linux-kernel - Re: [PATCH bpf v2 1/2] bpf, x86_32: Fix incorrect encoding in BPF

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Message-ID: <CAADnVQJHKXgXg3WQc5L5LvV7VvvgU92evjQZSfAOmSoCH9M-yQ@mail.gmail.com>
Date:   Fri, 24 Apr 2020 17:15:23 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Luke Nelson <lukenels@...washington.edu>
Cc:     bpf <bpf@...r.kernel.org>, Brian Gerst <brgerst@...il.com>,
        Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>, Wang YanQing <udknight@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf v2 1/2] bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension

On Wed, Apr 22, 2020 at 10:36 AM Luke Nelson <lukenels@...washington.edu> wrote:
>
> The current JIT uses the following sequence to zero-extend into the
> upper 32 bits of the destination register for BPF_LDX BPF_{B,H,W},
> when the destination register is not on the stack:
>
>   EMIT3(0xC7, add_1reg(0xC0, dst_hi), 0);
>
> The problem is that C7 /0 encodes a MOV instruction that requires a 4-byte
> immediate; the current code emits only 1 byte of the immediate. This
> means that the first 3 bytes of the next instruction will be treated as
> the rest of the immediate, breaking the stream of instructions.
>
> This patch fixes the problem by instead emitting "xor dst_hi,dst_hi"
> to clear the upper 32 bits. This fixes the problem and is more efficient
> than using MOV to load a zero immediate.
>
> This bug may not be currently triggerable as BPF_REG_AX is the only
> register not stored on the stack and the verifier uses it in a limited
> way, and the verifier implements a zero-extension optimization. But the
> JIT should avoid emitting incorrect encodings regardless.
>
> Fixes: 03f5781be2c7b ("bpf, x86_32: add eBPF JIT compiler for ia32")
> Signed-off-by: Xi Wang <xi.wang@...il.com>
> Signed-off-by: Luke Nelson <luke.r.nels@...il.com>

Applied. Thanks