bpf: Introduce function-by-function verification

New llvm and old llvm with libbpf help produce BTF that distinguish global and
static functions. Unlike arguments of static function the arguments of global
functions cannot be removed or optimized away by llvm. The compiler has to use
exactly the arguments specified in a function prototype. The argument type
information allows the verifier validate each global function independently.
For now only supported argument types are pointer to context and scalars. In
the future pointers to structures, sizes, pointer to packet data can be
supported as well. Consider the following example:

static int f1(int ...)
{
  ...
}

int f3(int b);

int f2(int a)
{
  f1(a) + f3(a);
}

int f3(int b)
{
  ...
}

int main(...)
{
  f1(...) + f2(...) + f3(...);
}

The verifier will start its safety checks from the first global function f2().
It will recursively descend into f1() because it's static. Then it will check
that arguments match for the f3() invocation inside f2(). It will not descend
into f3(). It will finish f2() that has to be successfully verified for all
possible values of 'a'. Then it will proceed with f3(). That function also has
to be safe for all possible values of 'b'. Then it will start subprog 0 (which
is main() function). It will recursively descend into f1() and will skip full
check of f2() and f3(), since they are global. The order of processing global
functions doesn't affect safety, since all global functions must be proven safe
based on their arguments only.

Such function by function verification can drastically improve speed of the
verification and reduce complexity.

Note that the stack limit of 512 still applies to the call chain regardless whether
functions were static or global. The nested level of 8 also still applies. The
same recursion prevention checks are in place as well.

The type information and static/global kind is preserved after the verification
hence in the above example global function f2() and f3() can be replaced later
by equivalent functions with the same types that are loaded and verified later
without affecting safety of this main() program. Such replacement (re-linking)
of global functions is a subject of future patches.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200110064124.1760511-3-ast@kernel.org

include/linux/bpf.h

@@ -566,6 +566,7 @@ static inline void bpf_dispatcher_change_prog(struct bpf_dispatcher *d,
 #endif
 
 struct bpf_func_info_aux {
+	u16 linkage;
 	bool unreliable;
 };
 
@@ -1081,7 +1082,11 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
 			   const char *func_name,
 			   struct btf_func_model *m);
 
-int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog);
+struct bpf_reg_state;
+int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog,
+			     struct bpf_reg_state *regs);
+int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
+			  struct bpf_reg_state *reg);
 
 struct bpf_prog *bpf_prog_by_id(u32 id);
 

include/linux/bpf_verifier.h

@@ -304,11 +304,13 @@ struct bpf_insn_aux_data {
 	u64 map_key_state; /* constant (32 bit) key tracking for maps */
 	int ctx_field_size; /* the ctx field size for load insn, maybe 0 */
 	int sanitize_stack_off; /* stack slot to be cleared */
-	bool seen; /* this insn was processed by the verifier */
+	u32 seen; /* this insn was processed by the verifier at env->pass_cnt */
 	bool zext_dst; /* this insn zero extends dst reg */
 	u8 alu_state; /* used in combination with alu_limit */
-	bool prune_point;
+
+	/* below fields are initialized once */
 	unsigned int orig_idx; /* original instruction index */
+	bool prune_point;
 };
 
 #define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */
@@ -379,6 +381,7 @@ struct bpf_verifier_env {
 		int *insn_stack;
 		int cur_stack;
 	} cfg;
+	u32 pass_cnt; /* number of times do_check() was called */
 	u32 subprog_cnt;
 	/* number of instructions analyzed by the verifier */
 	u32 prev_insn_processed, insn_processed;
@@ -428,4 +431,7 @@ bpf_prog_offload_replace_insn(struct bpf_verifier_env *env, u32 off,
 void
 bpf_prog_offload_remove_insns(struct bpf_verifier_env *env, u32 off, u32 cnt);
 
+int check_ctx_reg(struct bpf_verifier_env *env,
+		  const struct bpf_reg_state *reg, int regno);
+
 #endif /* _LINUX_BPF_VERIFIER_H */

include/uapi/linux/btf.h

@@ -146,6 +146,12 @@ enum {
 	BTF_VAR_GLOBAL_EXTERN = 2,
 };
 
+enum btf_func_linkage {
+	BTF_FUNC_STATIC = 0,
+	BTF_FUNC_GLOBAL = 1,
+	BTF_FUNC_EXTERN = 2,
+};
+
 /* BTF_KIND_VAR is followed by a single "struct btf_var" to describe
  * additional information related to the variable such as its linkage.
  */

kernel/bpf/btf.c

@@ -2651,8 +2651,8 @@ static s32 btf_func_check_meta(struct btf_verifier_env *env,
 		return -EINVAL;
 	}
 
-	if (btf_type_vlen(t)) {
-		btf_verifier_log_type(env, t, "vlen != 0");
+	if (btf_type_vlen(t) > BTF_FUNC_GLOBAL) {
+		btf_verifier_log_type(env, t, "Invalid func linkage");
 		return -EINVAL;
 	}
 
@@ -3506,7 +3506,8 @@ static u8 bpf_ctx_convert_map[] = {
 
 static const struct btf_member *
 btf_get_prog_ctx_type(struct bpf_verifier_log *log, struct btf *btf,
-		      const struct btf_type *t, enum bpf_prog_type prog_type)
+		      const struct btf_type *t, enum bpf_prog_type prog_type,
+		      int arg)
 {
 	const struct btf_type *conv_struct;
 	const struct btf_type *ctx_struct;
@@ -3527,12 +3528,13 @@ btf_get_prog_ctx_type(struct bpf_verifier_log *log, struct btf *btf,
 		 * is not supported yet.
 		 * BPF_PROG_TYPE_RAW_TRACEPOINT is fine.
 		 */
-		bpf_log(log, "BPF program ctx type is not a struct\n");
+		if (log->level & BPF_LOG_LEVEL)
+			bpf_log(log, "arg#%d type is not a struct\n", arg);
 		return NULL;
 	}
 	tname = btf_name_by_offset(btf, t->name_off);
 	if (!tname) {
-		bpf_log(log, "BPF program ctx struct doesn't have a name\n");
+		bpf_log(log, "arg#%d struct doesn't have a name\n", arg);
 		return NULL;
 	}
 	/* prog_type is valid bpf program type. No need for bounds check. */
@@ -3565,11 +3567,12 @@ btf_get_prog_ctx_type(struct bpf_verifier_log *log, struct btf *btf,
 static int btf_translate_to_vmlinux(struct bpf_verifier_log *log,
 				     struct btf *btf,
 				     const struct btf_type *t,
-				     enum bpf_prog_type prog_type)
+				     enum bpf_prog_type prog_type,
+				     int arg)
 {
 	const struct btf_member *prog_ctx_type, *kern_ctx_type;
 
-	prog_ctx_type = btf_get_prog_ctx_type(log, btf, t, prog_type);
+	prog_ctx_type = btf_get_prog_ctx_type(log, btf, t, prog_type, arg);
 	if (!prog_ctx_type)
 		return -ENOENT;
 	kern_ctx_type = prog_ctx_type + 1;
@@ -3731,7 +3734,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	info->reg_type = PTR_TO_BTF_ID;
 
 	if (tgt_prog) {
-		ret = btf_translate_to_vmlinux(log, btf, t, tgt_prog->type);
+		ret = btf_translate_to_vmlinux(log, btf, t, tgt_prog->type, arg);
 		if (ret > 0) {
 			info->btf_id = ret;
 			return true;
@@ -4112,11 +4115,16 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
 	return 0;
 }
 
-int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog)
+/* Compare BTF of a function with given bpf_reg_state.
+ * Returns:
+ * EFAULT - there is a verifier bug. Abort verification.
+ * EINVAL - there is a type mismatch or BTF is not available.
+ * 0 - BTF matches with what bpf_reg_state expects.
+ * Only PTR_TO_CTX and SCALAR_VALUE states are recognized.
+ */
+int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog,
+			     struct bpf_reg_state *reg)
 {
-	struct bpf_verifier_state *st = env->cur_state;
-	struct bpf_func_state *func = st->frame[st->curframe];
-	struct bpf_reg_state *reg = func->regs;
 	struct bpf_verifier_log *log = &env->log;
 	struct bpf_prog *prog = env->prog;
 	struct btf *btf = prog->aux->btf;
@@ -4126,27 +4134,30 @@ int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog)
 	const char *tname;
 
 	if (!prog->aux->func_info)
-		return 0;
+		return -EINVAL;
 
 	btf_id = prog->aux->func_info[subprog].type_id;
 	if (!btf_id)
-		return 0;
+		return -EFAULT;
 
 	if (prog->aux->func_info_aux[subprog].unreliable)
-		return 0;
+		return -EINVAL;
 
 	t = btf_type_by_id(btf, btf_id);
 	if (!t || !btf_type_is_func(t)) {
-		bpf_log(log, "BTF of subprog %d doesn't point to KIND_FUNC\n",
+		/* These checks were already done by the verifier while loading
+		 * struct bpf_func_info
+		 */
+		bpf_log(log, "BTF of func#%d doesn't point to KIND_FUNC\n",
 			subprog);
-		return -EINVAL;
+		return -EFAULT;
 	}
 	tname = btf_name_by_offset(btf, t->name_off);
 
 	t = btf_type_by_id(btf, t->type);
 	if (!t || !btf_type_is_func_proto(t)) {
-		bpf_log(log, "Invalid type of func %s\n", tname);
-		return -EINVAL;
+		bpf_log(log, "Invalid BTF of func %s\n", tname);
+		return -EFAULT;
 	}
 	args = (const struct btf_param *)(t + 1);
 	nargs = btf_type_vlen(t);
@@ -4172,25 +4183,127 @@ int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog)
 				bpf_log(log, "R%d is not a pointer\n", i + 1);
 				goto out;
 			}
-			/* If program is passing PTR_TO_CTX into subprogram
-			 * check that BTF type matches.
+			/* If function expects ctx type in BTF check that caller
+			 * is passing PTR_TO_CTX.
 			 */
-			if (reg[i + 1].type == PTR_TO_CTX &&
-			    !btf_get_prog_ctx_type(log, btf, t, prog->type))
-				goto out;
-			/* All other pointers are ok */
-			continue;
+			if (btf_get_prog_ctx_type(log, btf, t, prog->type, i)) {
+				if (reg[i + 1].type != PTR_TO_CTX) {
+					bpf_log(log,
+						"arg#%d expected pointer to ctx, but got %s\n",
+						i, btf_kind_str[BTF_INFO_KIND(t->info)]);
+					goto out;
+				}
+				if (check_ctx_reg(env, &reg[i + 1], i + 1))
+					goto out;
+				continue;
+			}
 		}
-		bpf_log(log, "Unrecognized argument type %s\n",
-			btf_kind_str[BTF_INFO_KIND(t->info)]);
+		bpf_log(log, "Unrecognized arg#%d type %s\n",
+			i, btf_kind_str[BTF_INFO_KIND(t->info)]);
 		goto out;
 	}
 	return 0;
 out:
-	/* LLVM optimizations can remove arguments from static functions. */
-	bpf_log(log,
-		"Type info disagrees with actual arguments due to compiler optimizations\n");
+	/* Compiler optimizations can remove arguments from static functions
+	 * or mismatched type can be passed into a global function.
+	 * In such cases mark the function as unreliable from BTF point of view.
+	 */
 	prog->aux->func_info_aux[subprog].unreliable = true;
+	return -EINVAL;
+}
+
+/* Convert BTF of a function into bpf_reg_state if possible
+ * Returns:
+ * EFAULT - there is a verifier bug. Abort verification.
+ * EINVAL - cannot convert BTF.
+ * 0 - Successfully converted BTF into bpf_reg_state
+ * (either PTR_TO_CTX or SCALAR_VALUE).
+ */
+int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
+			  struct bpf_reg_state *reg)
+{
+	struct bpf_verifier_log *log = &env->log;
+	struct bpf_prog *prog = env->prog;
+	struct btf *btf = prog->aux->btf;
+	const struct btf_param *args;
+	const struct btf_type *t;
+	u32 i, nargs, btf_id;
+	const char *tname;
+
+	if (!prog->aux->func_info ||
+	    prog->aux->func_info_aux[subprog].linkage != BTF_FUNC_GLOBAL) {
+		bpf_log(log, "Verifier bug\n");
+		return -EFAULT;
+	}
+
+	btf_id = prog->aux->func_info[subprog].type_id;
+	if (!btf_id) {
+		bpf_log(log, "Global functions need valid BTF\n");
+		return -EFAULT;
+	}
+
+	t = btf_type_by_id(btf, btf_id);
+	if (!t || !btf_type_is_func(t)) {
+		/* These checks were already done by the verifier while loading
+		 * struct bpf_func_info
+		 */
+		bpf_log(log, "BTF of func#%d doesn't point to KIND_FUNC\n",
+			subprog);
+		return -EFAULT;
+	}
+	tname = btf_name_by_offset(btf, t->name_off);
+
+	if (log->level & BPF_LOG_LEVEL)
+		bpf_log(log, "Validating %s() func#%d...\n",
+			tname, subprog);
+
+	if (prog->aux->func_info_aux[subprog].unreliable) {
+		bpf_log(log, "Verifier bug in function %s()\n", tname);
+		return -EFAULT;
+	}
+
+	t = btf_type_by_id(btf, t->type);
+	if (!t || !btf_type_is_func_proto(t)) {
+		bpf_log(log, "Invalid type of function %s()\n", tname);
+		return -EFAULT;
+	}
+	args = (const struct btf_param *)(t + 1);
+	nargs = btf_type_vlen(t);
+	if (nargs > 5) {
+		bpf_log(log, "Global function %s() with %d > 5 args. Buggy compiler.\n",
+			tname, nargs);
+		return -EINVAL;
+	}
+	/* check that function returns int */
+	t = btf_type_by_id(btf, t->type);
+	while (btf_type_is_modifier(t))
+		t = btf_type_by_id(btf, t->type);
+	if (!btf_type_is_int(t) && !btf_type_is_enum(t)) {
+		bpf_log(log,
+			"Global function %s() doesn't return scalar. Only those are supported.\n",
+			tname);
+		return -EINVAL;
+	}
+	/* Convert BTF function arguments into verifier types.
+	 * Only PTR_TO_CTX and SCALAR are supported atm.
+	 */
+	for (i = 0; i < nargs; i++) {
+		t = btf_type_by_id(btf, args[i].type);
+		while (btf_type_is_modifier(t))
+			t = btf_type_by_id(btf, t->type);
+		if (btf_type_is_int(t) || btf_type_is_enum(t)) {
+			reg[i + 1].type = SCALAR_VALUE;
+			continue;
+		}
+		if (btf_type_is_ptr(t) &&
+		    btf_get_prog_ctx_type(log, btf, t, prog->type, i)) {
+			reg[i + 1].type = PTR_TO_CTX;
+			continue;
+		}
+		bpf_log(log, "Arg#%d type %s in %s() is not supported yet.\n",
+			i, btf_kind_str[BTF_INFO_KIND(t->info)], tname);
+		return -EINVAL;
+	}
 	return 0;
 }