On Sun, Mar 20, 2022 at 6:34 PM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> Add a few fields for each arg (argN_release) that when set to true,
> tells verifier that for a release function, that argument's register
> will be the one for which meta.ref_obj_id will be set, and which will
> then be released using release_reference. To capture the regno,
> introduce a release_regno field in bpf_call_arg_meta.
>
> This would be required in the next patch, where we may either pass NULL
> or a refcounted pointer as an argument to the release function
> bpf_kptr_xchg. Just releasing only when meta.ref_obj_id is set is not
> enough, as there is a case where the type of argument needed matches,
> but the ref_obj_id is set to 0. Hence, we must enforce that whenever
> meta.ref_obj_id is zero, the register that is to be released can only
> be NULL for a release function.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> ---
> include/linux/bpf.h | 10 ++++++++++
> kernel/bpf/ringbuf.c | 2 ++
> kernel/bpf/verifier.c | 39 +++++++++++++++++++++++++++++++++------
> net/core/filter.c | 1 +
> 4 files changed, 46 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index f35920d279dd..48ddde854d67 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -487,6 +487,16 @@ struct bpf_func_proto {
> };
> u32 *arg_btf_id[5];
> };
> + union {
> + struct {
> + bool arg1_release;
> + bool arg2_release;
> + bool arg3_release;
> + bool arg4_release;
> + bool arg5_release;
> + };
> + bool arg_release[5];
> + };
Instead of having the new fields "argx_release" for each arg, what are
your thoughts on using PTR_RELEASE as an "enum bpf_type_flag" to the
existing "argx_type" field? For example, instead of
.arg1_type = ARG_PTR_TO_ALLOC_MEM,
.arg1_release = true,
could we do something like
.arg1_type = ARG_PTR_TO_ALLOC_MEM | PTR_RELEASE
In the verifier, we could determine whether an argument register
releases a reference by checking whether this PTR_RELEASE flag is set.
Would this be a little cleaner? Curious to hear your thoughts.
> int *ret_btf_id; /* return value btf_id */
> bool (*allowed)(const struct bpf_prog *prog);
> };
> diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
> index 710ba9de12ce..f40ce718630e 100644
> --- a/kernel/bpf/ringbuf.c
> +++ b/kernel/bpf/ringbuf.c
> @@ -405,6 +405,7 @@ const struct bpf_func_proto bpf_ringbuf_submit_proto = {
> .func = bpf_ringbuf_submit,
> .ret_type = RET_VOID,
> .arg1_type = ARG_PTR_TO_ALLOC_MEM,
> + .arg1_release = true,
> .arg2_type = ARG_ANYTHING,
> };
>
> @@ -418,6 +419,7 @@ const struct bpf_func_proto bpf_ringbuf_discard_proto = {
> .func = bpf_ringbuf_discard,
> .ret_type = RET_VOID,
> .arg1_type = ARG_PTR_TO_ALLOC_MEM,
> + .arg1_release = true,
> .arg2_type = ARG_ANYTHING,
> };
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 744b7362e52e..b8cd34607215 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -245,6 +245,7 @@ struct bpf_call_arg_meta {
> struct bpf_map *map_ptr;
> bool raw_mode;
> bool pkt_access;
> + u8 release_regno;
> int regno;
> int access_size;
> int mem_size;
> @@ -6101,12 +6102,31 @@ static bool check_btf_id_ok(const struct bpf_func_proto *fn)
> return true;
> }
>
> -static int check_func_proto(const struct bpf_func_proto *fn, int func_id)
> +static bool check_release_regno(const struct bpf_func_proto *fn, int func_id,
> + struct bpf_call_arg_meta *meta)
> +{
> + int i;
> +
> + for (i = 0; i < ARRAY_SIZE(fn->arg_release); i++) {
> + if (fn->arg_release[i]) {
> + if (!is_release_function(func_id))
> + return false;
> + if (meta->release_regno)
> + return false;
> + meta->release_regno = i + 1;
> + }
> + }
> + return !is_release_function(func_id) || meta->release_regno;
> +}
> +
> +static int check_func_proto(const struct bpf_func_proto *fn, int func_id,
> + struct bpf_call_arg_meta *meta)
> {
> return check_raw_mode_ok(fn) &&
> check_arg_pair_ok(fn) &&
> check_btf_id_ok(fn) &&
> - check_refcount_ok(fn, func_id) ? 0 : -EINVAL;
> + check_refcount_ok(fn, func_id) &&
> + check_release_regno(fn, func_id, meta) ? 0 : -EINVAL;
> }
>
> /* Packet data might have moved, any old PTR_TO_PACKET[_META,_END]
> @@ -6785,7 +6805,7 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> memset(&meta, 0, sizeof(meta));
> meta.pkt_access = fn->pkt_access;
>
> - err = check_func_proto(fn, func_id);
> + err = check_func_proto(fn, func_id, &meta);
> if (err) {
> verbose(env, "kernel subsystem misconfigured func %s#%d\n",
> func_id_name(func_id), func_id);
> @@ -6818,8 +6838,17 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> return err;
> }
>
> + regs = cur_regs(env);
> +
> if (is_release_function(func_id)) {
> - err = release_reference(env, meta.ref_obj_id);
> + err = -EINVAL;
> + if (meta.ref_obj_id)
> + err = release_reference(env, meta.ref_obj_id);
> + /* meta.ref_obj_id can only be 0 if register that is meant to be
> + * released is NULL, which must be > R0.
> + */
> + else if (meta.release_regno && register_is_null(®s[meta.release_regno]))
> + err = 0;
If I'm understanding this correctly, in this patch we will call
check_release_regno on every function to determine if any / which of
the argument registers release a reference. Given that in the majority
of cases the function will not be a release function, what are your
thoughts on moving that check to be within the scope of this if
function? So if it is a release function, and meta.ref_obj_id is not
set, then we do the checking for which argument register is a release
register and whether that register is null. Curious to hear your
thoughts.
> if (err) {
> verbose(env, "func %s#%d reference has not been acquired before\n",
> func_id_name(func_id), func_id);
> @@ -6827,8 +6856,6 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> }
> }
>
> - regs = cur_regs(env);
> -
> switch (func_id) {
> case BPF_FUNC_tail_call:
> err = check_reference_leak(env);
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 03655f2074ae..17eff4731b06 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -6622,6 +6622,7 @@ static const struct bpf_func_proto bpf_sk_release_proto = {
> .gpl_only = false,
> .ret_type = RET_INTEGER,
> .arg1_type = ARG_PTR_TO_BTF_ID_SOCK_COMMON,
> + .arg1_release = true,
> };
>
> BPF_CALL_5(bpf_xdp_sk_lookup_udp, struct xdp_buff *, ctx,
> --
> 2.35.1
>
