netdev - Re: [PATCH net] bpf: fix verifier memory corruption

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Date:	Wed, 15 Apr 2015 17:59:35 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net] bpf: fix verifier memory corruption

On Di, 2015-04-14 at 15:57 -0700, Alexei Starovoitov wrote:
> Due to missing bounds check the DAG pass of the BPF verifier can corrupt
> the memory which can cause random crashes during program loading:
> 
> [8.449451] BUG: unable to handle kernel paging request at ffffffffffffffff
> [8.451293] IP: [<ffffffff811de33d>] kmem_cache_alloc_trace+0x8d/0x2f0
> [8.452329] Oops: 0000 [#1] SMP
> [8.452329] Call Trace:
> [8.452329]  [<ffffffff8116cc82>] bpf_check+0x852/0x2000
> [8.452329]  [<ffffffff8116b7e4>] bpf_prog_load+0x1e4/0x310
> [8.452329]  [<ffffffff811b190f>] ? might_fault+0x5f/0xb0
> [8.452329]  [<ffffffff8116c206>] SyS_bpf+0x806/0xa30
> 
> Fixes: f1bca824dabb ("bpf: add search pruning optimization to verifier")
> Signed-off-by: Alexei Starovoitov <ast@...mgrid.com>
> ---
> Many things need to align for this crash to be seen, yet I managed to hit it.
> In my case JA was last insn, 't' was 255 and explored_states array
> had 256 elements. I've double checked other similar paths and all seems clean.
> 
>  kernel/bpf/verifier.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index a28e09c7825d..36508e69e92a 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1380,7 +1380,8 @@ peek_stack:
>  			/* tell verifier to check for equivalent states
>  			 * after every call and jump
>  			 */
> -			env->explored_states[t + 1] = STATE_LIST_MARK;
> +			if (t + 1 < insn_cnt)
> +				env->explored_states[t + 1] = STATE_LIST_MARK;
>  		} else {
>  			/* conditional jump with two edges */
>  			ret = push_insn(t, t + 1, FALLTHROUGH, env);

Quick review:

We have env->explored_states[t+1] access in the 

                } else {
                        /* conditional jump with two edges */
                        ret = push_insn(t, t + 1, FALLTHROUGH, env);
                        if (ret == 1)
                                goto peek_stack;
                        else if (ret < 0)
                                goto err_free;

>>>                     ret = push_insn(t, t + insns[t].off + 1, BRANCH, env);
                        if (ret == 1)
                                goto peek_stack;
                        else if (ret < 0)
                                goto err_free;
                }
        } else {


push_insn call. At this point insn[t].off could be 0, no?

Thanks,
Hannes


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