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Date: Thu, 10 Sep 2015 22:11:32 -0700 (PDT)
From: David Miller <davem@...emloft.net>
To: daniel@...earbox.net
Cc: chamaken@...il.com, fw@...len.de, netdev@...r.kernel.org
Subject: Re: [PATCH net] netlink, mmap: transform mmap skb into full skb on
taps
From: Daniel Borkmann <daniel@...earbox.net>
Date: Thu, 10 Sep 2015 20:05:46 +0200
> Ken-ichirou reported that running netlink in mmap mode for receive in
> combination with nlmon will throw a NULL pointer dereference in
> __kfree_skb() on nlmon_xmit(), in my case I can also trigger an "unable
> to handle kernel paging request". The problem is the skb_clone() in
> __netlink_deliver_tap_skb() for skbs that are mmaped.
>
> I.e. the cloned skb doesn't have a destructor, whereas the mmap netlink
> skb has it pointed to netlink_skb_destructor(), set in the handler
> netlink_ring_setup_skb(). There, skb->head is being set to NULL, so
> that in such cases, __kfree_skb() doesn't perform a skb_release_data()
> via skb_release_all(), where skb->head is possibly being freed through
> kfree(head) into slab allocator, although netlink mmap skb->head points
> to the mmap buffer. Similarly, the same has to be done also for large
> netlink skbs where the data area is vmalloced. Therefore, as discussed,
> make a copy for these rather rare cases for now. This fixes the issue
> on my and Ken-ichirou's test-cases.
>
> Reference: http://thread.gmane.org/gmane.linux.network/371129
> Fixes: bcbde0d449ed ("net: netlink: virtual tap device management")
> Reported-by: Ken-ichirou MATSUZAWA <chamaken@...il.com>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> Tested-by: Ken-ichirou MATSUZAWA <chamaken@...il.com>
Looking more deeply into this, I think we have the same exact problem
with netlink skbs that use vmalloc memory at skb->head.
We have a special hack, but only when netlink_skb_clone() is used,
to preserve the destructor. But these skbs can escape anywhere
and be eventually cloned as we have seen with the mmap stuff.
I'm wondering if we should do something more precise to fix this,
and in a way that handles both the mmap and vmalloc cases.
How about we burn the last flag bit in sk_buff:
diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 639e9b8..47d5875 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -97,22 +97,6 @@ int netlink_attachskb(struct sock *sk, struct sk_buff *skb,
void netlink_detachskb(struct sock *sk, struct sk_buff *skb);
int netlink_sendskb(struct sock *sk, struct sk_buff *skb);
-static inline struct sk_buff *
-netlink_skb_clone(struct sk_buff *skb, gfp_t gfp_mask)
-{
- struct sk_buff *nskb;
-
- nskb = skb_clone(skb, gfp_mask);
- if (!nskb)
- return NULL;
-
- /* This is a large skb, set destructor callback to release head */
- if (is_vmalloc_addr(skb->head))
- nskb->destructor = skb->destructor;
-
- return nskb;
-}
-
/*
* skb should fit one page. This choice is good for headerless malloc.
* But we should limit to 8K so that userspace does not have to
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 2738d35..77b804c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -584,8 +584,9 @@ struct sk_buff {
fclone:2,
peeked:1,
head_frag:1,
- xmit_more:1;
- /* one bit hole */
+ xmit_more:1,
+ clone_preserves_destructor;
+
kmemcheck_bitfield_end(flags1);
/* fields enclosed in headers_start/headers_end are copied
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index dad4dd3..4a7b8e3 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -825,7 +825,8 @@ static struct sk_buff *__skb_clone(struct sk_buff *n, struct sk_buff *skb)
n->hdr_len = skb->nohdr ? skb_headroom(skb) : skb->hdr_len;
n->cloned = 1;
n->nohdr = 0;
- n->destructor = NULL;
+ if (!skb->clone_preserves_destructor)
+ n->destructor = NULL;
C(tail);
C(end);
C(head);
diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index 6fcbd21..2ec5425 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1075,7 +1075,7 @@ static void nl_fib_input(struct sk_buff *skb)
nlmsg_len(nlh) < sizeof(*frn))
return;
- skb = netlink_skb_clone(skb, GFP_KERNEL);
+ skb = skb_clone(skb, GFP_KERNEL);
if (!skb)
return;
nlh = nlmsg_hdr(skb);
diff --git a/net/netfilter/nfnetlink.c b/net/netfilter/nfnetlink.c
index 70277b1..4c612481 100644
--- a/net/netfilter/nfnetlink.c
+++ b/net/netfilter/nfnetlink.c
@@ -291,7 +291,7 @@ static void nfnetlink_rcv_batch(struct sk_buff *skb, struct nlmsghdr *nlh,
replay:
status = 0;
- skb = netlink_skb_clone(oskb, GFP_KERNEL);
+ skb = skb_clone(oskb, GFP_KERNEL);
if (!skb)
return netlink_ack(oskb, nlh, -ENOMEM);
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 7f86d3b..214f1a1 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -719,6 +719,7 @@ static void netlink_ring_setup_skb(struct sk_buff *skb, struct sock *sk,
skb->end = skb->tail + size;
skb->len = 0;
+ skb->clone_preserves_destructor = 1;
skb->destructor = netlink_skb_destructor;
NETLINK_CB(skb).flags |= NETLINK_SKB_MMAPED;
NETLINK_CB(skb).sk = sk;
@@ -854,6 +855,14 @@ static void netlink_ring_set_copied(struct sock *sk, struct sk_buff *skb)
#define netlink_mmap_sendmsg(sk, msg, dst_portid, dst_group, scm) 0
#endif /* CONFIG_NETLINK_MMAP */
+static bool skb_can_release_head(struct sk_buff *skb)
+{
+ if (!skb->cloned ||
+ !atomic_dec_return(&(skb_shinfo(skb)->dataref)))
+ return true;
+ return false;
+}
+
static void netlink_skb_destructor(struct sk_buff *skb)
{
#ifdef CONFIG_NETLINK_MMAP
@@ -866,31 +875,35 @@ static void netlink_skb_destructor(struct sk_buff *skb)
* the status. In the direction userspace to kernel, the status is
* always reset here after the packet was processed and freed.
*/
- if (netlink_skb_is_mmaped(skb)) {
- hdr = netlink_mmap_hdr(skb);
- sk = NETLINK_CB(skb).sk;
+ if (!netlink_skb_is_mmaped(skb))
+ goto not_mmaped;
- if (NETLINK_CB(skb).flags & NETLINK_SKB_TX) {
- netlink_set_status(hdr, NL_MMAP_STATUS_UNUSED);
- ring = &nlk_sk(sk)->tx_ring;
- } else {
- if (!(NETLINK_CB(skb).flags & NETLINK_SKB_DELIVERED)) {
- hdr->nm_len = 0;
- netlink_set_status(hdr, NL_MMAP_STATUS_VALID);
- }
- ring = &nlk_sk(sk)->rx_ring;
- }
+ if (!skb_can_release_head(skb))
+ goto clone_refs_remain;
- WARN_ON(atomic_read(&ring->pending) == 0);
- atomic_dec(&ring->pending);
- sock_put(sk);
+ hdr = netlink_mmap_hdr(skb);
+ sk = NETLINK_CB(skb).sk;
- skb->head = NULL;
+ if (NETLINK_CB(skb).flags & NETLINK_SKB_TX) {
+ netlink_set_status(hdr, NL_MMAP_STATUS_UNUSED);
+ ring = &nlk_sk(sk)->tx_ring;
+ } else {
+ if (!(NETLINK_CB(skb).flags & NETLINK_SKB_DELIVERED)) {
+ hdr->nm_len = 0;
+ netlink_set_status(hdr, NL_MMAP_STATUS_VALID);
+ }
+ ring = &nlk_sk(sk)->rx_ring;
}
+
+ WARN_ON(atomic_read(&ring->pending) == 0);
+ atomic_dec(&ring->pending);
+ sock_put(sk);
+clone_refs_remain:
+ skb->head = NULL;
+not_mmaped:
#endif
if (is_vmalloc_addr(skb->head)) {
- if (!skb->cloned ||
- !atomic_dec_return(&(skb_shinfo(skb)->dataref)))
+ if (skb_can_release_head(skb))
vfree(skb->head);
skb->head = NULL;
@@ -1669,6 +1682,7 @@ static struct sk_buff *netlink_alloc_large_skb(unsigned int size,
if (data == NULL)
return NULL;
+ skb->clone_preserves_destructor = 1;
skb = __build_skb(data, size);
if (skb == NULL)
vfree(data);
--
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