Re: [PATCH bpf-next v7 02/24] bpf/verifier: allow kfunc to read user provided context

Kumar Kartikeya Dwivedi
2022-07-21T20:15:57Z

On Thu, 21 Jul 2022 at 17:36, 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.
>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
>
> ---
>

LGTM, with just a couple more nits.
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

> 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..d5fe7e618c52 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) {
> + enum bpf_access_type atype = meta->raw_mode ? BPF_WRITE : BPF_READ;
> + int offset = access_size - 1;
> +
> + /* Allow zero-byte read from NULL or PTR_TO_CTX */

This will not be handling the case for NULL, only for kfunc(ptr_to_ctx, 0)
A null pointer has its reg->type as scalar, so it will be handled by
the default case.

> + if (access_size == 0)
> + return zero_size_allowed ? 0 : -EINVAL;

We should use -EACCES, just to be consistent.

> +
> + return check_mem_access(env, env->insn_idx, regno, offset, BPF_B,
> + atype, -1, false);
> + }
> +
> + fallthrough;
> default: /* scalar_value or invalid ptr */
> /* Allow zero-byte read from NULL, regardless of pointer type */
> if (zero_size_allowed && access_size == 0 &&
> @@ -5335,6 +5355,7 @@ int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state
> WARN_ON_ONCE(regno < BPF_REG_2 || regno > BPF_REG_5);
>
> memset(&meta, 0, sizeof(meta));
> + meta.is_kfunc = true;
>
> if (may_be_null) {
> saved_reg = *mem_reg;
> --
> 2.36.1
>