Re: net: GPF in eth_header
From: Andrey Konovalov <hidden>
Date: 2016-11-29 15:31:13
Also in:
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On Tue, Nov 29, 2016 at 3:58 PM, Eric Dumazet [off-list ref] wrote:
On Tue, 2016-11-29 at 11:26 +0100, Andrey Konovalov wrote:quoted
On Sat, Nov 26, 2016 at 9:05 PM, Eric Dumazet [off-list ref] wrote:quoted
quoted
I actually see multiple places where skb_network_offset() is used as an argument to skb_pull(). So I guess every place can potentially be buggy.Well, I think the intent is to accept a negative number.I'm not sure that was the intent since it results in a signedness issue which leads to an out-of-bounds.Hey, I already mentioned where was the bug. You missed the investigation where I pointed it to FLorian ?quoted
A quick grep shows that the same issue can potentially happen in multiple places across the kernel: net/ipv6/ip6_output.c:1655: __skb_pull(skb, skb_network_offset(skb)); net/packet/af_packet.c:2043: skb_pull(skb, skb_network_offset(skb)); net/packet/af_packet.c:2165: skb_pull(skb, skb_network_offset(skb)); net/core/neighbour.c:1301: __skb_pull(skb, skb_network_offset(skb)); net/core/neighbour.c:1331: __skb_pull(skb, skb_network_offset(skb)); net/core/dev.c:3157: __skb_pull(skb, skb_network_offset(skb)); net/sched/sch_teql.c:337: __skb_pull(skb, skb_network_offset(skb)); net/sched/sch_atm.c:479: skb_pull(skb, skb_network_offset(skb)); net/ipv4/ip_output.c:1385: __skb_pull(skb, skb_network_offset(skb)); net/ipv4/ip_fragment.c:391: if (!pskb_pull(skb, skb_network_offset(skb) + ihl)) drivers/net/vxlan.c:1440: __skb_pull(reply, skb_network_offset(reply)); drivers/net/vxlan.c:1902: __skb_pull(skb, skb_network_offset(skb)); drivers/net/vrf.c:220: __skb_pull(skb, skb_network_offset(skb)); drivers/net/vrf.c:314: __skb_pull(skb, skb_network_offset(skb)); A similar thing also happened to somebody else (on a receive path!): https://forums.grsecurity.net/viewtopic.php?f=3&t=4550 Does it make sense to check skb_network_offset() before passing it to skb_pull() everywhere?Well, sure, we could add safety checks everywhere and slow the kernel when debugging is requested. But skb_network_offset() is not the problem here. Why are you focusing on it ? The real problem is in __skb_pull() or __skb_push() and all similar helpers. Lots of added checks and slowdowns.
The issue is not with skb_network_offset(), but with __skb_pull() using skb_network_offset() as an argument. I'm not sure what would be the beast way to fix this, to add a check before every __skb_pull(skb_network_offset()), to fix __skb_pull() to work with signed ints, to add BUG_ON()'s in __skb_pull, or something else. What I meant is that you fixed this very instance of the bug, and I'm pointing out that a similar one might hit us again.
quoted hunk ↗ jump to hunk
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index 9c535fbccf2c7dbfae04cee393460e86d588c26b..d6116e37d054fc1536114347ed3c41fc7dc7a882 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h@@ -1923,6 +1923,7 @@ static inline unsigned char *__skb_put(struct sk_buff *skb, unsigned int len) unsigned char *skb_push(struct sk_buff *skb, unsigned int len); static inline unsigned char *__skb_push(struct sk_buff *skb, unsigned int len) { + BUG_ON((int)len < 0); skb->data -= len; skb->len += len; return skb->data;@@ -1931,6 +1932,7 @@ static inline unsigned char *__skb_push(struct sk_buff *skb, unsigned int len) unsigned char *skb_pull(struct sk_buff *skb, unsigned int len); static inline unsigned char *__skb_pull(struct sk_buff *skb, unsigned int len) { + BUG_ON((int)len < 0); skb->len -= len; BUG_ON(skb->len < skb->data_len); return skb->data += len;@@ -1938,6 +1940,7 @@ static inline unsigned char *__skb_pull(struct sk_buff *skb, unsigned int len) static inline unsigned char *skb_pull_inline(struct sk_buff *skb, unsigned int len) { + BUG_ON((int)len < 0); return unlikely(len > skb->len) ? NULL : __skb_pull(skb, len); }diff --git a/net/core/skbuff.c b/net/core/skbuff.c index d1d1a5a5ad24ded523fc12ffba8c602b03bd0830..7bf098c848fd857ba5d287fc91d43f62f381bd55 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c@@ -1450,6 +1450,7 @@ EXPORT_SYMBOL(skb_put); */ unsigned char *skb_push(struct sk_buff *skb, unsigned int len) { + BUG_ON((int)len < 0); skb->data -= len; skb->len += len; if (unlikely(skb->data<skb->head)) --You received this message because you are subscribed to the Google Groups "syzkaller" group. To unsubscribe from this group and stop receiving emails from it, send an email to syzkaller+unsubscribe@googlegroups.com. For more options, visit https://groups.google.com/d/optout.