linux-kernel - Re: [PATCH bpf v2 2/2] bpf, x86_32: Fix clobbering of dst for BPF

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Message-ID: <20200423041016.GA1153@udknight>
Date:   Thu, 23 Apr 2020 12:10:16 +0800
From:   Wang YanQing <udknight@...il.com>
To:     Luke Nelson <lukenels@...washington.edu>
Cc:     bpf@...r.kernel.org, Brian Gerst <brgerst@...il.com>,
        Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...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@...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>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf v2 2/2] bpf, x86_32: Fix clobbering of dst for
 BPF_JSET

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@...il.com>
> Signed-off-by: Luke Nelson <luke.r.nels@...il.com>
Acked-by: Wang YanQing <udknight@...il.com>