[PATCH 4.17 211/324] bpf: fix sk_skb programs without skb->dev assigned

Greg Kroah-Hartman
2018-08-23T08:59:18Z

From: John Fastabend <john.fastabend@gmail.com>

[ Upstream commit 0c6bc6e531a6db36f49622f1f115770160f7afb0 ]

Multiple BPF helpers in use by sk_skb programs calculate the max
skb length using the __bpf_skb_max_len function. However, this
calculates the max length using the skb->dev pointer which can be
NULL when an sk_skb program is paired with an sk_msg program.

To force this a sk_msg program needs to redirect into the ingress
path of a sock with an attach sk_skb program. Then the the sk_skb
program would need to call one of the helpers that adjust the skb
size.

To fix the null ptr dereference use SKB_MAX_ALLOC size if no dev
is available.

Fixes: 8934ce2fd081 ("bpf: sockmap redirect ingress support")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/core/filter.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2516,7 +2516,8 @@ static int bpf_skb_net_shrink(struct sk_

static u32 __bpf_skb_max_len(const struct sk_buff *skb)
{
- return skb->dev->mtu + skb->dev->hard_header_len;
+ return skb->dev ? skb->dev->mtu + skb->dev->hard_header_len :
+ SKB_MAX_ALLOC;
}

static int bpf_skb_adjust_net(struct sk_buff *skb, s32 len_diff)