Re: [PATCH bpf-next v9 04/23] bpf/verifier: allow kfunc to return an allocated mem
From: Kumar Kartikeya Dwivedi
Date: Thu Aug 25 2022 - 21:25:21 EST
On Wed, 24 Aug 2022 at 15:41, Benjamin Tissoires
<benjamin.tissoires@xxxxxxxxxx> wrote:
>
> For drivers (outside of network), the incoming data is not statically
> defined in a struct. Most of the time the data buffer is kzalloc-ed
> and thus we can not rely on eBPF and BTF to explore the data.
>
> This commit allows to return an arbitrary memory, previously allocated by
> the driver.
> An interesting extra point is that the kfunc can mark the exported
> memory region as read only or read/write.
>
> So, when a kfunc is not returning a pointer to a struct but to a plain
> type, we can consider it is a valid allocated memory assuming that:
> - one of the arguments is either called rdonly_buf_size or
> rdwr_buf_size
> - and this argument is a const from the caller point of view
>
> We can then use this parameter as the size of the allocated memory.
>
> The memory is either read-only or read-write based on the name
> of the size parameter.
>
> Acked-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
>
> ---
>
> changes in v9:
> - updated to match upstream (replaced kfunc_flag by a field in
> kfunc_meta)
>
> no changes in v8
>
> changes in v7:
> - ensures btf_type_is_struct_ptr() checks for a ptr first
> (squashed from next commit)
> - remove multiple_ref_obj_id need
> - use btf_type_skip_modifiers instead of manually doing it in
> btf_type_is_struct_ptr()
> - s/strncmp/strcmp/ in btf_is_kfunc_arg_mem_size()
> - check for tnum_is_const when retrieving the size value
> - have only one check for "Ensure only one argument is referenced
> PTR_TO_BTF_ID"
> - add some more context to the commit message
>
> changes in v6:
> - code review from Kartikeya:
> - remove comment change that had no reasons to be
> - remove handling of PTR_TO_MEM with kfunc releases
> - introduce struct bpf_kfunc_arg_meta
> - do rdonly/rdwr_buf_size check in btf_check_kfunc_arg_match
> - reverted most of the changes in verifier.c
> - make sure kfunc acquire is using a struct pointer, not just a plain
> pointer
> - also forward ref_obj_id to PTR_TO_MEM in kfunc to not use after free
> the allocated memory
>
> changes in v5:
> - updated PTR_TO_MEM comment in btf.c to match upstream
> - make it read-only or read-write based on the name of size
>
> new in v4
>
> change btf.h
>
> fix allow kfunc to return an allocated mem
> ---
> include/linux/bpf.h | 9 +++-
> include/linux/btf.h | 10 +++++
> kernel/bpf/btf.c | 98 ++++++++++++++++++++++++++++++++++---------
> kernel/bpf/verifier.c | 43 +++++++++++++------
> 4 files changed, 128 insertions(+), 32 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 39bd36359c1e..90dd218e0199 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1932,13 +1932,20 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
> const char *func_name,
> struct btf_func_model *m);
> [...]
> +
> static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> const struct btf *btf, u32 func_id,
> struct bpf_reg_state *regs,
> bool ptr_to_mem_ok,
> - u32 kfunc_flags)
> + struct bpf_kfunc_arg_meta *kfunc_meta)
> {
> enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
> bool rel = false, kptr_get = false, trusted_arg = false;
> @@ -6207,12 +6232,12 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> return -EINVAL;
> }
>
> - if (is_kfunc) {
> + if (is_kfunc && kfunc_meta) {
> /* Only kfunc can be release func */
> - rel = kfunc_flags & KF_RELEASE;
> - kptr_get = kfunc_flags & KF_KPTR_GET;
> - trusted_arg = kfunc_flags & KF_TRUSTED_ARGS;
> - sleepable = kfunc_flags & KF_SLEEPABLE;
> + rel = kfunc_meta->flags & KF_RELEASE;
> + kptr_get = kfunc_meta->flags & KF_KPTR_GET;
> + trusted_arg = kfunc_meta->flags & KF_TRUSTED_ARGS;
> + sleepable = kfunc_meta->flags & KF_SLEEPABLE;
> }
>
> /* check that BTF function arguments match actual types that the
> @@ -6225,6 +6250,35 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>
> t = btf_type_skip_modifiers(btf, args[i].type, NULL);
> if (btf_type_is_scalar(t)) {
> + if (is_kfunc && kfunc_meta) {
> + bool is_buf_size = false;
> +
> + /* check for any const scalar parameter of name "rdonly_buf_size"
> + * or "rdwr_buf_size"
> + */
> + if (btf_is_kfunc_arg_mem_size(btf, &args[i], reg,
> + "rdonly_buf_size")) {
> + kfunc_meta->r0_rdonly = true;
> + is_buf_size = true;
> + } else if (btf_is_kfunc_arg_mem_size(btf, &args[i], reg,
> + "rdwr_buf_size"))
> + is_buf_size = true;
> +
> + if (is_buf_size) {
> + if (kfunc_meta->r0_size) {
> + bpf_log(log, "2 or more rdonly/rdwr_buf_size parameters for kfunc");
> + return -EINVAL;
> + }
> +
> + if (!tnum_is_const(reg->var_off)) {
> + bpf_log(log, "R%d is not a const\n", regno);
> + return -EINVAL;
> + }
> +
> + kfunc_meta->r0_size = reg->var_off.value;
Sorry for not pointing it out before, but you will need a call to
mark_chain_precision here after this, since the value of the scalar is
being used to decide the size of the returned pointer.
> + }
> + }
> +
> if (reg->type == SCALAR_VALUE)
> continue;
> bpf_log(log, "R%d is not a scalar\n", regno);
> @@ -6255,6 +6309,19 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> if (ret < 0)
> return ret;
>
> + if (is_kfunc && reg->type == PTR_TO_BTF_ID) {
I think you can drop this extra check 'reg->type == PTR_TO_BTF_ID),
this condition of only one ref_obj_id should hold regardless of the
type.
> [...]