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

[Eric Dumazet]

On Tue, Mar 12, 2024 at 2:21 PM Florian Westphal <fw@xxxxxxxxx> wrote:
>
> Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> > > so skb->sk gets propagated down to __ip_finish_output(), long
> > > after connrack defrag has called skb_orphan().
> > >
> > > No idea yet how to fix it,
> >
> > My plan was to refine "inet: frag: Always orphan skbs inside
> > ip_defrag()" and only do the skb_orphan()
> > for skb added to a frag_list.
> >
> > The head skb would keep a reference to the socket.
>
> I tried to follow this but its beyond my abilities.
>
> Defrag messes with skb->truesize, and I do not know how to
> fix that up safely so later calls to destructor won't underflow sk
> accouting.
>
> Furthermore, depending on delivery order, the skb that gets
> passed to rest of stack might not be the head skb (the one with
> full l4 header and sk reference), its always the last one that arrived.
>
> Existing code skb_morphs() this, see inet_frag_reasm_prepare() and also
> the ->truesize munging (which is fine only because all skbs are
> orphans...).
>
> So in order to not pass already-released sk to inet output somehow
> the skb->sk reference needs to be stolen and moved from one sk
> to another.
>
> No idea how to do this, let alone do regression testing for this.
> see e.g. 48cac18ecf1de82f76259a54402c3adb7839ad01 which added
> unconditional orphaning in ipv6 netfilter defrag.
>
> ATM the only "solution" I see is to completely remove netfilter defrag
> support for outgoing packets.

Thanks for taking a look Florian.

Perhaps not messing with truesize at all would help ?

Something based on this POC :

diff --git a/include/net/inet_frag.h b/include/net/inet_frag.h
index 153960663ce4c2389259e0dc2bb4ba0b011dc698..8fd688df4b24dec7b5a1ea365d5b34d81fe6e0e5
100644
--- a/include/net/inet_frag.h
+++ b/include/net/inet_frag.h
@@ -94,6 +94,7 @@ struct inet_frag_queue {
        struct rb_root          rb_fragments;
        struct sk_buff          *fragments_tail;
        struct sk_buff          *last_run_head;
+       struct sock             *sk;
        ktime_t                 stamp;
        int                     len;
        int                     meat;
diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 7072fc0783ef56e59c886a2f2516e7db7d10c942..aee706226ef8d9e9514fcf7d60e4d278ff7178fa
100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -297,6 +297,8 @@ void inet_frag_destroy(struct inet_frag_queue *q)
                        SKB_CONSUMED;
        WARN_ON(del_timer(&q->timer) != 0);

+       if (q->sk)
+               sock_put(q->sk);
        /* Release all fragment data. */
        fqdir = q->fqdir;
        f = fqdir->f;
diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index a4941f53b523725cd777d213500b8f6918287920..198a4c35cd1232278678a20bf0f2a49c63f548fc
100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -377,6 +377,10 @@ static int ip_frag_queue(struct ipq *qp, struct
sk_buff *skb)

                skb->_skb_refdst = 0UL;
                err = ip_frag_reasm(qp, skb, prev_tail, dev);
+               if (qp->q.sk) {
+                       swap(qp->q.sk, skb->sk);
+                       skb->destructor = sock_edemux;
+               }
                skb->_skb_refdst = orefdst;
                if (err)
                        inet_frag_kill(&qp->q);
@@ -384,6 +388,7 @@ 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 +492,6 @@ 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);
@@ -495,7 +499,12 @@ int ip_defrag(struct net *net, struct sk_buff
*skb, u32 user)
                int ret;

                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;
+               }
                ret = ip_frag_queue(qp, skb);

                spin_unlock(&qp->q.lock);