Subject: Re: [PATCH bpf v3 1/2] bpf: Fix verifier tracking scalars on spill

From: Maxim Mikityanskiy <[email-masked]>
Date: Wed, 7 Jun 2023 10:36:03 +0300

On Tue, 06 Jun 2023 at 18:32:37 -0700, Yonghong Song wrote:
> 
> 
> On 6/6/23 2:42 PM, Maxim Mikityanskiy wrote:
> > From: Maxim Mikityanskiy <[email-masked]>
> > 
> > The following scenario describes a verifier bypass in privileged mode
> > (CAP_BPF or CAP_SYS_ADMIN):
> > 
> > 1. Prepare a 32-bit rogue number.
> > 2. Put the rogue number into the upper half of a 64-bit register, and
> >     roll a random (unknown to the verifier) bit in the lower half. The
> >     rest of the bits should be zero (although variations are possible).
> > 3. Assign an ID to the register by MOVing it to another arbitrary
> >     register.
> > 4. Perform a 32-bit spill of the register, then perform a 32-bit fill to
> >     another register. Due to a bug in the verifier, the ID will be
> >     preserved, although the new register will contain only the lower 32
> >     bits, i.e. all zeros except one random bit.
> > 
> > At this point there are two registers with different values but the same
> > ID, which means the integrity of the verifier state has been corrupted.
> > Next steps show the actual bypass:
> > 
> > 5. Compare the new 32-bit register with 0. In the branch where it's
> >     equal to 0, the verifier will believe that the original 64-bit
> >     register is also 0, because it has the same ID, but its actual value
> >     still contains the rogue number in the upper half.
> >     Some optimizations of the verifier prevent the actual bypass, so
> >     extra care is needed: the comparison must be between two registers,
> >     and both branches must be reachable (this is why one random bit is
> >     needed). Both branches are still suitable for the bypass.
> > 6. Right shift the original register by 32 bits to pop the rogue number.
> > 7. Use the rogue number as an offset with any pointer. The verifier will
> >     believe that the offset is 0, while in reality it's the given number.
> > 
> > The fix is similar to the 32-bit BPF_MOV handling in check_alu_op for
> > SCALAR_VALUE. If the spill is narrowing the actual register value, don't
> > keep the ID, make sure it's reset to 0.
> > 
> > Fixes: 354e8f1970f8 ("bpf: Support <8-byte scalar spill and refill")
> > Signed-off-by: Maxim Mikityanskiy <[email-masked]>
> 
> LGTM with a small nit below.
> 
> Acked-by: Yonghong Song <[email-masked]>
> 
> > ---
> >   kernel/bpf/verifier.c | 7 +++++++
> >   1 file changed, 7 insertions(+)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 5871aa78d01a..7be23eced561 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -3856,6 +3856,8 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> >   	mark_stack_slot_scratched(env, spi);
> >   	if (reg && !(off % BPF_REG_SIZE) && register_is_bounded(reg) &&
> >   	    !register_is_null(reg) && env->bpf_capable) {
> > +		bool reg_value_fits;
> > +
> >   		if (dst_reg != BPF_REG_FP) {
> >   			/* The backtracking logic can only recognize explicit
> >   			 * stack slot address like [fp - 8]. Other spill of
> > @@ -3867,7 +3869,12 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> >   			if (err)
> >   				return err;
> >   		}
> > +
> > +		reg_value_fits = fls64(reg->umax_value) <= BITS_PER_BYTE * size;
> >   		save_register_state(state, spi, reg, size);
> > +		/* Break the relation on a narrowing spill. */
> > +		if (!reg_value_fits)
> > +			state->stack[spi].spilled_ptr.id = 0;
> 
> I think the code can be simplied like below:
> 
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4230,6 +4230,8 @@ static int check_stack_write_fixed_off(struct
> bpf_verifier_env *env,
>                                 return err;
>                 }
>                 save_register_state(state, spi, reg, size);
> +               if (fls64(reg->umax_value) > BITS_PER_BYTE * size)
> +                       state->stack[spi].spilled_ptr.id = 0;
>         } else if (!reg && !(off % BPF_REG_SIZE) && is_bpf_st_mem(insn) &&
>                    insn->imm != 0 && env->bpf_capable) {
>                 struct bpf_reg_state fake_reg = {};
> 

That's true, I kept the variable to avoid churn when I send a follow-up
improvement:

+               /* Make sure that reg had an ID to build a relation on spill. */
+               if (reg_value_fits && !reg->id)
+                       reg->id = ++env->id_gen;
                save_register_state(state, spi, reg, size);

But yeah, I agree, let's simplify it for now, there is no guarantee that
the follow-up patch will be accepted as is. Thanks for the review!

> >   	} else if (!reg && !(off % BPF_REG_SIZE) && is_bpf_st_mem(insn) &&
> >   		   insn->imm != 0 && env->bpf_capable) {
> >   		struct bpf_reg_state fake_reg = {};