On Tue, 12 Jul 2022 at 17:02, Benjamin Tissoires
<benjamin.tissoires@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> ---
>
> new in v6
> ---
> kernel/bpf/verifier.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 328cfab3af60..f6af57a84247 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 = ®s[regno];
> + enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
> u32 *max_access;
>
> switch (base_type(reg->type)) {
> @@ -5223,6 +5225,19 @@ 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 access_t = meta->raw_mode ? BPF_WRITE : BPF_READ;
small nit: _t suffix is used for types, so you could probably rename
this. maybe atype?
> +
> + return check_mem_access(env, env->insn_idx, regno, access_size, BPF_B,
> + access_t, -1, false);
If I read the code correctly, this makes the max_ctx_offset of prog
access_size + 1 (off + size_to_bytes(BPF_B)), which is 1 more than the
actual size being accessed.
This also messes up check_helper_mem_access when it allows NULL, 0
pair to pass (because check is against actual size + 1). We do allow
passing NULL when size is 0 for kfuncs (see zero_size_allowed is true
in check_mem_size_reg), so your hid_hw_request function is missing
that NULL check for buf too.
In the selftest that checks for failure in loading
+ bpf_kfunc_call_test_mem_len_pass1(&args->data, sizeof(*args) + 1);
so it will still fail with just sizeof(*args).
Also please add coverage for this case in the next version.
> + }
> +
> + 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 +5350,7 @@ 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
>
