bpf: Add support for forcing kfunc args to be trusted

Teach the verifier to detect a new KF_TRUSTED_ARGS kfunc flag, which
means each pointer argument must be trusted, which we define as a
pointer that is referenced (has non-zero ref_obj_id) and also needs to
have its offset unchanged, similar to how release functions expect their
argument. This allows a kfunc to receive pointer arguments unchanged
from the result of the acquire kfunc.

This is required to ensure that kfunc that operate on some object only
work on acquired pointers and not normal PTR_TO_BTF_ID with same type
which can be obtained by pointer walking. The restrictions applied to
release arguments also apply to trusted arguments. This implies that
strict type matching (not deducing type by recursively following members
at offset) and OBJ_RELEASE offset checks (ensuring they are zero) are
used for trusted pointer arguments.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220721134245.2450-5-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/btf.h

@@ -17,6 +17,38 @@
 #define KF_RELEASE	(1 << 1) /* kfunc is a release function */
 #define KF_RET_NULL	(1 << 2) /* kfunc returns a pointer that may be NULL */
 #define KF_KPTR_GET	(1 << 3) /* kfunc returns reference to a kptr */
+/* Trusted arguments are those which are meant to be referenced arguments with
+ * unchanged offset. It is used to enforce that pointers obtained from acquire
+ * kfuncs remain unmodified when being passed to helpers taking trusted args.
+ *
+ * Consider
+ *	struct foo {
+ *		int data;
+ *		struct foo *next;
+ *	};
+ *
+ *	struct bar {
+ *		int data;
+ *		struct foo f;
+ *	};
+ *
+ *	struct foo *f = alloc_foo(); // Acquire kfunc
+ *	struct bar *b = alloc_bar(); // Acquire kfunc
+ *
+ * If a kfunc set_foo_data() wants to operate only on the allocated object, it
+ * will set the KF_TRUSTED_ARGS flag, which will prevent unsafe usage like:
+ *
+ *	set_foo_data(f, 42);	   // Allowed
+ *	set_foo_data(f->next, 42); // Rejected, non-referenced pointer
+ *	set_foo_data(&f->next, 42);// Rejected, referenced, but wrong type
+ *	set_foo_data(&b->f, 42);   // Rejected, referenced, but bad offset
+ *
+ * In the final case, usually for the purposes of type matching, it is deduced
+ * by looking at the type of the member at the offset, but due to the
+ * requirement of trusted argument, this deduction will be strict and not done
+ * for this case.
+ */
+#define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer arguments */
 
 struct btf;
 struct btf_member;

kernel/bpf/btf.c

@@ -6174,10 +6174,10 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 				    u32 kfunc_flags)
 {
 	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
+	bool rel = false, kptr_get = false, trusted_arg = false;
 	struct bpf_verifier_log *log = &env->log;
 	u32 i, nargs, ref_id, ref_obj_id = 0;
 	bool is_kfunc = btf_is_kernel(btf);
-	bool rel = false, kptr_get = false;
 	const char *func_name, *ref_tname;
 	const struct btf_type *t, *ref_t;
 	const struct btf_param *args;
@@ -6211,6 +6211,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 		/* 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;
 	}
 
 	/* check that BTF function arguments match actual types that the
@@ -6235,10 +6236,19 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 			return -EINVAL;
 		}
 
+		/* Check if argument must be a referenced pointer, args + i has
+		 * been verified to be a pointer (after skipping modifiers).
+		 */
+		if (is_kfunc && trusted_arg && !reg->ref_obj_id) {
+			bpf_log(log, "R%d must be referenced\n", regno);
+			return -EINVAL;
+		}
+
 		ref_t = btf_type_skip_modifiers(btf, t->type, &ref_id);
 		ref_tname = btf_name_by_offset(btf, ref_t->name_off);
 
-		if (rel && reg->ref_obj_id)
+		/* Trusted args have the same offset checks as release arguments */
+		if (trusted_arg || (rel && reg->ref_obj_id))
 			arg_type |= OBJ_RELEASE;
 		ret = check_func_arg_reg_off(env, reg, regno, arg_type);
 		if (ret < 0)
@@ -6336,7 +6346,8 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 			reg_ref_tname = btf_name_by_offset(reg_btf,
 							   reg_ref_t->name_off);
 			if (!btf_struct_ids_match(log, reg_btf, reg_ref_id,
-						  reg->off, btf, ref_id, rel && reg->ref_obj_id)) {
+						  reg->off, btf, ref_id,
+						  trusted_arg || (rel && reg->ref_obj_id))) {
 				bpf_log(log, "kernel function %s args#%d expected pointer to %s %s but R%d has a pointer to %s %s\n",
 					func_name, i,
 					btf_type_str(ref_t), ref_tname,

net/bpf/test_run.c

@@ -691,6 +691,10 @@ noinline void bpf_kfunc_call_test_mem_len_fail2(u64 *mem, int len)
 {
 }
 
+noinline void bpf_kfunc_call_test_ref(struct prog_test_ref_kfunc *p)
+{
+}
+
 __diag_pop();
 
 ALLOW_ERROR_INJECTION(bpf_modify_return_test, ERRNO);
@@ -714,6 +718,7 @@ BTF_ID_FLAGS(func, bpf_kfunc_call_test_fail3)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_pass1)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_fail1)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_fail2)
+BTF_ID_FLAGS(func, bpf_kfunc_call_test_ref, KF_TRUSTED_ARGS)
 BTF_SET8_END(test_sk_check_kfunc_ids)
 
 static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,