netdev - Re: Self-XORing BPF registers is undefined behavior

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Date:   Wed, 27 May 2020 10:14:51 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Michal Kubecek <mkubecek@...e.cz>,
        Alexei Starovoitov <ast@...nel.org>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: Self-XORing BPF registers is undefined behavior

On Wed, May 27, 2020 at 10:12 AM Alexander Potapenko <glider@...gle.com> wrote:
>
> On Wed, May 27, 2020 at 6:58 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Wed, May 27, 2020 at 8:52 AM Alexander Potapenko <glider@...gle.com> wrote:
> > >
> > > This basically means that BPF's output register was uninitialized when
> > > ___bpf_prog_run() returned.
> > >
> > > When I replace the lines initializing registers A and X in net/core/filter.c:
> > >
> > > -               *new_insn++ = BPF_ALU32_REG(BPF_XOR, BPF_REG_A, BPF_REG_A);
> > > -               *new_insn++ = BPF_ALU32_REG(BPF_XOR, BPF_REG_X, BPF_REG_X);
> > >
> > > with
> > >
> > > +               *new_insn++ = BPF_MOV32_IMM(BPF_REG_A, 0);
> > > +               *new_insn++ = BPF_MOV32_IMM(BPF_REG_X, 0);
> > >
> > > , the bug goes away, therefore I think it's being caused by XORing the
> > > initially uninitialized registers with themselves.
> > >
> > > kernel/bpf/core.c:1408, where the uninitialized value was stored to
> > > memory, points to the "ALU(ADD,  +)" macro in ___bpf_prog_run().
> > > But the debug info seems to be incorrect here: if I comment this line
> > > out and unroll the macro manually, KMSAN points to "ALU(SUB,  -)".
> > > Most certainly it's actually one of the XOR instruction declarations.
> > >
> > > Do you think it makes sense to use the UB-proof BPF_MOV32_IMM
> > > instructions to initialize the registers?
> >
> > I think it's better for UBsan to get smarter about xor-ing.
>
> Could you please elaborate on this? How exactly should KMSAN handle
> this situation?
> Note that despite the source says "BPF_ALU32_REG(BPF_XOR, BPF_REG_A,
> BPF_REG_A);", it doesn't necessarily boil down to an expression like A
> = A ^ A. It's more likely that temporary values will be involved,
> making it quite hard to figure out whether the two operands are really
> the same.

I really don't know who to make it smarter. It's your area of expertise.