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Date:   Thu, 23 Apr 2020 12:10:16 +0800
From:   Wang YanQing <udknight@gmail.com>
To:     Luke Nelson <lukenels@cs.washington.edu>
Cc:     bpf@vger.kernel.org, Brian Gerst <brgerst@gmail.com>,
        Luke Nelson <luke.r.nels@gmail.com>,
        Xi Wang <xi.wang@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf v2 2/2] bpf, x86_32: Fix clobbering of dst for
 BPF_JSET
Message-ID: <20200423041016.GA1153@udknight>
Mail-Followup-To: Wang YanQing <udknight@gmail.com>,
        Luke Nelson <lukenels@cs.washington.edu>, bpf@vger.kernel.org,
        Brian Gerst <brgerst@gmail.com>,
        Luke Nelson <luke.r.nels@gmail.com>, Xi Wang <xi.wang@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
        Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20200422173630.8351-1-luke.r.nels@gmail.com>
 <20200422173630.8351-2-luke.r.nels@gmail.com>
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On Wed, Apr 22, 2020 at 10:36:30AM -0700, Luke Nelson wrote:
> The current JIT clobbers the destination register for BPF_JSET BPF_X
> and BPF_K by using "and" and "or" instructions. This is fine when the
> destination register is a temporary loaded from a register stored on
> the stack but not otherwise.
> 
> This patch fixes the problem (for both BPF_K and BPF_X) by always loading
> the destination register into temporaries since BPF_JSET should not
> modify the destination register.
> 
> 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.
> 
> Fixes: 03f5781be2c7b ("bpf, x86_32: add eBPF JIT compiler for ia32")
> Signed-off-by: Xi Wang <xi.wang@gmail.com>
> Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Acked-by: Wang YanQing <udknight@gmail.com>