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

[Eric Dumazet <eric.dumazet@gmail.com>]

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.