netdev - Re: [PATCH net] net: filter: initialize A and X registers

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Message-ID: <CAMEtUux=mOyZkfxMWi3eA01TvmJrfJpeBpGcHvheptcckA=dzQ@mail.gmail.com>
Date:	Tue, 22 Apr 2014 21:59:32 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	David Miller <davem@...emloft.net>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: filter: initialize A and X registers

On Tue, Apr 22, 2014 at 8:57 PM, David Miller <davem@...emloft.net> wrote:
> From: Alexei Starovoitov <ast@...mgrid.com>
> Date: Tue, 22 Apr 2014 20:18:57 -0700
>
>> exisiting BPF verifier allows uninitialized access to registers,
>> 'ret A' is considered to be a valid filter.
>> So initialize A and X to zero to prevent leaking kernel memory
>> In the future BPF verifier will be rejecting such filters
>>
>> Signed-off-by: Alexei Starovoitov <ast@...mgrid.com>
>
> Has the code always been like this?

the previous interpreter had:
unsigned int sk_run_filter(const struct sk_buff *skb,
                           const struct sock_filter *fentry)
{
        void *ptr;
        u32 A = 0;                      /* Accumulator */
        u32 X = 0;                      /* Index Register */
...
so it wasn't affected.

> Did the eBPF changes introduce this problem either directly or
> indirectly?

this bug is an oversight on my side.
I believe in the net-next it should be fixed by making verifier smarter
instead of wasting run time cycles to initialize regs.
For now the same fix is needed for both net and net-next.
We can remove extra assignments when verifier becomes smarter.
ebpf verifier that I posted earlier had checks for uninitialized regs and stack.
I missed lack of uninit regs part in classic verifier when ebpf verifier and jit
were dropped from the patch set.
Sorry about this slip.
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