D. Wythe <[email protected]> wrote:
> From: "D. Wythe" <[email protected]>
>
> A malicious eBPF program can interrupt the subsequent processing of
> a skb by returning an exceptional retval, and no one will be responsible
> for releasing the very skb.
How? The bpf verifier is supposed to reject nf bpf programs that
return a value other than accept or drop.
If this is a real bug, please also figure out why
006c0e44ed92 ("selftests/bpf: add missing netfilter return value and ctx access tests")
failed to catch it.
> Moreover, normal programs can also have the demand to return NF_STOLEN,
No, this should be disallowed already.
> net/netfilter/nf_bpf_link.c | 19 ++++++++++++++++++-
> 1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
> index e502ec0..03c47d6 100644
> --- a/net/netfilter/nf_bpf_link.c
> +++ b/net/netfilter/nf_bpf_link.c
> @@ -12,12 +12,29 @@ static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb,
> const struct nf_hook_state *s)
> {
> const struct bpf_prog *prog = bpf_prog;
> + unsigned int verdict;
> struct bpf_nf_ctx ctx = {
> .state = s,
> .skb = skb,
> };
>
> - return bpf_prog_run(prog, &ctx);
> + verdict = bpf_prog_run(prog, &ctx);
> + switch (verdict) {
> + case NF_STOLEN:
> + consume_skb(skb);
> + fallthrough;
This can't be right. STOLEN really means STOLEN (free'd,
redirected, etc, "skb" MUST be "leaked".
Which is also why the bpf program is not allowed to return it.
On 11/29/23 9:18 PM, Florian Westphal wrote:
> D. Wythe <[email protected]> wrote:
>> From: "D. Wythe" <[email protected]>
>>
>> A malicious eBPF program can interrupt the subsequent processing of
>> a skb by returning an exceptional retval, and no one will be responsible
>> for releasing the very skb.
> How? The bpf verifier is supposed to reject nf bpf programs that
> return a value other than accept or drop.
>
> If this is a real bug, please also figure out why
> 006c0e44ed92 ("selftests/bpf: add missing netfilter return value and ctx access tests")
> failed to catch it.
Hi Florian,
You are right, i make a mistake.. , it's not a bug..
And my origin intention was to allow ebpf progs to return NF_STOLEN, we
are trying to modify some netfilter modules via ebpf,
and some scenarios require the use of NF_STOLEN, but from your
description, it seems that at least currently,
you do not want to return NF_STOLEN, until there is a helper for
sonsume_skb(), right ?
Again, very sorry to bother you.
Best wishes,
D. Wythe.
>> Moreover, normal programs can also have the demand to return NF_STOLEN,
> No, this should be disallowed already.
>
>> net/netfilter/nf_bpf_link.c | 19 ++++++++++++++++++-
>> 1 file changed, 18 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
>> index e502ec0..03c47d6 100644
>> --- a/net/netfilter/nf_bpf_link.c
>> +++ b/net/netfilter/nf_bpf_link.c
>> @@ -12,12 +12,29 @@ static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb,
>> const struct nf_hook_state *s)
>> {
>> const struct bpf_prog *prog = bpf_prog;
>> + unsigned int verdict;
>> struct bpf_nf_ctx ctx = {
>> .state = s,
>> .skb = skb,
>> };
>>
>> - return bpf_prog_run(prog, &ctx);
>> + verdict = bpf_prog_run(prog, &ctx);
>> + switch (verdict) {
>> + case NF_STOLEN:
>> + consume_skb(skb);
>> + fallthrough;
> This can't be right. STOLEN really means STOLEN (free'd,
> redirected, etc, "skb" MUST be "leaked".
>
> Which is also why the bpf program is not allowed to return it.
D. Wythe <[email protected]> wrote:
> And my origin intention was to allow ebpf progs to return NF_STOLEN, we are
> trying to modify some netfilter modules via ebpf,
> and some scenarios require the use of NF_STOLEN, but from your description,
NF_STOLEN can only be supported via a trusted helper, as least as far as
I understand.
Otherwise verifier would have to guarantee that any branch that returns
NF_STOLEN has released the skb, or passed it to a function that will
release the skb in the near future.
On 11/29/23 10:47 PM, Florian Westphal wrote:
> D. Wythe <[email protected]> wrote:
>> And my origin intention was to allow ebpf progs to return NF_STOLEN, we are
>> trying to modify some netfilter modules via ebpf,
>> and some scenarios require the use of NF_STOLEN, but from your description,
> NF_STOLEN can only be supported via a trusted helper, as least as far as
> I understand.
>
> Otherwise verifier would have to guarantee that any branch that returns
> NF_STOLEN has released the skb, or passed it to a function that will
> release the skb in the near future.
Thank you very much for your help. I now understand the difficulty here.
The verifier cannot determine whether the consume_skb() was executed or not,
when the return value goes to NF_STOLEN.
We may use NF_DROP at first, it won't be make much difference for us now.
Also, do you have any plans to support this helper?
Best wishes,
D. Wythe