On Sun, Jan 1, 2023 at 12:34 AM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> The root of the problem is missing liveness marking for STACK_DYNPTR
> slots. This leads to all kinds of problems inside stacksafe.
>
> The verifier by default inside stacksafe ignores spilled_ptr in stack
> slots which do not have REG_LIVE_READ marks. Since this is being checked
> in the 'old' explored state, it must have already done clean_live_states
> for this old bpf_func_state. Hence, it won't be receiving any more
> liveness marks from to be explored insns (it has received REG_LIVE_DONE
> marking from liveness point of view).
>
> What this means is that verifier considers that it's safe to not compare
> the stack slot if was never read by children states. While liveness
> marks are usually propagated correctly following the parentage chain for
> spilled registers (SCALAR_VALUE and PTR_* types), the same is not the
> case for STACK_DYNPTR.
>
> clean_live_states hence simply rewrites these stack slots to the type
> STACK_INVALID since it sees no REG_LIVE_READ marks.
>
> The end result is that we will never see STACK_DYNPTR slots in explored
> state. Even if verifier was conservatively matching !REG_LIVE_READ
> slots, very next check continuing the stacksafe loop on seeing
> STACK_INVALID would again prevent further checks.
>
> Now as long as verifier stores an explored state which we can compare to
> when reaching a pruning point, we can abuse this bug to make verifier
> prune search for obviously unsafe paths using STACK_DYNPTR slots
> thinking they are never used hence safe.
>
> Doing this in unprivileged mode is a bit challenging. add_new_state is
> only set when seeing BPF_F_TEST_STATE_FREQ (which requires privileges)
> or when jmps_processed difference is >= 2 and insn_processed difference
> is >= 8. So coming up with the unprivileged case requires a little more
> work, but it is still totally possible. The test case being discussed
> below triggers the heuristic even in unprivileged mode.
>
> However, it no longer works since commit
> 8addbfc7b308 ("bpf: Gate dynptr API behind CAP_BPF").
>
> Let's try to study the test step by step.
>
> Consider the following program (C style BPF ASM):
>
> 0 r0 = 0;
> 1 r6 = &ringbuf_map;
> 3 r1 = r6;
> 4 r2 = 8;
> 5 r3 = 0;
> 6 r4 = r10;
> 7 r4 -= -16;
> 8 call bpf_ringbuf_reserve_dynptr;
> 9 if r0 == 0 goto pc+1;
> 10 goto pc+1;
> 11 *(r10 - 16) = 0xeB9F;
> 12 r1 = r10;
> 13 r1 -= -16;
> 14 r2 = 0;
> 15 call bpf_ringbuf_discard_dynptr;
> 16 r0 = 0;
> 17 exit;
>
> We know that insn 12 will be a pruning point, hence if we force
> add_new_state for it, it will first verify the following path as
> safe in straight line exploration:
> 0 1 3 4 5 6 7 8 9 -> 10 -> (12) 13 14 15 16 17
>
> Then, when we arrive at insn 12 from the following path:
> 0 1 3 4 5 6 7 8 9 -> 11 (12)
>
> We will find a state that has been verified as safe already at insn 12.
> Since register state is same at this point, regsafe will pass. Next, in
> stacksafe, for spi = 0 and spi = 1 (location of our dynptr) is skipped
> seeing !REG_LIVE_READ. The rest matches, so stacksafe returns true.
> Next, refsafe is also true as reference state is unchanged in both
> states.
>
> The states are considered equivalent and search is pruned.
>
> Hence, we are able to construct a dynptr with arbitrary contents and use
> the dynptr API to operate on this arbitrary pointer and arbitrary size +
> offset.
>
> To fix this, first define a mark_dynptr_read function that propagates
> liveness marks whenever a valid initialized dynptr is accessed by dynptr
> helpers. REG_LIVE_WRITTEN is marked whenever we initialize an
> uninitialized dynptr. This is done in mark_stack_slots_dynptr. It allows
> screening off mark_reg_read and not propagating marks upwards from that
> point.
>
> This ensures that we either set REG_LIVE_READ64 on both dynptr slots, or
> none, so clean_live_states either sets both slots to STACK_INVALID or
> none of them. This is the invariant the checks inside stacksafe rely on.
>
> Next, do a complete comparison of both stack slots whenever they have
> STACK_DYNPTR. Compare the dynptr type stored in the spilled_ptr, and
> also whether both form the same first_slot. Only then is the later path
> safe.
>
> Fixes: 97e03f521050 ("bpf: Add verifier support for dynptrs")
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> ---
> kernel/bpf/verifier.c | 73 +++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 73 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 4a25375ebb0d..f7248235e119 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -781,6 +781,9 @@ static int mark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_reg_
> state->stack[spi - 1].spilled_ptr.ref_obj_id = id;
> }
>
> + state->stack[spi].spilled_ptr.live |= REG_LIVE_WRITTEN;
> + state->stack[spi - 1].spilled_ptr.live |= REG_LIVE_WRITTEN;
> +
> return 0;
> }
>
> @@ -805,6 +808,26 @@ static int unmark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_re
>
> __mark_reg_not_init(env, &state->stack[spi].spilled_ptr);
> __mark_reg_not_init(env, &state->stack[spi - 1].spilled_ptr);
> +
> + /* Why do we need to set REG_LIVE_WRITTEN for STACK_INVALID slot?
> + *
> + * While we don't allow reading STACK_INVALID, it is still possible to
> + * do <8 byte writes marking some but not all slots as STACK_MISC. Then,
> + * helpers or insns can do partial read of that part without failing,
> + * but check_stack_range_initialized, check_stack_read_var_off, and
> + * check_stack_read_fixed_off will do mark_reg_read for all 8-bytes of
> + * the slot conservatively. Hence we need to screen off those liveness
> + * marking walks.
> + *
> + * This was not a problem before because STACK_INVALID is only set by
> + * default, or in clean_live_states after REG_LIVE_DONE, not randomly
> + * during verifier state exploration. Hence, for this case parentage
Where does it get set randomly during verifier state exploration for this case?
> + * chain will still be live, while earlier reg->parent was NULL, so we
What does "live" in "parentage chain will still be live" here mean?
what does "earlier" in "earlier reg->parent" refer to here, and why
was the earlier reg->parent NULL?
> + * need REG_LIVE_WRITTEN to screen off read marker propagation.
> + */
> + state->stack[spi].spilled_ptr.live |= REG_LIVE_WRITTEN;
> + state->stack[spi - 1].spilled_ptr.live |= REG_LIVE_WRITTEN;
> +
> return 0;
> }
>
> @@ -2388,6 +2411,30 @@ static int mark_reg_read(struct bpf_verifier_env *env,
> return 0;
> }
>
> +static int mark_dynptr_read(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
> +{
> + struct bpf_func_state *state = func(env, reg);
> + int spi, ret;
> +
> + /* For CONST_PTR_TO_DYNPTR, it must have already been done by
> + * check_reg_arg in check_helper_call and mark_btf_func_reg_size in
> + * check_kfunc_call.
> + */
> + if (reg->type == CONST_PTR_TO_DYNPTR)
> + return 0;
> + spi = get_spi(reg->off);
> + /* Caller ensures dynptr is valid and initialized, which means spi is in
> + * bounds and spi is the first dynptr slot. Simply mark stack slot as
> + * read.
> + */
> + ret = mark_reg_read(env, &state->stack[spi].spilled_ptr,
> + state->stack[spi].spilled_ptr.parent, REG_LIVE_READ64);
> + if (ret)
> + return ret;
> + return mark_reg_read(env, &state->stack[spi - 1].spilled_ptr,
> + state->stack[spi - 1].spilled_ptr.parent, REG_LIVE_READ64);
> +}
> +
> /* This function is supposed to be used by the following 32-bit optimization
> * code only. It returns TRUE if the source or destination register operates
> * on 64-bit, otherwise return FALSE.
> @@ -5928,6 +5975,7 @@ int process_dynptr_func(struct bpf_verifier_env *env, int regno,
> enum bpf_arg_type arg_type, struct bpf_call_arg_meta *meta)
> {
> struct bpf_reg_state *regs = cur_regs(env), *reg = ®s[regno];
> + int err;
>
> /* MEM_UNINIT and MEM_RDONLY are exclusive, when applied to an
> * ARG_PTR_TO_DYNPTR (or ARG_PTR_TO_DYNPTR | DYNPTR_TYPE_*):
> @@ -6008,6 +6056,10 @@ int process_dynptr_func(struct bpf_verifier_env *env, int regno,
> err_extra, regno);
> return -EINVAL;
> }
> +
> + err = mark_dynptr_read(env, reg);
> + if (err)
> + return err;
> }
> return 0;
> }
> @@ -13204,6 +13256,27 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
> * return false to continue verification of this path
> */
> return false;
> + /* Both are same slot_type, but STACK_DYNPTR requires more
> + * checks before it can considered safe.
> + */
> + if (old->stack[spi].slot_type[i % BPF_REG_SIZE] == STACK_DYNPTR) {
> + /* If both are STACK_DYNPTR, type must be same */
> + if (old->stack[spi].spilled_ptr.dynptr.type != cur->stack[spi].spilled_ptr.dynptr.type)
> + return false;
> + /* Both should also have first slot at same spi */
> + if (old->stack[spi].spilled_ptr.dynptr.first_slot != cur->stack[spi].spilled_ptr.dynptr.first_slot)
> + return false;
> + /* ids should be same */
> + if (!!old->stack[spi].spilled_ptr.ref_obj_id != !!cur->stack[spi].spilled_ptr.ref_obj_id)
> + return false;
> + if (old->stack[spi].spilled_ptr.ref_obj_id &&
> + !check_ids(old->stack[spi].spilled_ptr.ref_obj_id,
> + cur->stack[spi].spilled_ptr.ref_obj_id, idmap))
> + return false;
> + WARN_ON_ONCE(i % BPF_REG_SIZE);
> + i += BPF_REG_SIZE - 1;
> + continue;
> + }
> if (i % BPF_REG_SIZE != BPF_REG_SIZE - 1)
> continue;
> if (!is_spilled_reg(&old->stack[spi]))
> --
> 2.39.0
>
