Re: [PATCH bpf-next v1 3/8] bpf: Fix partial dynptr stack slot reads/writes

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Sat, Jan 07, 2023 at 12:46:23AM IST, Joanne Koong wrote:
> On Sun, Jan 1, 2023 at 12:34 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.
> >
> > Fixes: 97e03f521050 ("bpf: Add verifier support for dynptrs")
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> > ---
> >  kernel/bpf/verifier.c | 73 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 73 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index ca970f80e395..b985d90505cc 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -769,6 +769,8 @@ static void mark_dynptr_cb_reg(struct bpf_reg_state *reg,
> >         __mark_dynptr_reg(reg, type, true);
> >  }
> >
> > +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)
> > @@ -858,6 +860,44 @@ 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;
>
> nit: an empty line here helps readability
>

Ok.

> > +       /* 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.
> > +        */
>
> I agree with Andrii's point - I think it'd be more helpful if we error
> out here if the dynptr is refcounted. It'd be easy to check too, we
> already have dynptr_type_refcounted().
>

Ack, I'll change it to return an error early.

> > +       __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 we should also invalidate any data slices associated with the
> dynptrs? It seems natural that once a dynptr is invalidated, none of
> its data slices should be usable.
>

Great catch, will fix.

> > +}
> > +
> >  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);
> > @@ -3384,6 +3424,8 @@ 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);
> > +
> >         mark_stack_slot_scratched(env, spi);
> >         if (reg && !(off % BPF_REG_SIZE) && register_is_bounded(reg) &&
> >             !register_is_null(reg) && env->bpf_capable) {
> > @@ -3497,6 +3539,13 @@ 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;
>
> I think you can just use __get_spi() here
>

Ack.

> > +               destroy_stack_slots_dynptr(env, state, spi);
>
> I think here too,
>
> if (state->stack[spi].slot_type[0] == STACK_DYNPTR)
>     destroy_stack_slots_dynptr(env, state, spi)
>
> makes it more readable.
>
> And if it is a STACK_DYNPTR, we can also fast-forward i.
>

No issues with such a change, but it's going to precede almost every call to
this function. I don't have a strong preference, but we could also call it
destroy_if_dynptr_stack_slot to make it more clear the destructon is conditional
and move it inside the function.

> [...]