Re: [PATCH bpf-next v8 02/24] bpf/verifier: allow kfunc to read user provided context

Alexei Starovoitov
2022-07-22T16:16:50Z

On Fri, Jul 22, 2022 at 1:46 AM Benjamin Tissoires
<benjamin.tissoires@redhat.com> wrote:
>
> When a kfunc was trying to access data from context in a syscall eBPF
> program, the verifier was rejecting the call.
> 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 and allow such situation to happen.
>
> Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
>
> ---
>
> 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/verifier.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7c1e056624f9..c807c5d7085a 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -248,6 +248,7 @@ struct bpf_call_arg_meta {
> struct bpf_map *map_ptr;
> bool raw_mode;
> bool pkt_access;
> + bool is_kfunc;
> u8 release_regno;
> int regno;
> int access_size;
> @@ -5170,6 +5171,7 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
> struct bpf_call_arg_meta *meta)
> {
> struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
> + enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
> u32 *max_access;
>
> switch (base_type(reg->type)) {
> @@ -5223,6 +5225,24 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
> env,
> regno, reg->off, access_size,
> zero_size_allowed, ACCESS_HELPER, meta);
> + case PTR_TO_CTX:
> + /* in case of a kfunc called in a program of type SYSCALL, the context is
> + * user supplied, so not computed statically.
> + * Dynamically check it now
> + */
> + if (prog_type == BPF_PROG_TYPE_SYSCALL && meta && meta->is_kfunc) {

prog_type check looks a bit odd here.
Can we generalize with
if (!env->ops->convert_ctx_access

In other words any program type that doesn't have ctx rewrites can
use helpers to access ctx fields ?

Also why kfunc only?
It looks safe to allow normal helpers as well.