Re: [PATCH bpf-next v1 2/8] bpf: Fix missing var_off check for ARG_PTR_TO_DYNPTR

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On Fri, Jan 06, 2023 at 06:27:06AM IST, Joanne Koong wrote:
> On Sun, Jan 1, 2023 at 12:34 AM Kumar Kartikeya Dwivedi
> <memxor@xxxxxxxxx> wrote:
> >
> > Currently, the dynptr function is not checking the variable offset part
> > of PTR_TO_STACK that it needs to check. The fixed offset is considered
> > when computing the stack pointer index, but if the variable offset was
> > not a constant (such that it could not be accumulated in reg->off), we
> > will end up a discrepency where runtime pointer does not point to the
> > actual stack slot we mark as STACK_DYNPTR.
> >
> > It is impossible to precisely track dynptr state when variable offset is
> > not constant, hence, just like bpf_timer, kptr, bpf_spin_lock, etc.
> > simply reject the case where reg->var_off is not constant. Then,
> > consider both reg->off and reg->var_off.value when computing the stack
> > pointer index.
> >
> > A new helper dynptr_get_spi is introduced to hide over these details
> > since the dynptr needs to be located in multiple places outside the
> > process_dynptr_func checks, hence once we know it's a PTR_TO_STACK, we
> > need to enforce these checks in all places.
> >
> > Note that it is disallowed for unprivileged users to have a non-constant
> > var_off, so this problem should only be possible to trigger from
> > programs having CAP_PERFMON. However, its effects can vary.
> >
> > Without the fix, it is possible to replace the contents of the dynptr
> > arbitrarily by making verifier mark different stack slots than actual
> > location and then doing writes to the actual stack address of dynptr at
> > runtime.
> >
> > Fixes: 97e03f521050 ("bpf: Add verifier support for dynptrs")
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> > ---
> >  kernel/bpf/verifier.c                         | 83 ++++++++++++++-----
> >  .../bpf/prog_tests/kfunc_dynptr_param.c       |  2 +-
> >  .../testing/selftests/bpf/progs/dynptr_fail.c |  6 +-
> >  3 files changed, 66 insertions(+), 25 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index f7248235e119..ca970f80e395 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -638,11 +638,34 @@ static void print_liveness(struct bpf_verifier_env *env,
> >                 verbose(env, "D");
> >  }
> >
> > -static int get_spi(s32 off)
> > +static int __get_spi(s32 off)
> >  {
> >         return (-off - 1) / BPF_REG_SIZE;
> >  }
> >
> > +static int dynptr_get_spi(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
> > +{
> > +       int off, spi;
> > +
> > +       if (!tnum_is_const(reg->var_off)) {
> > +               verbose(env, "dynptr has to be at the constant offset\n");
> > +               return -EINVAL;
> > +       }
> > +
> > +       off = reg->off + reg->var_off.value;
> > +       if (off % BPF_REG_SIZE) {
> > +               verbose(env, "cannot pass in dynptr at an offset=%d\n", reg->off);
>
> I think you meant off instead of reg->off?
>

Ack.

> > +               return -EINVAL;
> > +       }
> > +
> > +       spi = __get_spi(off);
> > +       if (spi < 1) {
> > +               verbose(env, "cannot pass in dynptr at an offset=%d\n", (int)(off + reg->var_off.value));
>
> I think you meant off instead of off + reg->var_off.value
>

Ack.

> > +               return -EINVAL;
> > +       }
>
> I think this if (spi < 1) check should have the same logic
> is_spi_bounds_valid() does (eg checking against total allocated slots
> as well). I think we can combine is_spi_bounds_valid() with this
> function and then every place we call is_spi_bounds_valid()
>

Ok, I'll combine both.

> > +       return spi;
> > +}
> > +
> >  static bool is_spi_bounds_valid(struct bpf_func_state *state, int spi, int nr_slots)
> >  {
> >         int allocated_slots = state->allocated_stack / BPF_REG_SIZE;
> > @@ -754,7 +777,9 @@ static int mark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_reg_
> >         enum bpf_dynptr_type type;
> >         int spi, i, id;
> >
> > -       spi = get_spi(reg->off);
> > +       spi = dynptr_get_spi(env, reg);
> > +       if (spi < 0)
> > +               return spi;
> >
> >         if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS))
> >                 return -EINVAL;
> > @@ -792,7 +817,9 @@ static int unmark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_re
> >         struct bpf_func_state *state = func(env, reg);
> >         int spi, i;
> >
> > -       spi = get_spi(reg->off);
> > +       spi = dynptr_get_spi(env, reg);
> > +       if (spi < 0)
> > +               return spi;
> >
> >         if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS))
> >                 return -EINVAL;
> > @@ -839,7 +866,11 @@ static bool is_dynptr_reg_valid_uninit(struct bpf_verifier_env *env, struct bpf_
> >         if (reg->type == CONST_PTR_TO_DYNPTR)
> >                 return false;
> >
> > -       spi = get_spi(reg->off);
> > +       spi = dynptr_get_spi(env, reg);
> > +       if (spi < 0)
> > +               return spi;
> > +
> > +       /* We will do check_mem_access to check and update stack bounds later */
> >         if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS))
> >                 return true;
> >
> > @@ -855,14 +886,15 @@ static bool is_dynptr_reg_valid_uninit(struct bpf_verifier_env *env, struct bpf_
> >  static bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
> >  {
> >         struct bpf_func_state *state = func(env, reg);
> > -       int spi;
> > -       int i;
> > +       int spi, i;
> >
> >         /* This already represents first slot of initialized bpf_dynptr */
> >         if (reg->type == CONST_PTR_TO_DYNPTR)
> >                 return true;
> >
> > -       spi = get_spi(reg->off);
> > +       spi = dynptr_get_spi(env, reg);
> > +       if (spi < 0)
> > +               return false;
> >         if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS) ||
> >             !state->stack[spi].spilled_ptr.dynptr.first_slot)
> >                 return false;
> > @@ -891,7 +923,9 @@ static bool is_dynptr_type_expected(struct bpf_verifier_env *env, struct bpf_reg
> >         if (reg->type == CONST_PTR_TO_DYNPTR) {
> >                 return reg->dynptr.type == dynptr_type;
> >         } else {
> > -               spi = get_spi(reg->off);
> > +               spi = dynptr_get_spi(env, reg);
> > +               if (WARN_ON_ONCE(spi < 0))
> > +                       return false;
> >                 return state->stack[spi].spilled_ptr.dynptr.type == dynptr_type;
> >         }
> >  }
> > @@ -2422,7 +2456,9 @@ static int mark_dynptr_read(struct bpf_verifier_env *env, struct bpf_reg_state *
> >          */
> >         if (reg->type == CONST_PTR_TO_DYNPTR)
> >                 return 0;
> > -       spi = get_spi(reg->off);
> > +       spi = dynptr_get_spi(env, reg);
> > +       if (WARN_ON_ONCE(spi < 0))
> > +               return spi;
> >         /* 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.
> > @@ -5946,6 +5982,11 @@ static int process_kptr_func(struct bpf_verifier_env *env, int regno,
> >         return 0;
> >  }
> >
> > +static bool arg_type_is_release(enum bpf_arg_type type)
> > +{
> > +       return type & OBJ_RELEASE;
> > +}
>
> nit: I dont think you need this arg_type_is_release() change
>

Ack.

> > +
> >  /* There are two register types representing a bpf_dynptr, one is PTR_TO_STACK
> >   * which points to a stack slot, and the other is CONST_PTR_TO_DYNPTR.
> >   *
> > @@ -5986,12 +6027,14 @@ int process_dynptr_func(struct bpf_verifier_env *env, int regno,
> >         }
> >         /* CONST_PTR_TO_DYNPTR already has fixed and var_off as 0 due to
> >          * check_func_arg_reg_off's logic. We only need to check offset
> > -        * alignment for PTR_TO_STACK.
> > +        * and its alignment for PTR_TO_STACK.
> >          */
> > -       if (reg->type == PTR_TO_STACK && (reg->off % BPF_REG_SIZE)) {
> > -               verbose(env, "cannot pass in dynptr at an offset=%d\n", reg->off);
> > -               return -EINVAL;
>
> > +       if (reg->type == PTR_TO_STACK) {
> > +               err = dynptr_get_spi(env, reg);
> > +               if (err < 0)
> > +                       return err;
> >         }
>
> nit: if we do something like
>
> If (reg->type == PTR_TO_STACK) {
>     spi = dynptr_get_spi(env, reg);
>     if (spi < 0)
>         return spi;
> } else {
>     spi = __get_spi(reg->off);
> }
>
> then we can just pass in spi to is_dynptr_reg_valid_uninit() and
> is_dynptr_reg_valid_init() instead of having to recompute/check them
> again
>

Seems a little misleading to set it to something in the else branch (where stack
pointer index has no meaning), but I do see your point, I guess it can be
ignored for the other case and set to 0 by default.

> [...]



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