Re: kernel BUG at ./include/linux/skbuff.h:LINE!

[Steffen Klassert]

On Thu, Jan 04, 2018 at 07:58:01AM -0800, syzbot wrote:
> Hello,
> 
> syzkaller hit the following crash on
> ead68f216110170ec729e2c4dec0aad6d38259d7
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached
> Raw console output is attached.
> C reproducer is attached
> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> for information about syzkaller reproducers
> 
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+82bbd65569c49c6c0c4d@xxxxxxxxxxxxxxxxxxxxxxxxx
> It will help syzbot understand when the bug is fixed. See footer for
> details.
> If you forward the report, please keep this part and the footer.
> 
> ------------[ cut here ]------------
> kernel BUG at ./include/linux/skbuff.h:2068!

The reproducer does not trigger here, but I'm pretty sure
it is this:

Subject: [PATCH RFC ipsec] esp: Fix GRO when the headers not fully on the linear part of
 the skb.

The GRO layer does not necessarily pull the complete headers
into the linear part of the skb, a part may remain on the
first page fragment. This can lead to a crash if we try to
pull the headers, so make sure we have them on the linear
part before pulling.

Fixes: 7785bba299a8 ("esp: Add a software GRO codepath")
Reported-by: syzbot+82bbd65569c49c6c0c4d@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
---
 net/ipv4/esp4_offload.c | 3 ++-
 net/ipv6/esp6_offload.c | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/esp4_offload.c b/net/ipv4/esp4_offload.c
index f8b918c766b0..dfc1e049aad5 100644
--- a/net/ipv4/esp4_offload.c
+++ b/net/ipv4/esp4_offload.c
@@ -38,7 +38,8 @@ static struct sk_buff **esp4_gro_receive(struct sk_buff **head,
 	__be32 spi;
 	int err;
 
-	skb_pull(skb, offset);
+	if (!pskb_pull(skb, offset))
+		return NULL; 
 
 	if ((err = xfrm_parse_spi(skb, IPPROTO_ESP, &spi, &seq)) != 0)
 		goto out;
diff --git a/net/ipv6/esp6_offload.c b/net/ipv6/esp6_offload.c
index 333a478aa161..dd9627490c7c 100644
--- a/net/ipv6/esp6_offload.c
+++ b/net/ipv6/esp6_offload.c
@@ -60,7 +60,8 @@ static struct sk_buff **esp6_gro_receive(struct sk_buff **head,
 	int nhoff;
 	int err;
 
-	skb_pull(skb, offset);
+	if (!pskb_pull(skb, offset))
+		return NULL;
 
 	if ((err = xfrm_parse_spi(skb, IPPROTO_ESP, &spi, &seq)) != 0)
 		goto out;
-- 
2.14.1