On Mon, Jan 9, 2023 at 3:05 AM Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
>
> On Thu, Jan 05, 2023 at 03:54:03AM IST, Andrii Nakryiko wrote:
> > 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
> > > + * chain will still be live, while earlier reg->parent was NULL, so we
> > > + * 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) {
> >
> > how about moving this check right after `if (i % BPF_REG_SIZE !=
> > BPF_REG_SIZE - 1)` ? Then we can actually generalize this to a switch
> > to handle STACK_SPILL and STACK_DYNPTR separately. I'm adding
> > STACK_ITER in my upcoming patch set, so this will have all the things
> > ready for that?
> >
> > switch (old->stack[spi].slot_type[BPF_REG_SIZE - 1]) {
> > case STACK_SPILL:
> > if (!regsafe(...))
> > return false;
> > break;
> > case STACK_DYNPTR:
> > ...
> > break;
> > /* and then eventually */
> > case STACK_ITER:
> > ...
> >
> > WDYT?
> >
>
> I can do this, it certainly makes sense with your upcoming changes, and it does
> look cleaner.
>
> > > + /* If both are STACK_DYNPTR, type must be same */
> > > + if (old->stack[spi].spilled_ptr.dynptr.type != cur->stack[spi].spilled_ptr.dynptr.type)
> >
> > struct bpf_reg_state *old_reg, *cur_reg;
> >
> > old_reg = &old->stack[spi].spilled_ptr;
> > cur_reg = &cur->stack[spi].spilled_ptr;
> >
> > and then use old_reg and cur_reg in one simple if
> >
> > here's how I have it locally:
> >
> > case STACK_DYNPTR:
> > old_reg = &old->stack[spi].spilled_ptr;
> > cur_reg = &cur->stack[spi].spilled_ptr;
> > if (old_reg->dynptr.type != cur_reg->dynptr.type ||
> > old_reg->dynptr.first_slot !=
> > cur_reg->dynptr.first_slot ||
> > !check_ids(old_reg->ref_obj_id,
> > cur_reg->ref_obj_id, idmap))
> > return false;
> > break;
> >
> > seems a bit cleaner?
> >
>
> Yep.
>
> > I'm also thinking of getting rid of first_slot field and instead have
> > a rule that first slot has proper type set, but the next one has
> > BPF_DYNPTR_TYPE_INVALID as type. This should simplify things a bit, I
> > think. At least it seems that way for STACK_ITER state I'm adding. But
> > that's a separate refactoring, probably.
> >
>
> Yeah, I'd rather not mix that into this set. Let me know if you think that's
> better and I can follow up after the next iteration with that change.
So far it looks better for me, but we can always fix it later.
>
> > > + 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))
> >
> > my previous change to tech check_ids to enforce that either id have to
> > be zeroes or non-zeroes at the same time already landed, so you don't
> > need to check `old->stack[spi].spilled_ptr.ref_obj_id`. Even more, it
> > seems wrong to do this check like this, because if cur has ref_obj_id
> > set we'll ignore it, right?
>
> The check before that ensures either both are set or both are unset. If there is
> a mismatch we return false. I see that check_ids does it now, so yes it wouldn't
> be needed anymore. I am not sure about the last part, I don't think it will be
> ignored?
Right, I forgot about that `if with !!old.ref_obj_id != !!cur.ref_obj_id`.
