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

From: "Kurt Manucredo" <[email-masked]>
Date: Sun, 06 Jun 2021 21:15:46 +0200

On Sat, 5 Jun 2021 10:55:25 -0700, 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 %lldn", umax_val);
> > +		return -EINVAL;
> > +	}
> 
> I think your fix is good. I would like to move after
> the following code though:
> 
>          if (!src_known &&
>              opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
>                  __mark_reg_unknown(env, dst_reg);
>                  return 0;
>          }
> 

It can only be right before that code not after. That's the latest. In the
case of the syzbot bug, opcode == BPF_LSH and !src_known. Therefore it
needs to be before that block of code.

> > +
> >   	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.
> 

Shouldn't the src reg be changed so that the shift-out-of-bounds can't
occur, if return -EINVAL is not what we want here? Changing the dst reg
might not help. If I look into kernel/bpf/core.c I can see:
	DST = DST OP SRC;

> Please also add a test at tools/testing/selftests/bpf/verifier/.
> 
I'm going to look into selftests,

kind regards
thanks,

Kurt Manucredo