)
-			return NULL;
+			goto out_restore_sk;
 		skb_shinfo(clone)->frag_list = skb_shinfo(head)->frag_list;
 		skb_frag_list_init(head);
 		for (i = 0; i < skb_shinfo(head)->nr_frags; i++)
@@ -509,6 +527,12 @@ void *inet_frag_reasm_prepare(struct inet_frag_queue *q, struct sk_buff *skb,
 		nextp = &skb_shinfo(head)->frag_list;
 	}
 
+out_restore_sk:
+	if (sk) {
+		head->sk = sk;
+		head->destructor = sock_edemux;
+	}
+
 	return nextp;
 }
 EXPORT_SYMBOL(inet_frag_reasm_prepare);
diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index a4941f53b523..fb947d1613fe 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -384,6 +384,7 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
 	}
 
 	skb_dst_drop(skb);
+	skb_orphan(skb);
 	return -EINPROGRESS;
 
 insert_error:
@@ -487,7 +488,6 @@ int ip_defrag(struct net *net, struct sk_buff *skb, u32 user)
 	struct ipq *qp;
 
 	__IP_INC_STATS(net, IPSTATS_MIB_REASMREQDS);
-	skb_orphan(skb);
 
 	/* Lookup (or create) queue header */
 	qp = ip_find(net, ip_hdr(skb), user, vif);
diff --git a/net/ipv6/netfilter/nf_conntrack_reasm.c b/net/ipv6/netfilter/nf_conntrack_reasm.c
index 1a51a44571c3..d0dcbaca1994 100644
--- a/net/ipv6/netfilter/nf_conntrack_reasm.c
+++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
@@ -294,6 +294,7 @@ static int nf_ct_frag6_queue(struct frag_queue *fq, struct sk_buff *skb,
 	}
 
 	skb_dst_drop(skb);
+	skb_orphan(skb);
 	return -EINPROGRESS;
 
 insert_error:
@@ -469,7 +470,6 @@ int nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
 	hdr = ipv6_hdr(skb);
 	fhdr = (struct frag_hdr *)skb_transport_header(skb);
 
-	skb_orphan(skb);
 	fq = fq_find(net, fhdr->identification, user, hdr,
 		     skb->dev ? skb->dev->ifindex : 0);
 	if (fq == NULL) {