[PATCH 4.1 129/159] net: Fix skb_set_peeked use-after-free bug

Greg Kroah-Hartman
2015-09-26T21:28:47Z

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

[ Upstream commit a0a2a6602496a45ae838a96db8b8173794b5d398 ]

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 <bblanco@plumgrid.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Brenden Blanco <bblanco@plumgrid.com>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/core/datagram.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

--- 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

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
done:
skb->peeked = 1;

- return 0;
+ return skb;
}

/**
@@ -229,8 +229,9 @@ struct sk_buff *__skb_recv_datagram(stru
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);