Re: [PATCH bpf-next v1 06/13] bpf: Fix missing var_off check for ARG_PTR_TO_DYNPTR

Kumar Kartikeya Dwivedi
2022-10-21T01:54:16Z

On Fri, Oct 21, 2022 at 06:16:27AM IST, Alexei Starovoitov wrote:
> On Thu, Oct 20, 2022 at 08:53:45AM +0530, Kumar Kartikeya Dwivedi wrote:
> > On Thu, Oct 20, 2022 at 08:26:44AM IST, Alexei Starovoitov wrote:
> > > On Wed, Oct 19, 2022 at 7:40 PM Kumar Kartikeya Dwivedi
> > > <memxor@gmail.com> wrote:
> > > >
> > > > On Thu, Oct 20, 2022 at 07:43:16AM IST, Alexei Starovoitov wrote:
> > > > > On Wed, Oct 19, 2022 at 6:04 PM Kumar Kartikeya Dwivedi
> > > > > <memxor@gmail.com> wrote:
> > > > > >
> > > > > > On Thu, Oct 20, 2022 at 12:22:56AM IST, Alexei Starovoitov wrote:
> > > > > > > On Tue, Oct 18, 2022 at 6:59 AM Kumar Kartikeya Dwivedi
> > > > > > > <memxor@gmail.com> 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@gmail.com>
> > > > > > > > ---
> > > > > > > > kernel/bpf/verifier.c | 80 +++++++++++++++----
> > > > > > > > .../testing/selftests/bpf/prog_tests/dynptr.c | 6 +-
> > > > > > > > .../bpf/prog_tests/kfunc_dynptr_param.c | 2 +-
> > > > > > > > 3 files changed, 67 insertions(+), 21 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > > > > > > index 8f667180f70f..0fd73f96c5e2 100644
> > > > > > > > --- a/kernel/bpf/verifier.c
> > > > > > > > +++ b/kernel/bpf/verifier.c
> > > > > > > > @@ -610,11 +610,34 @@ 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 spi;
> > > > > > > > +
> > > > > > > > + if (reg->off % BPF_REG_SIZE) {
> > > > > > > > + verbose(env, "cannot pass in dynptr at an offset=%d\n", reg->off);
> > > > > > > > + return -EINVAL;
> > > > > > > > + }
> > > > > > >
> > > > > > > I think this cannot happen.
> > > > > > >
> > > > > >
> > > > > > There are existing selftests that trigger this.
> > > > >
> > > > > Really. Which one is that?
> > > > > Those that you've modified in this patch are hitting
> > > > > "cannot pass in dynptr..." message from the check below, no?
> > > > >
> > > >
> > > > Just taking one example, invalid_read2 which does:
> > > >
> > > > bpf_dynptr_read(read_data, sizeof(read_data), (void *)&ptr + 1, 0, 0);
> > > >
> > > > does hit this one, it passes fp-15, no var_off.
> > > >
> > > > Same with invalid_helper2 that was updated.
> > > > Same with invalid_offset that was updated.
> > > > invalid_write3 gained coverage from this patch, earlier it was probably just
> > > > being rejected because of arg_type_is_release checking spilled_ptr.id.
> > > > not_valid_dynptr is also hitting this one, not the one below.
> > > >
> > > > The others now started hitting this error as the order of checks was changed in
> > > > the verifier. Since arg_type_is_release checking happens before
> > > > process_dynptr_func, it uses dynptr_get_spi to check ref_obj_id of spilled_ptr.
> > > > At that point no checks have been made of the dynptr argument, so dynptr_get_spi
> > > > is required to ensure spi is in bounds.
> > > >
> > > > The reg->off % BPF_REG_SIZE was earlier in check_func_arg_reg_off but that alone
> > > > is not sufficient. This is why I wrapped everything into dynptr_get_spi.
> > >
> > > I see. That was not obvious at all that some other patch
> > > is removing that check from check_func_arg_reg_off.
> > >
> >
> > It is done in patch 4. There I move that check from the check_func_arg_reg_off
> > to process_dynptr_func.
>
> "Finally, since check_func_arg_reg_off is meant to be generic, move
> dynptr specific check into process_dynptr_func."
>
> It's a sign that patch 4 is doing too much. It should be at least two patches.
>
> >
> > > Why is the check there not sufficient?
> > >
> >
> > I wanted to keep check_func_arg_reg_off free of assumptions for helper specific
> > checks. It just ensures a few rules:
>
> Currently it's
> case PTR_TO_STACK:
> if (arg_type_is_dynptr(arg_type) && reg->off % BPF_REG_SIZE) {
> it's not really helper specific.
>
> process_dynptr_func may be the right palce to check for alignment,
> but imo the patch set is doing way too much.
> Instead of fixing dynptr specific issues it goes into massive refactoring.
> Please do one or the other.
> One patch set for refactoring only with no functional changes.
> Another patch set with fixes.
> Either order is fine.

Ok, I will split it into two. First send the refactorings (and incorporate
feedback based on the discussion), and then the fixes on top of that.

Thanks.