Subject: Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

From: Kees Cook <[email-masked]>
Date: Wed, 9 Jun 2021 11:20:33 -0700

On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote:
> On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov
> <[email-masked]> wrote:
> > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <[email-masked]> 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: [email-masked]
> > > > Signed-off-by: Kurt Manucredo <[email-masked]>
> > > > ---
> > > >
> > > > 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

Can check_shl_overflow() be used at all? Best to just make things
readable and compiler-happy, whatever the implementation. :)

-- 
Kees Cook