[PATCH v5 bpf-next 2/5] bpf: kfunc support for ARG_PTR_TO_CONST_STR

[KP Singh]

kfuncs can handle pointers to memory when the next argument is
the size of the memory that can be read and verify these as
ARG_CONST_SIZE_OR_ZERO

Similarly add support for string constants (const char *) and
verify it similar to ARG_PTR_TO_CONST_STR.

Signed-off-by: KP Singh <kpsingh at kernel.org>
---
 include/linux/bpf_verifier.h |  2 +
 kernel/bpf/btf.c             | 30 ++++++++++++
 kernel/bpf/verifier.c        | 89 +++++++++++++++++++++---------------
 3 files changed, 83 insertions(+), 38 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 81b19669efba..f6d8898270d5 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -560,6 +560,8 @@ int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state
 			     u32 regno);
 int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
 		   u32 regno, u32 mem_size);
+int check_const_str(struct bpf_verifier_env *env,
+		    const struct bpf_reg_state *reg, int regno);
 
 /* this lives here instead of in bpf.h because it needs to dereference tgt_prog */
 static inline u64 bpf_trampoline_compute_key(const struct bpf_prog *tgt_prog,
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 37bc77b3b499..9b9d6117deae 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6162,6 +6162,27 @@ static bool is_kfunc_arg_mem_size(const struct btf *btf,
 	return true;
 }
 
+static bool btf_param_is_const_str_ptr(const struct btf *btf,
+				       const struct btf_param *param)
+{
+	const struct btf_type *t;
+	bool is_const = false;
+
+	t = btf_type_by_id(btf, param->type);
+	if (!btf_type_is_ptr(t))
+		return false;
+
+	t = btf_type_by_id(btf, t->type);
+	while (btf_type_is_modifier(t)) {
+		if (BTF_INFO_KIND(t->info) == BTF_KIND_CONST)
+			is_const = true;
+		t = btf_type_by_id(btf, t->type);
+	}
+
+	return (is_const &&
+		!strcmp(btf_name_by_offset(btf, t->name_off), "char"));
+}
+
 static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 				    const struct btf *btf, u32 func_id,
 				    struct bpf_reg_state *regs,
@@ -6344,10 +6365,19 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 		} else if (ptr_to_mem_ok) {
 			const struct btf_type *resolve_ret;
 			u32 type_size;
+			int err;
 
 			if (is_kfunc) {
 				bool arg_mem_size = i + 1 < nargs && is_kfunc_arg_mem_size(btf, &args[i + 1], &regs[regno + 1]);
 
+
+				if (btf_param_is_const_str_ptr(btf, &args[i])) {
+					err = check_const_str(env, reg, regno);
+					if (err < 0)
+						return err;
+					continue;
+				}
+
 				/* Permit pointer to mem, but only when argument
 				 * type is pointer to scalar, or struct composed
 				 * (recursively) of scalars.
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4938477912cd..8ce5d2f86c1e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5840,6 +5840,56 @@ static u32 stack_slot_get_id(struct bpf_verifier_env *env, struct bpf_reg_state
 	return state->stack[spi].spilled_ptr.id;
 }
 
+int check_const_str(struct bpf_verifier_env *env,
+		    const struct bpf_reg_state *reg, int regno)
+{
+	struct bpf_map *map;
+	int map_off;
+	u64 map_addr;
+	char *str_ptr;
+	int err;
+
+	if (reg->type != PTR_TO_MAP_VALUE)
+		return -EACCES;
+
+	map = reg->map_ptr;
+	if (!bpf_map_is_rdonly(map)) {
+		verbose(env, "R%d does not point to a readonly map'\n", regno);
+		return -EACCES;
+	}
+
+	if (!tnum_is_const(reg->var_off)) {
+		verbose(env, "R%d is not a constant address'\n", regno);
+		return -EACCES;
+	}
+
+	if (!map->ops->map_direct_value_addr) {
+		verbose(env,
+			"no direct value access support for this map type\n");
+		return -EACCES;
+	}
+
+	err = check_map_access(env, regno, reg->off, map->value_size - reg->off,
+			       false, ACCESS_HELPER);
+	if (err)
+		return err;
+
+	map_off = reg->off + reg->var_off.value;
+	err = map->ops->map_direct_value_addr(map, &map_addr, map_off);
+	if (err) {
+		verbose(env, "direct value access on string failed\n");
+		return err;
+	}
+
+	str_ptr = (char *)(long)(map_addr);
+	if (!strnchr(str_ptr + map_off, map->value_size - map_off, 0)) {
+		verbose(env, "string is not zero-terminated\n");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			  struct bpf_call_arg_meta *meta,
 			  const struct bpf_func_proto *fn)
@@ -6074,44 +6124,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			return err;
 		err = check_ptr_alignment(env, reg, 0, size, true);
 	} else if (arg_type == ARG_PTR_TO_CONST_STR) {
-		struct bpf_map *map = reg->map_ptr;
-		int map_off;
-		u64 map_addr;
-		char *str_ptr;
-
-		if (!bpf_map_is_rdonly(map)) {
-			verbose(env, "R%d does not point to a readonly map'\n", regno);
-			return -EACCES;
-		}
-
-		if (!tnum_is_const(reg->var_off)) {
-			verbose(env, "R%d is not a constant address'\n", regno);
-			return -EACCES;
-		}
-
-		if (!map->ops->map_direct_value_addr) {
-			verbose(env, "no direct value access support for this map type\n");
-			return -EACCES;
-		}
-
-		err = check_map_access(env, regno, reg->off,
-				       map->value_size - reg->off, false,
-				       ACCESS_HELPER);
-		if (err)
-			return err;
-
-		map_off = reg->off + reg->var_off.value;
-		err = map->ops->map_direct_value_addr(map, &map_addr, map_off);
-		if (err) {
-			verbose(env, "direct value access on string failed\n");
-			return err;
-		}
-
-		str_ptr = (char *)(long)(map_addr);
-		if (!strnchr(str_ptr + map_off, map->value_size - map_off, 0)) {
-			verbose(env, "string is not zero-terminated\n");
-			return -EINVAL;
-		}
+		err = check_const_str(env, reg, regno);
 	} else if (arg_type == ARG_PTR_TO_KPTR) {
 		if (process_kptr_func(env, regno, meta))
 			return -EACCES;
-- 
2.37.0.rc0.161.g10f37bed90-goog