Re: [PATCH bpf-next v1 07/13] bpf: Fix partial dynptr stack slot reads/writes

Kumar Kartikeya Dwivedi
2022-10-22T04:08:43Z

On Sat, Oct 22, 2022 at 04:20:28AM IST, Joanne Koong wrote:
> On Tue, Oct 18, 2022 at 6:59 AM Kumar Kartikeya Dwivedi
> <memxor@gmail.com> wrote:
> >
> > Currently, while reads are disallowed for dynptr stack slots, writes are
> > not. Reads don't work from both direct access and helpers, while writes
> > do work in both cases, but have the effect of overwriting the slot_type.
> >
> > While this is fine, handling for a few edge cases is missing. Firstly,
> > a user can overwrite the stack slots of dynptr partially.
> >
> > Consider the following layout:
> > spi: [d][d][?]
> > 2 1 0
> >
> > First slot is at spi 2, second at spi 1.
> > Now, do a write of 1 to 8 bytes for spi 1.
> >
> > This will essentially either write STACK_MISC for all slot_types or
> > STACK_MISC and STACK_ZERO (in case of size < BPF_REG_SIZE partial write
> > of zeroes). The end result is that slot is scrubbed.
> >
> > Now, the layout is:
> > spi: [d][m][?]
> > 2 1 0
> >
> > Suppose if user initializes spi = 1 as dynptr.
> > We get:
> > spi: [d][d][d]
> > 2 1 0
> >
> > But this time, both spi 2 and spi 1 have first_slot = true.
> >
> > Now, when passing spi 2 to dynptr helper, it will consider it as
> > initialized as it does not check whether second slot has first_slot ==
> > false. And spi 1 should already work as normal.
> >
> > This effectively replaced size + offset of first dynptr, hence allowing
> > invalid OOB reads and writes.
> >
> > Make a few changes to protect against this:
> > When writing to PTR_TO_STACK using BPF insns, when we touch spi of a
> > STACK_DYNPTR type, mark both first and second slot (regardless of which
> > slot we touch) as STACK_INVALID. Reads are already prevented.
> >
> > Second, prevent writing to stack memory from helpers if the range may
> > contain any STACK_DYNPTR slots. Reads are already prevented.
> >
> > For helpers, we cannot allow it to destroy dynptrs from the writes as
> > depending on arguments, helper may take uninit_mem and dynptr both at
> > the same time. This would mean that helper may write to uninit_mem
> > before it reads the dynptr, which would be bad.
> >
> > PTR_TO_MEM: [?????dd]
> >
> > Depending on the code inside the helper, it may end up overwriting the
> > dynptr contents first and then read those as the dynptr argument.
> >
> > Verifier would only simulate destruction when it does byte by byte
> > access simulation in check_helper_call for meta.access_size, and
> > fail to catch this case, as it happens after argument checks.
> >
> > The same would need to be done for any other non-trivial objects created
> > on the stack in the future, such as bpf_list_head on stack, or
> > bpf_rb_root on stack.
> >
> > A common misunderstanding in the current code is that MEM_UNINIT means
> > writes, but note that writes may also be performed even without
> > MEM_UNINIT in case of helpers, in that case the code after handling meta
> > && meta->raw_mode will complain when it sees STACK_DYNPTR. So that
> > invalid read case also covers writes to potential STACK_DYNPTR slots.
> > The only loophole was in case of meta->raw_mode which simulated writes
> > through instructions which could overwrite them.
> >
> > A future series sequenced after this will focus on the clean up of
> > helper access checks and bugs around that.
>
> thanks for your work on this (and on the rest of the stack, which I'm
> still working on reviewing)
>
> Regarding writes leading to partial dynptr stack slots, I'm regretting
> not having the verifier flat-out reject this in the first place
> (instead of it being allowed but internally the stack slot gets marked
> as invalid) - I think it overall ends up being more confusing to end
> users, where there it's not obvious at all that writing to the dynptr
> on the stack automatically invalidates it. I'm not sure whether it's
> too late from a public API behavior perspective to change this or not.

It would be incorrect to reject writes into dynptrs whose reference is not
tracked by the verifier (so bpf_dynptr_from_mem), because the compiler would be
free to reuse the stack space for some other variable when the local dynptr
variable's lifetime ends, and the verifier would have no way to know when the
variable went out of scope.

I feel it is also incorrect to refuse bpf_dynptr_from_mem where unref dynptr
already exists as well. Right now it sees STACK_DYNPTR in the slot_type and
fails. But consider something like this:

void prog(void)
{
{
struct bpf_dynptr ptr;
bpf_dynptr_from_mem(...);
...
}

...

{
struct bpf_dynptr ptr;
bpf_dynptr_from_mem(...);
}
}

The program is valid, but if ptr in both scopes share the same stack slots, the
call in the second scope would fail because verifier would see STACK_DYNPTR in
slot_type.

It is fine though to simply reject writes in case of dynptrs obtained from
bpf_ringbuf_reserve_dynptr, because if they are overwritten before being
released, it will end up being an error later due to unreleased reference state.
The lifetime of the object in this case is being controlled using BPF helpers
explicitly.

So I think it is ok to do in the second case, and it is unaffected by backward
compatibility constraints. It wouldn't have been possible for the unref case
even when you started out with this.

> ANyways, assuming it is too late, I left a few comments below.
>
> >
> > Fixes: 97e03f521050 ("bpf: Add verifier support for dynptrs")
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> > ---
> > kernel/bpf/verifier.c | 76 +++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 76 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 0fd73f96c5e2..89ae384ea6a7 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -740,6 +740,8 @@ static void mark_dynptr_cb_reg(struct bpf_reg_state *reg1,
> > __mark_dynptr_regs(reg1, NULL, type);
> > }
> >
> > +static void destroy_stack_slots_dynptr(struct bpf_verifier_env *env,
> > + struct bpf_func_state *state, int spi);
> >
> > static int mark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
> > enum bpf_arg_type arg_type, int insn_idx)
> > @@ -755,6 +757,9 @@ static int mark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_reg_
> > if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS))
> > return -EINVAL;
> >
> > + destroy_stack_slots_dynptr(env, state, spi);
> > + destroy_stack_slots_dynptr(env, state, spi - 1);
>
> I don't think we need these two lines. mark_stack_slots_dynptr() is
> called only in the case where an uninitialized dynptr is getting
> initialized; is_dynptr_reg_valid_uninit() will have already been
> called prior to this (in check_func_arg()), where
> is_dynptr_reg_valid_uninit() will have checked that for any
> uninitialized dynptr, the stack slot has not already been marked as
> STACK_DYNTPR. Maybe I'm missing something in this analysis? What are
> your thoughts?
>

You're right, it shouldn't be needed here now.
In case of insn writes we already destroy both slots of a pair.

If we decide to allow mark_stack_slots_dynptr on STACK_DYNPTR that is
unreferenced, per the discussion above, I will keep it, because it would be
needed then, otherwise I will drop it.

> > +
> > for (i = 0; i < BPF_REG_SIZE; i++) {
> > state->stack[spi].slot_type[i] = STACK_DYNPTR;
> > state->stack[spi - 1].slot_type[i] = STACK_DYNPTR;
> > @@ -829,6 +834,44 @@ static int unmark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_re
> > return 0;
> > }
> >
> > +static void destroy_stack_slots_dynptr(struct bpf_verifier_env *env,
> > + struct bpf_func_state *state, int spi)
> > +{
> > + int i;
> > +
> > + /* We always ensure that STACK_DYNPTR is never set partially,
> > + * hence just checking for slot_type[0] is enough. This is
> > + * different for STACK_SPILL, where it may be only set for
> > + * 1 byte, so code has to use is_spilled_reg.
> > + */
> > + if (state->stack[spi].slot_type[0] != STACK_DYNPTR)
> > + return;
> > + /* Reposition spi to first slot */
> > + if (!state->stack[spi].spilled_ptr.dynptr.first_slot)
> > + spi = spi + 1;
> > +
> > + mark_stack_slot_scratched(env, spi);
> > + mark_stack_slot_scratched(env, spi - 1);
> > +
> > + /* Writing partially to one dynptr stack slot destroys both. */
> > + for (i = 0; i < BPF_REG_SIZE; i++) {
> > + state->stack[spi].slot_type[i] = STACK_INVALID;
> > + state->stack[spi - 1].slot_type[i] = STACK_INVALID;
> > + }
> > +
> > + /* Do not release reference state, we are destroying dynptr on stack,
> > + * not using some helper to release it. Just reset register.
> > + */
> > + __mark_reg_not_init(env, &state->stack[spi].spilled_ptr);
> > + __mark_reg_not_init(env, &state->stack[spi - 1].spilled_ptr);
> > +
> > + /* Same reason as unmark_stack_slots_dynptr above */
> > + state->stack[spi].spilled_ptr.live |= REG_LIVE_WRITTEN;
> > + state->stack[spi - 1].spilled_ptr.live |= REG_LIVE_WRITTEN;
> > +
> > + return;
> > +}
>
> I think it'd be cleaner if we combined this and
> unmark_stack_slots_dynptr() into one function. The logic is pretty
> much the same except for if the reference state should be released.
>

Ack, will do. I can put this logic in a common function and both could be
callers of that, passing true/false, so it remains readable while avoiding the
duplication.

> > +
> > static bool is_dynptr_reg_valid_uninit(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
> > {
> > struct bpf_func_state *state = func(env, reg);
> > @@ -3183,6 +3226,8 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> > env->insn_aux_data[insn_idx].sanitize_stack_spill = true;
> > }
> >
> > + destroy_stack_slots_dynptr(env, state, spi);
>
> If the stack slot is a dynptr, I think we can just return after this
> call, else we do extra work and mark the stack slots as STACK_MISC
> (3rd case in the if statement).
>

That is the intention here. The destroy_stack_slots_dynptr overwrites two slots,
while we still simulate the write to the slot being written to.

[?][d][d]
2 1 0

If I wrote to spi = 1, it would now be [?][m][?].
Earlier it would have been [?][m][d].

Any stray write (either fixed or variable offset) to a dynptr slot ends the
lifetime of the dynptr object, so both slots representing the dynptr object need
to be invalidated.

But the write itself needs to happen, and its state has to be reflected in the
stack state for those particular slot(s).

The main point here is to prevent partial destruction, which allows manifesting
the case described in the commit log. Writing to one slot of the two
representing a dynptr invalidates both.

> > +
> > mark_stack_slot_scratched(env, spi);
> > if (reg && !(off % BPF_REG_SIZE) && register_is_bounded(reg) &&
> > !register_is_null(reg) && env->bpf_capable) {
> > @@ -3296,6 +3341,13 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env,
> > if (err)
> > return err;
> >
> > + for (i = min_off; i < max_off; i++) {
> > + int slot, spi;
> > +
> > + slot = -i - 1;
> > + spi = slot / BPF_REG_SIZE;
> > + destroy_stack_slots_dynptr(env, state, spi);
> > + }
> >
>
> Instead of calling destroy_stack_slots_dynptr() in
> check_stack_write_fixed_off() and check_stack_write_var_off(), I think
> calling it from check_stack_write() would be a better place. I think
> that'd be more efficient as well where if it is a write to a dynptr,
> we can directly return after invalidating the stack slot.
>

We cannot directly return, as explained above.

> > /* Variable offset writes destroy any spilled pointers in range. */
> > for (i = min_off; i < max_off; i++) {
> > @@ -5257,6 +5309,30 @@ static int check_stack_range_initialized(
> > }
> >
> > if (meta && meta->raw_mode) {
> > + /* Ensure we won't be overwriting dynptrs when simulating byte
> > + * by byte access in check_helper_call using meta.access_size.
> > + * This would be a problem if we have a helper in the future
> > + * which takes:
> > + *
> > + * helper(uninit_mem, len, dynptr)
> > + *
> > + * Now, uninint_mem may overlap with dynptr pointer. Hence, it
> > + * may end up writing to dynptr itself when touching memory from
> > + * arg 1. This can be relaxed on a case by case basis for known
> > + * safe cases, but reject due to the possibilitiy of aliasing by
> > + * default.
> > + */
> > + for (i = min_off; i < max_off + access_size; i++) {
> > + slot = -i - 1;
> > + spi = slot / BPF_REG_SIZE;
>
> I think we can just use get_spi(i) here
>

Ack.

> > + /* raw_mode may write past allocated_stack */
> > + if (state->allocated_stack <= slot)
> > + continue;
>
> break?
>

I think you realised why it's continue in your other reply :).