From 72e6f0680249f5e0a87f2b282d033baefd90d84e Mon Sep 17 00:00:00 2001 From: Herbert Xu Date: Mon, 13 Jul 2015 16:04:13 +0800 Subject: net: Clone skb before setting peeked flag commit 738ac1ebb96d02e0d23bc320302a6ea94c612dec upstream. Shared skbs must not be modified and this is crucial for broadcast and/or multicast paths where we use it as an optimisation to avoid unnecessary cloning. The function skb_recv_datagram breaks this rule by setting peeked without cloning the skb first. This causes funky races which leads to double-free. This patch fixes this by cloning the skb and replacing the skb in the list when setting skb->peeked. Fixes: a59322be07c9 ("[UDP]: Only increment counter on first peek/recv") Reported-by: Konstantin Khlebnikov Signed-off-by: Herbert Xu Signed-off-by: David S. Miller [bwh: Backported to 3.2: adjust context] Signed-off-by: Ben Hutchings --- net/core/datagram.c | 42 ++++++++++++++++++++++++++++++++++++++---- 1 file changed, 38 insertions(+), 4 deletions(-) diff --git a/net/core/datagram.c b/net/core/datagram.c index 6f54d0a..eaa1d32 100644 --- a/net/core/datagram.c +++ b/net/core/datagram.c @@ -128,6 +128,35 @@ out_noerr: goto out; } +static int skb_set_peeked(struct sk_buff *skb) +{ + struct sk_buff *nskb; + + if (skb->peeked) + return 0; + + /* We have to unshare an skb before modifying it. */ + if (!skb_shared(skb)) + goto done; + + nskb = skb_clone(skb, GFP_ATOMIC); + if (!nskb) + return -ENOMEM; + + skb->prev->next = nskb; + skb->next->prev = nskb; + nskb->prev = skb->prev; + nskb->next = skb->next; + + consume_skb(skb); + skb = nskb; + +done: + skb->peeked = 1; + + return 0; +} + /** * __skb_recv_datagram - Receive a datagram skbuff * @sk: socket @@ -160,7 +189,9 @@ out_noerr: struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned flags, int *peeked, int *err) { + struct sk_buff_head *queue = &sk->sk_receive_queue; struct sk_buff *skb; + unsigned long cpu_flags; long timeo; /* * Caller is allowed not to check sk->sk_err before skb_recv_datagram() @@ -179,15 +210,16 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned flags, * Look at current nfs client by the way... * However, this function was correct in any case. 8) */ - unsigned long cpu_flags; - struct sk_buff_head *queue = &sk->sk_receive_queue; - spin_lock_irqsave(&queue->lock, cpu_flags); skb = skb_peek(queue); if (skb) { *peeked = skb->peeked; if (flags & MSG_PEEK) { - skb->peeked = 1; + + error = skb_set_peeked(skb); + if (error) + goto unlock_err; + atomic_inc(&skb->users); } else __skb_unlink(skb, queue); @@ -206,6 +238,8 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned flags, return NULL; +unlock_err: + spin_unlock_irqrestore(&queue->lock, cpu_flags); no_packet: *err = error; return NULL; -- cgit v1.1