linux-kernel - Re: [PATCH net] tun: Remove nested call to bpf_net_ctx_set() in do_xdp

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Message-ID: <e263f723-0b9c-4059-982d-2bb4b5636759@redhat.com>
Date: Thu, 25 Jul 2024 12:44:27 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Jeongjun Park <aha310510@...il.com>, jasowang@...hat.com
Cc: syzbot+44623300f057a28baf1e@...kaller.appspotmail.com,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, jiri@...nulli.us,
 bigeasy@...utronix.de, linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 bpf@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH net] tun: Remove nested call to bpf_net_ctx_set() in
 do_xdp_generic()

On 7/25/24 04:43, Willem de Bruijn wrote:
> Jeongjun Park wrote:
>> In the previous commit, bpf_net_context handling was added to
>> tun_sendmsg() and do_xdp_generic(), but if you write code like this,
>> bpf_net_context overlaps in the call trace below, causing various
>> memory corruptions.
> 
> I'm no expert on this code, but commit 401cb7dae813 that introduced
> bpf_net_ctx_set explicitly states that nested calls are allowed.
> 
> And the function does imply that:
> 
> static inline struct bpf_net_context *bpf_net_ctx_set(struct bpf_net_context *bpf_net_ctx)
> {
>          struct task_struct *tsk = current;
> 
>          if (tsk->bpf_net_context != NULL)
>                  return NULL;
>          bpf_net_ctx->ri.kern_flags = 0;
> 
>          tsk->bpf_net_context = bpf_net_ctx;
>          return bpf_net_ctx;
> }

I agree with Willem, the ctx nesting looks legit generally speaking. 
@Jeongjun: you need to track down more accurately the issue root cause 
and include such info into the commit message.

Skimming over the code I *think* do_xdp_generic() is not cleaning the 
nested context in all the paths before return and that could cause the 
reported issue.

Thanks,

Paolo