bpf: add __arg_trusted global func arg tag

Add support for passing PTR_TO_BTF_ID registers to global subprogs.
Currently only PTR_TRUSTED flavor of PTR_TO_BTF_ID is supported.
Non-NULL semantics is assumed, so caller will be forced to prove
PTR_TO_BTF_ID can't be NULL.

Note, we disallow global subprogs to destroy passed in PTR_TO_BTF_ID
arguments, even the trusted one. We achieve that by not setting
ref_obj_id when validating subprog code. This basically enforces (in
Rust terms) borrowing semantics vs move semantics. Borrowing semantics
seems to be a better fit for isolated global subprog validation
approach.

Implementation-wise, we utilize existing logic for matching
user-provided BTF type to kernel-side BTF type, used by BPF CO-RE logic
and following same matching rules. We enforce a unique match for types.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240130000648.2144827-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 86 insertions, 13 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 9ec08cfb2967..ed7a05815984 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6985,9 +6985,77 @@ static bool btf_is_dynptr_ptr(const struct btf *btf, const struct btf_type *t)
 	return false;
 }
 
+struct bpf_cand_cache {
+	const char *name;
+	u32 name_len;
+	u16 kind;
+	u16 cnt;
+	struct {
+		const struct btf *btf;
+		u32 id;
+	} cands[];
+};
+
+static DEFINE_MUTEX(cand_cache_mutex);
+
+static struct bpf_cand_cache *
+bpf_core_find_cands(struct bpf_core_ctx *ctx, u32 local_type_id);
+
+static int btf_get_ptr_to_btf_id(struct bpf_verifier_log *log, int arg_idx,
+				 const struct btf *btf, const struct btf_type *t)
+{
+	struct bpf_cand_cache *cc;
+	struct bpf_core_ctx ctx = {
+		.btf = btf,
+		.log = log,
+	};
+	u32 kern_type_id, type_id;
+	int err = 0;
+
+	/* skip PTR and modifiers */
+	type_id = t->type;
+	t = btf_type_by_id(btf, t->type);
+	while (btf_type_is_modifier(t)) {
+		type_id = t->type;
+		t = btf_type_by_id(btf, t->type);
+	}
+
+	mutex_lock(&cand_cache_mutex);
+	cc = bpf_core_find_cands(&ctx, type_id);
+	if (IS_ERR(cc)) {
+		err = PTR_ERR(cc);
+		bpf_log(log, "arg#%d reference type('%s %s') candidate matching error: %d\n",
+			arg_idx, btf_type_str(t), __btf_name_by_offset(btf, t->name_off),
+			err);
+		goto cand_cache_unlock;
+	}
+	if (cc->cnt != 1) {
+		bpf_log(log, "arg#%d reference type('%s %s') %s\n",
+			arg_idx, btf_type_str(t), __btf_name_by_offset(btf, t->name_off),
+			cc->cnt == 0 ? "has no matches" : "is ambiguous");
+		err = cc->cnt == 0 ? -ENOENT : -ESRCH;
+		goto cand_cache_unlock;
+	}
+	if (btf_is_module(cc->cands[0].btf)) {
+		bpf_log(log, "arg#%d reference type('%s %s') points to kernel module type (unsupported)\n",
+			arg_idx, btf_type_str(t), __btf_name_by_offset(btf, t->name_off));
+		err = -EOPNOTSUPP;
+		goto cand_cache_unlock;
+	}
+	kern_type_id = cc->cands[0].id;
+
+cand_cache_unlock:
+	mutex_unlock(&cand_cache_mutex);
+	if (err)
+		return err;
+
+	return kern_type_id;
+}
+
 enum btf_arg_tag {
 	ARG_TAG_CTX = 0x1,
 	ARG_TAG_NONNULL = 0x2,
+	ARG_TAG_TRUSTED = 0x4,
 };
 
 /* Process BTF of a function to produce high-level expectation of function
@@ -7089,6 +7157,8 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
 
 			if (strcmp(tag, "ctx") == 0) {
 				tags |= ARG_TAG_CTX;
+			} else if (strcmp(tag, "trusted") == 0) {
+				tags |= ARG_TAG_TRUSTED;
 			} else if (strcmp(tag, "nonnull") == 0) {
 				tags |= ARG_TAG_NONNULL;
 			} else {
@@ -7127,6 +7197,22 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
 			sub->args[i].arg_type = ARG_PTR_TO_DYNPTR | MEM_RDONLY;
 			continue;
 		}
+		if (tags & ARG_TAG_TRUSTED) {
+			int kern_type_id;
+
+			if (tags & ARG_TAG_NONNULL) {
+				bpf_log(log, "arg#%d has invalid combination of tags\n", i);
+				return -EINVAL;
+			}
+
+			kern_type_id = btf_get_ptr_to_btf_id(log, i, btf, t);
+			if (kern_type_id < 0)
+				return kern_type_id;
+
+			sub->args[i].arg_type = ARG_PTR_TO_BTF_ID | PTR_TRUSTED;
+			sub->args[i].btf_id = kern_type_id;
+			continue;
+		}
 		if (is_global) { /* generic user data pointer */
 			u32 mem_size;
 
@@ -8229,17 +8315,6 @@ size_t bpf_core_essential_name_len(const char *name)
 	return n;
 }
 
-struct bpf_cand_cache {
-	const char *name;
-	u32 name_len;
-	u16 kind;
-	u16 cnt;
-	struct {
-		const struct btf *btf;
-		u32 id;
-	} cands[];
-};
-
 static void bpf_free_cands(struct bpf_cand_cache *cands)
 {
 	if (!cands->cnt)
@@ -8260,8 +8335,6 @@ static struct bpf_cand_cache *vmlinux_cand_cache[VMLINUX_CAND_CACHE_SIZE];
 #define MODULE_CAND_CACHE_SIZE 31
 static struct bpf_cand_cache *module_cand_cache[MODULE_CAND_CACHE_SIZE];
 
-static DEFINE_MUTEX(cand_cache_mutex);
-
 static void __print_cand_cache(struct bpf_verifier_log *log,
 			       struct bpf_cand_cache **cache,
 			       int cache_size)