netdev - Re: [PATCH net] x86: bpf_jit: fix two bugs in eBPF JIT compiler

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Date:	Fri, 10 Oct 2014 20:12:41 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] x86: bpf_jit: fix two bugs in eBPF JIT compiler

On Fri, 2014-10-10 at 19:44 -0700, Alexei Starovoitov wrote:

> 2.
> while staring at the code realized that 64-byte buffer may not be enough
> when 1st insn is large, so increase it to 128 to avoid buffer overflow
> (theoretical maximum size of prologue+div is 109) and add runtime check.
> 


> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index d56cd1f..8266896 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -187,7 +187,8 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
>  {
>  	struct bpf_insn *insn = bpf_prog->insnsi;
>  	int insn_cnt = bpf_prog->len;
> -	u8 temp[64];
> +	bool seen_ld_abs = ctx->seen_ld_abs | (oldproglen == 0);
> +	u8 temp[128];

Hmmm. I would use some guard like :

#define BPF_MAX_INSN_SIZE 128
#define BPF_INSN_SAFETY   64

	u8 temp[MAX_INSN_SIZE + BPF_INSN_SAFETY];
 

> +		if (ilen >= sizeof(temp)) {

	if (ilen > BPF_MAX_INSN_SIZE) {
...

> +			pr_err("bpf_jit_compile fatal insn size error\n");
> +			return -EFAULT;
> +		}
> +

Otherwise, we might have corrupted stack and panic anyway.


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