On 04.08.2015 10:42, Herbert Xu wrote:
Brenden Blanco <bbla...@plumgrid.com> wrote:
[ 318.244596] BUG: unable to handle kernel NULL pointer dereference
at 000000000000008e
[ 318.245182] IP: [<ffffffff81455e7c>] __skb_recv_datagram+0xbc/0x5a0
Replying to myself, and adding commit interested parties...
I went through the git log for the function in question, and
positively identified that the following commit introduces the crash:
738ac1e net: Clone skb before setting peeked flag
Null dereference is at line 224 of net/core/datagram.c (according to
my objdump dis-assembly):
Sorry, brain fart on my part. Please try this patch.
---8<---
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 <bbla...@plumgrid.com>
Signed-off-by: Herbert Xu <herb...@gondor.apana.org.au>
Seems correct. You can add:
Reviewed-by: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
Your skb_set_peeked() doesn't set prev/next to NULL when
unlinks old skb from the queue unlike to __skb_unlink().
Isn't big deal but nulling might be useful.
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);
--
Konstantin
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