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Message-ID: <CACT4Y+b=si6NCx=nRHKm_pziXnVMmLo-eSuRajsxmx5+Hy_ycg@mail.gmail.com>
Date: Mon, 7 Jun 2021 09:38:43 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Yonghong Song <yhs@...com>, Kurt Manucredo <fuzzybritches0@...il.com>,
syzbot+bed360704c521841c85d@...kaller.appspotmail.com,
Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
Daniel Borkmann <daniel@...earbox.net>, "David S. Miller" <davem@...emloft.net>,
Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>,
Martin KaFai Lau <kafai@...com>, KP Singh <kpsingh@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>,
Song Liu <songliubraving@...com>, syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
nathan@...nel.org, Nick Desaulniers <ndesaulniers@...gle.com>,
Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
linux-kernel-mentees@...ts.linuxfoundation.org,
Shuah Khan <skhan@...uxfoundation.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run
On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@...com> wrote:
> > On 6/5/21 8:01 AM, Kurt Manucredo wrote:
> > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run()
> > > kernel/bpf/core.c:1414:2.
> >
> > This is not enough. We need more information on why this happens
> > so we can judge whether the patch indeed fixed the issue.
> >
> > >
> > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid
> > > missing them and return with error when detected.
> > >
> > > Reported-and-tested-by: syzbot+bed360704c521841c85d@...kaller.appspotmail.com
> > > Signed-off-by: Kurt Manucredo <fuzzybritches0@...il.com>
> > > ---
> > >
> > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231
> > >
> > > Changelog:
> > > ----------
> > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals.
> > > Fix commit message.
> > > v3 - Make it clearer what the fix is for.
> > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
> > > check in check_alu_op() in verifier.c.
> > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
> > > check in ___bpf_prog_run().
> > >
> > > thanks
> > >
> > > kind regards
> > >
> > > Kurt
> > >
> > > kernel/bpf/verifier.c | 30 +++++++++---------------------
> > > 1 file changed, 9 insertions(+), 21 deletions(-)
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 94ba5163d4c5..ed0eecf20de5 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> > > u32_min_val = src_reg.u32_min_value;
> > > u32_max_val = src_reg.u32_max_value;
> > >
> > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) &&
> > > + umax_val >= insn_bitness) {
> > > + /* Shifts greater than 31 or 63 are undefined.
> > > + * This includes shifts by a negative number.
> > > + */
> > > + verbose(env, "invalid shift %lld\n", umax_val);
> > > + return -EINVAL;
> > > + }
> >
> > I think your fix is good. I would like to move after
>
> I suspect such change will break valid programs that do shift by register.
>
> > the following code though:
> >
> > if (!src_known &&
> > opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
> > __mark_reg_unknown(env, dst_reg);
> > return 0;
> > }
> >
> > > +
> > > if (alu32) {
> > > src_known = tnum_subreg_is_const(src_reg.var_off);
> > > if ((src_known &&
> > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> > > scalar_min_max_xor(dst_reg, &src_reg);
> > > break;
> > > case BPF_LSH:
> > > - if (umax_val >= insn_bitness) {
> > > - /* Shifts greater than 31 or 63 are undefined.
> > > - * This includes shifts by a negative number.
> > > - */
> > > - mark_reg_unknown(env, regs, insn->dst_reg);
> > > - break;
> > > - }
> >
> > I think this is what happens. For the above case, we simply
> > marks the dst reg as unknown and didn't fail verification.
> > So later on at runtime, the shift optimization will have wrong
> > shift value (> 31/64). Please correct me if this is not right
> > analysis. As I mentioned in the early please write detailed
> > analysis in commit log.
>
> The large shift is not wrong. It's just undefined.
> syzbot has to ignore such cases.
Hi Alexei,
The report is produced by KUBSAN. I thought there was an agreement on
cleaning up KUBSAN reports from the kernel (the subset enabled on
syzbot at least).
What exactly cases should KUBSAN ignore?
+linux-hardening/kasan-dev for KUBSAN false positive
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