[RESEND,v2,bpf-next] bpf: Cleanup check_refcount_ok

Commit Message

Dave Marchevsky Aug. 8, 2022, 5:15 p.m. UTC

Discussion around a recently-submitted patch provided historical
context for check_refcount_ok [0]. Specifically, the function and its
helpers - may_be_acquire_function and arg_type_may_be_refcounted -
predate the OBJ_RELEASE type flag and the addition of many more helpers
with acquire/release semantics.

The purpose of check_refcount_ok is to ensure:
  1) Helper doesn't have multiple uses of return reg's ref_obj_id
  2) Helper with release semantics only has one arg needing to be
  released, since that's tracked using meta->ref_obj_id

With current verifier, it's safe to remove check_refcount_ok and its
helpers. Since addition of OBJ_RELEASE type flag, case 2) has been
handled by the arg_type_is_release check in check_func_arg. To ensure
case 1) won't result in verifier silently prioritizing one use of
ref_obj_id, this patch adds a helper_multiple_ref_obj_use check which
fails loudly if a helper passes > 1 test for use of ref_obj_id.

  [0]: lore.kernel.org/bpf/[e-mail]

Signed-off-by: Dave Marchevsky <[e-mail]>
Acked-by: Martin KaFai Lau <[e-mail]>
Acked-by: Joanne Koong <[e-mail]>
---
No extant helpers fail the helper_multiple_ref_obj_use check (as
expected). I validated this by adding BPF_FUNC_dynptr_data to
is_acquire_function check and observing that dynptr selftests failed
with expected error, then doing the same for is_ptr_cast_function.

v2 -> v2 Resend: lore.kernel.org/bpf/[e-mail]
  * Add Joanne ack
  * Rebase

v1 -> v2: lore.kernel.org/bpf/[e-mail]
  * EFAULT instead of EINVAL, minor edit to dynptr acquire comment (Joanne)
  * Add Martin Acked-by

 kernel/bpf/verifier.c | 74 +++++++++++++++++--------------------------
 1 file changed, 29 insertions(+), 45 deletions(-)

Comments

Kumar Kartikeya Dwivedi Aug. 8, 2022, 6:53 p.m. UTC | #1

On Mon, 8 Aug 2022 at 19:16, Dave Marchevsky <[e-mail]> wrote:
>
> Discussion around a recently-submitted patch provided historical
> context for check_refcount_ok [0]. Specifically, the function and its
> helpers - may_be_acquire_function and arg_type_may_be_refcounted -
> predate the OBJ_RELEASE type flag and the addition of many more helpers
> with acquire/release semantics.
>
> The purpose of check_refcount_ok is to ensure:
>   1) Helper doesn't have multiple uses of return reg's ref_obj_id
>   2) Helper with release semantics only has one arg needing to be
>   released, since that's tracked using meta->ref_obj_id
>
> With current verifier, it's safe to remove check_refcount_ok and its
> helpers. Since addition of OBJ_RELEASE type flag, case 2) has been
> handled by the arg_type_is_release check in check_func_arg. To ensure
> case 1) won't result in verifier silently prioritizing one use of
> ref_obj_id, this patch adds a helper_multiple_ref_obj_use check which
> fails loudly if a helper passes > 1 test for use of ref_obj_id.
>
>   [0]: lore.kernel.org/bpf/[e-mail]
>
> Signed-off-by: Dave Marchevsky <[e-mail]>
> Acked-by: Martin KaFai Lau <[e-mail]>
> Acked-by: Joanne Koong <[e-mail]>

Acked-by: Kumar Kartikeya Dwivedi <[e-mail]>

[e-mail] Aug. 9, 2022, 4:50 p.m. UTC | #2

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <[e-mail]>:

On Mon, 8 Aug 2022 10:15:59 -0700 you wrote:
> Discussion around a recently-submitted patch provided historical
> context for check_refcount_ok [0]. Specifically, the function and its
> helpers - may_be_acquire_function and arg_type_may_be_refcounted -
> predate the OBJ_RELEASE type flag and the addition of many more helpers
> with acquire/release semantics.
> 
> The purpose of check_refcount_ok is to ensure:
>   1) Helper doesn't have multiple uses of return reg's ref_obj_id
>   2) Helper with release semantics only has one arg needing to be
>   released, since that's tracked using meta->ref_obj_id
> 
> [...]

Here is the summary with links:
  - [RESEND,v2,bpf-next] bpf: Cleanup check_refcount_ok
    https://git.kernel.org/bpf/bpf-next/c/b2d8ef19c6e7

You are awesome, thank you!

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 843a966cd02b..01e7f48b4d8c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -467,25 +467,11 @@  static bool type_is_rdonly_mem(u32 type)
 	return type & MEM_RDONLY;
 }
 
-static bool arg_type_may_be_refcounted(enum bpf_arg_type type)
-{
-	return type == ARG_PTR_TO_SOCK_COMMON;
-}
-
 static bool type_may_be_null(u32 type)
 {
 	return type & PTR_MAYBE_NULL;
 }
 
-static bool may_be_acquire_function(enum bpf_func_id func_id)
-{
-	return func_id == BPF_FUNC_sk_lookup_tcp ||
-		func_id == BPF_FUNC_sk_lookup_udp ||
-		func_id == BPF_FUNC_skc_lookup_tcp ||
-		func_id == BPF_FUNC_map_lookup_elem ||
-	        func_id == BPF_FUNC_ringbuf_reserve;
-}
-
 static bool is_acquire_function(enum bpf_func_id func_id,
 				const struct bpf_map *map)
 {
@@ -518,6 +504,26 @@  static bool is_ptr_cast_function(enum bpf_func_id func_id)
 		func_id == BPF_FUNC_skc_to_tcp_request_sock;
 }
 
+static bool is_dynptr_acquire_function(enum bpf_func_id func_id)
+{
+	return func_id == BPF_FUNC_dynptr_data;
+}
+
+static bool helper_multiple_ref_obj_use(enum bpf_func_id func_id,
+					const struct bpf_map *map)
+{
+	int ref_obj_uses = 0;
+
+	if (is_ptr_cast_function(func_id))
+		ref_obj_uses++;
+	if (is_acquire_function(func_id, map))
+		ref_obj_uses++;
+	if (is_dynptr_acquire_function(func_id))
+		ref_obj_uses++;
+
+	return ref_obj_uses > 1;
+}
+
 static bool is_cmpxchg_insn(const struct bpf_insn *insn)
 {
 	return BPF_CLASS(insn->code) == BPF_STX &&
@@ -6453,33 +6459,6 @@  static bool check_arg_pair_ok(const struct bpf_func_proto *fn)
 	return true;
 }
 
-static bool check_refcount_ok(const struct bpf_func_proto *fn, int func_id)
-{
-	int count = 0;
-
-	if (arg_type_may_be_refcounted(fn->arg1_type))
-		count++;
-	if (arg_type_may_be_refcounted(fn->arg2_type))
-		count++;
-	if (arg_type_may_be_refcounted(fn->arg3_type))
-		count++;
-	if (arg_type_may_be_refcounted(fn->arg4_type))
-		count++;
-	if (arg_type_may_be_refcounted(fn->arg5_type))
-		count++;
-
-	/* A reference acquiring function cannot acquire
-	 * another refcounted ptr.
-	 */
-	if (may_be_acquire_function(func_id) && count)
-		return false;
-
-	/* We only support one arg being unreferenced at the moment,
-	 * which is sufficient for the helper functions we have right now.
-	 */
-	return count <= 1;
-}
-
 static bool check_btf_id_ok(const struct bpf_func_proto *fn)
 {
 	int i;
@@ -6502,8 +6481,7 @@  static int check_func_proto(const struct bpf_func_proto *fn, int func_id)
 {
 	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_btf_id_ok(fn) ? 0 : -EINVAL;
 }
 
 /* Packet data might have moved, any old PTR_TO_PACKET[_META,_END]
@@ -7473,6 +7451,12 @@  static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 	if (type_may_be_null(regs[BPF_REG_0].type))
 		regs[BPF_REG_0].id = ++env->id_gen;
 
+	if (helper_multiple_ref_obj_use(func_id, meta.map_ptr)) {
+		verbose(env, "verifier internal error: func %s#%d sets ref_obj_id more than once\n",
+			func_id_name(func_id), func_id);
+		return -EFAULT;
+	}
+
 	if (is_ptr_cast_function(func_id)) {
 		/* For release_reference() */
 		regs[BPF_REG_0].ref_obj_id = meta.ref_obj_id;
@@ -7485,10 +7469,10 @@  static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		regs[BPF_REG_0].id = id;
 		/* For release_reference() */
 		regs[BPF_REG_0].ref_obj_id = id;
-	} else if (func_id == BPF_FUNC_dynptr_data) {
+	} else if (is_dynptr_acquire_function(func_id)) {
 		int dynptr_id = 0, i;
 
-		/* Find the id of the dynptr we're acquiring a reference to */
+		/* Find the id of the dynptr we're tracking the reference of */
 		for (i = 0; i < MAX_BPF_FUNC_REG_ARGS; i++) {
 			if (arg_type_is_dynptr(fn->arg_type[i])) {
 				if (dynptr_id) {