netdev - Re: [PATCH bpf 1/2] bpf: fix x64 JIT code generation for jmp to 1st insn

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Date:   Wed, 31 Jul 2019 20:43:33 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf 1/2] bpf: fix x64 JIT code generation for jmp to 1st insn

On Wed, Jul 31, 2019 at 12:36 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Jul 30, 2019, at 6:38 PM, Alexei Starovoitov <ast@...nel.org> wrote:
> >
> > Introduction of bounded loops exposed old bug in x64 JIT.
> > JIT maintains the array of offsets to the end of all instructions to
> > compute jmp offsets.
> > addrs[0] - offset of the end of the 1st insn (that includes prologue).
> > addrs[1] - offset of the end of the 2nd insn.
> > JIT didn't keep the offset of the beginning of the 1st insn,
> > since classic BPF didn't have backward jumps and valid extended BPF
> > couldn't have a branch to 1st insn, because it didn't allow loops.
> > With bounded loops it's possible to construct a valid program that
> > jumps backwards to the 1st insn.
> > Fix JIT by computing:
> > addrs[0] - offset of the end of prologue == start of the 1st insn.
> > addrs[1] - offset of the end of 1st insn.
> >
> > Reported-by: syzbot+35101610ff3e83119b1b@...kaller.appspotmail.com
> > Fixes: 2589726d12a1 ("bpf: introduce bounded loops")
> > Fixes: 0a14842f5a3c ("net: filter: Just In Time compiler for x86-64")
> > Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>
> Acked-by: Song Liu <songliubraving@...com>
>
> Do we need similar fix for x86_32?

Right. x86_32 would need similar fix.

Applied to bpf tree.