> The commit 738ac1ebb96d02e0d23bc320302a6ea94c612dec ("net: Clone
> skb before setting peeked flag") introduced a use-after-free bug
> in skb_recv_datagram. This is because skb_set_peeked may create
> a new skb and free the existing one. As it stands the caller will
> continue to use the old freed skb.
>
> This patch fixes it by making skb_set_peeked return the new skb
> (or the old one if unchanged).
>
> Fixes: 738ac1ebb96d ("net: Clone skb before setting peeked flag")
> Reported-by: Brenden Blanco <[email protected]>
> Signed-off-by: Herbert Xu <[email protected]>
>
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index 4967262..617088a 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -131,12 +131,12 @@ out_noerr:
> goto out;
> }
>
> -static int skb_set_peeked(struct sk_buff *skb)
> +static struct sk_buff *skb_set_peeked(struct sk_buff *skb)
> {
> struct sk_buff *nskb;
>
> if (skb->peeked)
> - return 0;
> + return skb;
>
> /* We have to unshare an skb before modifying it. */
> if (!skb_shared(skb))
> @@ -144,7 +144,7 @@ static int skb_set_peeked(struct sk_buff *skb)
>
> nskb = skb_clone(skb, GFP_ATOMIC);
> if (!nskb)
> - return -ENOMEM;
> + return ERR_PTR(-ENOMEM);
>
> skb->prev->next = nskb;
> skb->next->prev = nskb;
> @@ -157,7 +157,7 @@ static int skb_set_peeked(struct sk_buff *skb)
> done:
> skb->peeked = 1;
>
> - return 0;
> + return skb;
> }
>
> /**
> @@ -229,8 +229,9 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk,
> unsigned int flags,
> continue;
> }
>
> - error = skb_set_peeked(skb);
> - if (error)
> + skb = skb_set_peeked(skb);
> + error = PTR_ERR(skb);
> + if (IS_ERR(skb))
> goto unlock_err;
>
> atomic_inc(&skb->users);
This patch holds good in my testing. Thanks!
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