netdev - Re: [PATCH] xfrm:fix access to the null pointer in __xfrm_state

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Date:   Sat, 29 Oct 2022 19:08:57 +0200
From:   Thomas Jarosch <thomas.jarosch@...ra2net.com>
To:     Zhihao Chen <chenzhihao@...zu.com>
Cc:     netdev@...r.kernel.org, baihaowen@...zu.com,
        steffen.klassert@...unet.com, Chonglong Xu <xuchonglong@...zu.com>
Subject: Re: [PATCH] xfrm:fix access to the null pointer in
 __xfrm_state_delete()

Hi Chen,

You wrote on Thu, Oct 27, 2022 at 02:24:55PM +0000:
> Validate the byseq node before removing it from the hlist of state_byseq.
> km.seq cannot be used to determine whether the SA is in the byseq hlist
> because xfrm_add_sa() may initialize km.seq to 0 and the SA is not inserted
> into hlist. In later network communication, the seq field will increase
> after the valid packet is received.
> 
> In the above case, the NULL pointer will be accessed and cause a kernel
> panic when the SA is being removed from hlist by checking km.seq field in
> __xfrm_state_delete().

thanks for your patch!

The solution is pretty close already, it's pfkey_send_new_mapping() from af_key.c 
messing with "x->km.seq" if a new NAT-T mapping is detected.

I'll send a patch for the root cause on Monday, the commit message will be 
the most delicate thing. I already checked different PF_KEYv2 based IPSec 
implementations how they behave regarding SADB_X_NAT_T_NEW_MAPPING.

Some details are public here already:
https://github.com/strongswan/strongswan/issues/992#issuecomment-1294651331

If you are using a modern IPSec userspace application like strongswan 5.x,
one can use the netlink xfrm interface and disable CONFIG_NET_KEY in
the kernel to completely avoid the issue with PF_KEYv2.
Also unloading the "af_key" kernel module helps.

Perhaps it might make sense to extend your patch to WARN_ON
in case we run into this situation again in the future?
Then we would not sweep the issue under the rug.

Cheers,
Thomas