bpf: Introduce allocated objects support

Introduce support for representing pointers to objects allocated by the
BPF program, i.e. PTR_TO_BTF_ID that point to a type in program BTF.
This is indicated by the presence of MEM_ALLOC type flag in reg->type to
avoid having to check btf_is_kernel when trying to match argument types
in helpers.

Whenever walking such types, any pointers being walked will always yield
a SCALAR instead of pointer. In the future we might permit kptr inside
such allocated objects (either kernel or program allocated), and it will
then form a PTR_TO_BTF_ID of the respective type.

For now, such allocated objects will always be referenced in verifier
context, hence ref_obj_id == 0 for them is a bug. It is allowed to write
to such objects, as long fields that are special are not touched
(support for which will be added in subsequent patches). Note that once
such a pointer is marked PTR_UNTRUSTED, it is no longer allowed to write
to it.

No PROBE_MEM handling is therefore done for loads into this type unless
PTR_UNTRUSTED is part of the register type, since they can never be in
an undefined state, and their lifetime will always be valid.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118015614.2013203-6-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -525,6 +525,11 @@ enum bpf_type_flag {
 	/* Size is known at compile time. */
 	MEM_FIXED_SIZE		= BIT(10 + BPF_BASE_TYPE_BITS),
 
+	/* MEM is of an allocated object of type in program BTF. This is used to
+	 * tag PTR_TO_BTF_ID allocated using bpf_obj_new.
+	 */
+	MEM_ALLOC		= BIT(11 + BPF_BASE_TYPE_BITS),
+
 	__BPF_TYPE_FLAG_MAX,
 	__BPF_TYPE_LAST_FLAG	= __BPF_TYPE_FLAG_MAX - 1,
 };
@@ -2792,4 +2797,10 @@ struct bpf_key {
 	bool has_ref;
 };
 #endif /* CONFIG_KEYS */
+
+static inline bool type_is_alloc(u32 type)
+{
+	return type & MEM_ALLOC;
+}
+
 #endif /* _LINUX_BPF_H */

kernel/bpf/btf.c

@@ -6034,6 +6034,11 @@ int btf_struct_access(struct bpf_verifier_log *log,
 
 		switch (err) {
 		case WALK_PTR:
+			/* For local types, the destination register cannot
+			 * become a pointer again.
+			 */
+			if (type_is_alloc(reg->type))
+				return SCALAR_VALUE;
 			/* If we found the pointer or scalar on t+off,
 			 * we're done.
 			 */

kernel/bpf/verifier.c

@@ -4687,14 +4687,27 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 		return -EACCES;
 	}
 
-	if (env->ops->btf_struct_access) {
+	if (env->ops->btf_struct_access && !type_is_alloc(reg->type)) {
+		if (!btf_is_kernel(reg->btf)) {
+			verbose(env, "verifier internal error: reg->btf must be kernel btf\n");
+			return -EFAULT;
+		}
 		ret = env->ops->btf_struct_access(&env->log, reg, off, size, atype, &btf_id, &flag);
 	} else {
-		if (atype != BPF_READ) {
+		/* Writes are permitted with default btf_struct_access for
+		 * program allocated objects (which always have ref_obj_id > 0),
+		 * but not for untrusted PTR_TO_BTF_ID | MEM_ALLOC.
+		 */
+		if (atype != BPF_READ && reg->type != (PTR_TO_BTF_ID | MEM_ALLOC)) {
 			verbose(env, "only read is supported\n");
 			return -EACCES;
 		}
 
+		if (type_is_alloc(reg->type) && !reg->ref_obj_id) {
+			verbose(env, "verifier internal error: ref_obj_id for allocated object must be non-zero\n");
+			return -EFAULT;
+		}
+
 		ret = btf_struct_access(&env->log, reg, off, size, atype, &btf_id, &flag);
 	}
 
@@ -5973,6 +5986,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
 	 * fixed offset.
 	 */
 	case PTR_TO_BTF_ID:
+	case PTR_TO_BTF_ID | MEM_ALLOC:
 		/* When referenced PTR_TO_BTF_ID is passed to release function,
 		 * it's fixed offset must be 0.	In the other cases, fixed offset
 		 * can be non-zero.
@@ -13690,6 +13704,13 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 			break;
 		case PTR_TO_BTF_ID:
 		case PTR_TO_BTF_ID | PTR_UNTRUSTED:
+		/* PTR_TO_BTF_ID | MEM_ALLOC always has a valid lifetime, unlike
+		 * PTR_TO_BTF_ID, and an active ref_obj_id, but the same cannot
+		 * be said once it is marked PTR_UNTRUSTED, hence we must handle
+		 * any faults for loads into such types. BPF_WRITE is disallowed
+		 * for this case.
+		 */
+		case PTR_TO_BTF_ID | MEM_ALLOC | PTR_UNTRUSTED:
 			if (type == BPF_READ) {
 				insn->code = BPF_LDX | BPF_PROBE_MEM |
 					BPF_SIZE((insn)->code);