[PATCH 4.1 127/159] net: Clone skb before setting peeked flag

Greg Kroah-Hartman
2015-09-26T21:29:12Z

From: Herbert Xu <herbert@gondor.apana.org.au>

[ Upstream commit 738ac1ebb96d02e0d23bc320302a6ea94c612dec ]

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 <khlebnikov@yandex-team.ru>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/core/datagram.c | 41 ++++++++++++++++++++++++++++++++++++++---
1 file changed, 38 insertions(+), 3 deletions(-)

--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -131,6 +131,35 @@ 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
@@ -165,7 +194,9 @@ out_noerr:
struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
int *peeked, int *off, int *err)
{
+ struct sk_buff_head *queue = &sk->sk_receive_queue;
struct sk_buff *skb, *last;
+ unsigned long cpu_flags;
long timeo;
/*
* Caller is allowed not to check sk->sk_err before skb_recv_datagram()
@@ -184,8 +215,6 @@ struct sk_buff *__skb_recv_datagram(stru
* 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;
int _off = *off;

last = (struct sk_buff *)queue;
@@ -199,7 +228,11 @@ struct sk_buff *__skb_recv_datagram(stru
_off -= skb->len;
continue;
}
- skb->peeked = 1;
+
+ error = skb_set_peeked(skb);
+ if (error)
+ goto unlock_err;
+
atomic_inc(&skb->users);
} else
__skb_unlink(skb, queue);
@@ -223,6 +256,8 @@ struct sk_buff *__skb_recv_datagram(stru

return NULL;

+unlock_err:
+ spin_unlock_irqrestore(&queue->lock, cpu_flags);
no_packet:
*err = error;
return NULL;