From: Daniel Borkmann <daniel@iogearbox.net>
To: Arthur Fabre <afabre@cloudflare.com>, marek@cloudflare.com
Cc: ast@kernel.org, netdev@vger.kernel.org
Subject: Re: SOCKET_FILTER regression - eBPF can't subtract when attached from unprivileged user
Date: Fri, 1 Mar 2019 13:51:00 +0100 [thread overview]
Message-ID: <ef8e597a-745b-6355-8814-5b863c95e232@iogearbox.net> (raw)
In-Reply-To: <20190301113901.29448-1-afabre@cloudflare.com>
On 03/01/2019 12:39 PM, Arthur Fabre wrote:
> I can reproduce this on 4.19.0-3-amd64 both with, and without the JIT enabled.
>
> Dumping the "root" and "non-root" programs with bpftool,
> the subtraction instructions differ:
>
> "non-root":
> 0: (85) call bpf_ktime_get_ns#74944
> 1: (bf) r7 = r0
> 2: (85) call bpf_ktime_get_ns#74944
> 3: (bf) r6 = r0
> 4: (bf) r8 = r6
> 5: (b4) w11 = -1
> 6: (1f) r11 -= r8
> 7: (4f) r11 |= r8
> 8: (87) r11 = -r11
> 9: (c7) r11 s>>= 63
> 10: (5f) r8 &= r11
>
> "root":
> 0: (85) call bpf_ktime_get_ns#74944
> 1: (bf) r7 = r0
> 2: (85) call bpf_ktime_get_ns#74944
> 3: (bf) r6 = r0
> 4: (bf) r8 = r6
>
> The remainder of the instructions are for writing the results in the map,
> and the instructions are identical.
>
> I believe the extra instructions come from "fixup_bpf_calls" in the verifier:
>
> if (isneg)
> *patch++ = BPF_ALU64_IMM(BPF_MUL, off_reg, -1);
> *patch++ = BPF_MOV32_IMM(BPF_REG_AX, aux->alu_limit - 1);
> *patch++ = BPF_ALU64_REG(BPF_SUB, BPF_REG_AX, off_reg);
> *patch++ = BPF_ALU64_REG(BPF_OR, BPF_REG_AX, off_reg);
> *patch++ = BPF_ALU64_IMM(BPF_NEG, BPF_REG_AX, 0);
> *patch++ = BPF_ALU64_IMM(BPF_ARSH, BPF_REG_AX, 63);
> if (issrc) {
> *patch++ = BPF_ALU64_REG(BPF_AND, BPF_REG_AX,
> off_reg);
> insn->src_reg = BPF_REG_AX;
> } else {
> *patch++ = BPF_ALU64_REG(BPF_AND, off_reg,
> BPF_REG_AX);
> }
>
> This was introduced by "bpf: prevent out of bounds speculation on pointer arithmetic"
> (https://patchwork.ozlabs.org/patch/1039606/).
> I don't yet understand what's going on.
Hmm, thanks for the report, I'll take a look right away! There's no map
involved here it seems, so there shouldn't be such fixup.
Cheers,
Daniel
next prev parent reply other threads:[~2019-03-01 12:51 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-01 1:06 SOCKET_FILTER regression - eBPF can't subtract when attached from unprivileged user Marek Majkowski
2019-03-01 11:39 ` Arthur Fabre
2019-03-01 12:51 ` Daniel Borkmann [this message]
2019-03-01 14:04 ` Daniel Borkmann
2019-03-01 14:10 ` Marek Majkowski
2019-03-01 14:22 ` Daniel Borkmann
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