Re: [PATCH bpf-next v9 01/23] bpf/verifier: allow all functions to read user provided context

[Alexei Starovoitov]

On Tue, Aug 30, 2022 at 7:29 AM Benjamin Tissoires
<benjamin.tissoires@xxxxxxxxxx> wrote:
>
> On Fri, Aug 26, 2022 at 3:51 AM Kumar Kartikeya Dwivedi
> <memxor@xxxxxxxxx> wrote:
> >
> > On Fri, 26 Aug 2022 at 03:42, Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > >
> > > On Wed, Aug 24, 2022 at 6:41 AM Benjamin Tissoires
> > > <benjamin.tissoires@xxxxxxxxxx> wrote:
> > > >
> > > > When a function was trying to access data from context in a syscall eBPF
> > > > program, the verifier was rejecting the call unless it was accessing the
> > > > first element.
> > > > This is because the syscall context is not known at compile time, and
> > > > so we need to check this when actually accessing it.
> > > >
> > > > Check for the valid memory access if there is no convert_ctx callback,
> > > > and allow such situation to happen.
> > > >
> > > > There is a slight hiccup with subprogs. btf_check_subprog_arg_match()
> > > > will check that the types are matching, which is a good thing, but to
> > > > have an accurate result, it hides the fact that the context register may
> > > > be null. This makes env->prog->aux->max_ctx_offset being set to the size
> > > > of the context, which is incompatible with a NULL context.
> > > >
> > > > Solve that last problem by storing max_ctx_offset before the type check
> > > > and restoring it after.
> > > >
> > > > Acked-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> > > > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> > > >
> > > > ---
> > > >
> > > > changes in v9:
> > > > - rewrote the commit title and description
> > > > - made it so all functions can make use of context even if there is
> > > >   no convert_ctx
> > > > - remove the is_kfunc field in bpf_call_arg_meta
> > > >
> > > > changes in v8:
> > > > - fixup comment
> > > > - return -EACCESS instead of -EINVAL for consistency
> > > >
> > > > changes in v7:
> > > > - renamed access_t into atype
> > > > - allow zero-byte read
> > > > - check_mem_access() to the correct offset/size
> > > >
> > > > new in v6
> > > > ---
> > > >  kernel/bpf/btf.c      | 11 ++++++++++-
> > > >  kernel/bpf/verifier.c | 19 +++++++++++++++++++
> > > >  2 files changed, 29 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > > index 903719b89238..386300f52b23 100644
> > > > --- a/kernel/bpf/btf.c
> > > > +++ b/kernel/bpf/btf.c
> > > > @@ -6443,8 +6443,8 @@ int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
> > > >  {
> > > >         struct bpf_prog *prog = env->prog;
> > > >         struct btf *btf = prog->aux->btf;
> > > > +       u32 btf_id, max_ctx_offset;
> > > >         bool is_global;
> > > > -       u32 btf_id;
> > > >         int err;
> > > >
> > > >         if (!prog->aux->func_info)
> > > > @@ -6457,9 +6457,18 @@ int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
> > > >         if (prog->aux->func_info_aux[subprog].unreliable)
> > > >                 return -EINVAL;
> > > >
> > > > +       /* subprogs arguments are not actually accessing the data, we need
> > > > +        * to check for the types if they match.
> > > > +        * Store the max_ctx_offset and restore it after btf_check_func_arg_match()
> > > > +        * given that this function will have a side effect of changing it.
> > > > +        */
> > > > +       max_ctx_offset = env->prog->aux->max_ctx_offset;
> > > > +
> > > >         is_global = prog->aux->func_info_aux[subprog].linkage == BTF_FUNC_GLOBAL;
> > > >         err = btf_check_func_arg_match(env, btf, btf_id, regs, is_global, 0);
> > > >
> > > > +       env->prog->aux->max_ctx_offset = max_ctx_offset;
> > >
> > > I don't understand this.
> > > If we pass a ctx into a helper and it's going to
> > > access [0..N] bytes from it why do we need to hide it?
> > > max_ctx_offset will be used later raw_tp, tp, syscall progs
> > > to determine whether it's ok to load them.
> > > By hiding the actual size of access somebody can construct
> > > a prog that reads out of bounds.
> > > How is this related to NULL-ness property?
> >
> > Same question, was just typing exactly the same thing.
>
> The test I have that is failing in patch 2/23 is the following, with
> args being set to NULL by userspace:
>
> SEC("syscall")
> int kfunc_syscall_test_null(struct syscall_test_args *args)
> {
>        bpf_kfunc_call_test_mem_len_pass1(args, 0);
>
>        return 0;
> }
>
> Basically:
> if userspace declares the following:
>  DECLARE_LIBBPF_OPTS(bpf_test_run_opts, syscall_topts,
>                .ctx_in = NULL,
>                .ctx_size_in = 0,
>        );
>
> The verifier is happy with the current released kernel:
> kfunc_syscall_test_fail() never dereferences the ctx pointer, it just
> passes it around to bpf_kfunc_call_test_mem_len_pass1(), which in turn
> is also happy because it says it is not accessing the data at all (0
> size memory parameter).
>
> In the current code, check_helper_mem_access() actually returns
> -EINVAL, but doesn't change max_ctx_offset (it's still at the value of
> 0 here). The program is now marked as unreliable, but the verifier
> goes on.
>
> When adding this patch, if we declare a syscall eBPF (or any other
> function that doesn't have env->ops->convert_ctx_access), the previous
> "test" is failing because this ensures the syscall program has to have
> a valid ctx pointer.
> btf_check_func_arg_match() now calls check_mem_access() which
> basically validates the fact that the program can dereference the ctx.
>
> So now, without the max_ctx_offset store/restore, the verifier
> enforces that the provided ctx is not null.
>
> What I thought that would happen was that if we were to pass a NULL
> context from userspace, but the eBPF program dereferences it (or in
> that case have a subprog or a function call that dereferences it),
> then max_ctx_offset would still be set to the proper value because of
> that internal dereference, and so the verifier would reject with
> -EINVAL the call to the eBPF program.
>
> If I add another test that has the following ebpf prog (with ctx_in
> being set to NULL by the userspace):
>
> SEC("syscall")
> int kfunc_syscall_test_null_fail(struct syscall_test_args *args)
> {
>        bpf_kfunc_call_test_mem_len_pass1(args, sizeof(*args));
>
>        return 0;
> }
>
> Then the call of the program is actually failing with -EINVAL, even
> with this patch.
>
> But again, if setting from userspace a ctx of NULL with a 0 size is
> not considered as valid, then we can just drop that hunk and add a
> test to enforce it.

PTR_TO_CTX in the verifier always means valid pointer.
All code paths in the verifier assumes that it's not NULL.
Pointer to skb, to xdp, to pt_regs, etc.
The syscall prog type is little bit special, since it
makes sense not to pass any argument to such prog.
So ctx_size_in == 0 is enforced after the verification:
if (ctx_size_in < prog->aux->max_ctx_offset ||
    ctx_size_in > U16_MAX)
          return -EINVAL;
The verifier should be able to proceed assuming ctx != NULL
and remember max max_ctx_offset.
If max_ctx_offset == 4 and ctx_size_in == 0 then
it doesn't matter whether the actual 'ctx' pointer is NULL
or points to a valid memory.
So it's ok for the verifier to assume ctx != NULL everywhere.

Back to the issue at hand.
With this patch the line:
    bpf_kfunc_call_test_mem_len_pass1(args, sizeof(*args));
will be seen as access_size == sizeof(*args), right?
So this part:
+                       if (access_size == 0)
+                               return zero_size_allowed ? 0 : -EACCES;

will be skipped and
the newly added check_mem_access() will call check_ctx_access()
which will call syscall_prog_is_valid_access() and it will say
that any off < U16_MAX is fine and will simply
record max max_ctx_offset.
The ctx_size_in < prog->aux->max_ctx_offset check is done later.

So when you're saying:
"call of the program is actually failing with -EINVAL"
that's the check you're referring to?

If so, everything works as expected.
The verifier thinks that bpf_kfunc_call_test_mem_len_pass1()
can read that many bytes from args,
so it has to reject running the loaded prog in bpf_prog_test_run_syscall().

So what are you trying to achieve ?
Make the verifier understand that ctx can be NULL ?
If so that is a probably huge undertaking.
Something else?