bpf: reject program if a __user tagged memory accessed in kernel way

BPF verifier supports direct memory access for BPF_PROG_TYPE_TRACING type
of bpf programs, e.g., a->b. If "a" is a pointer
pointing to kernel memory, bpf verifier will allow user to write
code in C like a->b and the verifier will translate it to a kernel
load properly. If "a" is a pointer to user memory, it is expected
that bpf developer should be bpf_probe_read_user() helper to
get the value a->b. Without utilizing BTF __user tagging information,
current verifier will assume that a->b is a kernel memory access
and this may generate incorrect result.

Now BTF contains __user information, it can check whether the
pointer points to a user memory or not. If it is, the verifier
can reject the program and force users to use bpf_probe_read_user()
helper explicitly.

In the future, we can easily extend btf_add_space for other
address space tagging, for example, rcu/percpu etc.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220127154606.654961-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -332,7 +332,10 @@ enum bpf_type_flag {
 	 */
 	MEM_ALLOC		= BIT(2 + BPF_BASE_TYPE_BITS),
 
-	__BPF_TYPE_LAST_FLAG	= MEM_ALLOC,
+	/* MEM is in user address space. */
+	MEM_USER		= BIT(3 + BPF_BASE_TYPE_BITS),
+
+	__BPF_TYPE_LAST_FLAG	= MEM_USER,
 };
 
 /* Max number of base types. */
@@ -588,7 +591,7 @@ struct bpf_verifier_ops {
 				 const struct btf *btf,
 				 const struct btf_type *t, int off, int size,
 				 enum bpf_access_type atype,
-				 u32 *next_btf_id);
+				 u32 *next_btf_id, enum bpf_type_flag *flag);
 };
 
 struct bpf_prog_offload_ops {
@@ -1780,7 +1783,7 @@ static inline bool bpf_tracing_btf_ctx_access(int off, int size,
 int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,
 		      const struct btf_type *t, int off, int size,
 		      enum bpf_access_type atype,
-		      u32 *next_btf_id);
+		      u32 *next_btf_id, enum bpf_type_flag *flag);
 bool btf_struct_ids_match(struct bpf_verifier_log *log,
 			  const struct btf *btf, u32 id, int off,
 			  const struct btf *need_btf, u32 need_type_id);

include/linux/btf.h

@@ -238,6 +238,11 @@ static inline bool btf_type_is_var(const struct btf_type *t)
 	return BTF_INFO_KIND(t->info) == BTF_KIND_VAR;
 }
 
+static inline bool btf_type_is_type_tag(const struct btf_type *t)
+{
+	return BTF_INFO_KIND(t->info) == BTF_KIND_TYPE_TAG;
+}
+
 /* union is only a special case of struct:
  * all its offsetof(member) == 0
  */

kernel/bpf/btf.c

@@ -4886,6 +4886,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	const char *tname = prog->aux->attach_func_name;
 	struct bpf_verifier_log *log = info->log;
 	const struct btf_param *args;
+	const char *tag_value;
 	u32 nr_args, arg;
 	int i, ret;
 
@@ -5038,6 +5039,13 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	info->btf = btf;
 	info->btf_id = t->type;
 	t = btf_type_by_id(btf, t->type);
+
+	if (btf_type_is_type_tag(t)) {
+		tag_value = __btf_name_by_offset(btf, t->name_off);
+		if (strcmp(tag_value, "user") == 0)
+			info->reg_type |= MEM_USER;
+	}
+
 	/* skip modifiers */
 	while (btf_type_is_modifier(t)) {
 		info->btf_id = t->type;
@@ -5064,12 +5072,12 @@ enum bpf_struct_walk_result {
 
 static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 			   const struct btf_type *t, int off, int size,
-			   u32 *next_btf_id)
+			   u32 *next_btf_id, enum bpf_type_flag *flag)
 {
 	u32 i, moff, mtrue_end, msize = 0, total_nelems = 0;
 	const struct btf_type *mtype, *elem_type = NULL;
 	const struct btf_member *member;
-	const char *tname, *mname;
+	const char *tname, *mname, *tag_value;
 	u32 vlen, elem_id, mid;
 
 again:
@@ -5253,7 +5261,8 @@ error:
 		}
 
 		if (btf_type_is_ptr(mtype)) {
-			const struct btf_type *stype;
+			const struct btf_type *stype, *t;
+			enum bpf_type_flag tmp_flag = 0;
 			u32 id;
 
 			if (msize != size || off != moff) {
@@ -5262,9 +5271,19 @@ error:
 					mname, moff, tname, off, size);
 				return -EACCES;
 			}
+
+			/* check __user tag */
+			t = btf_type_by_id(btf, mtype->type);
+			if (btf_type_is_type_tag(t)) {
+				tag_value = __btf_name_by_offset(btf, t->name_off);
+				if (strcmp(tag_value, "user") == 0)
+					tmp_flag = MEM_USER;
+			}
+
 			stype = btf_type_skip_modifiers(btf, mtype->type, &id);
 			if (btf_type_is_struct(stype)) {
 				*next_btf_id = id;
+				*flag = tmp_flag;
 				return WALK_PTR;
 			}
 		}
@@ -5291,13 +5310,14 @@ error:
 int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,
 		      const struct btf_type *t, int off, int size,
 		      enum bpf_access_type atype __maybe_unused,
-		      u32 *next_btf_id)
+		      u32 *next_btf_id, enum bpf_type_flag *flag)
 {
+	enum bpf_type_flag tmp_flag = 0;
 	int err;
 	u32 id;
 
 	do {
-		err = btf_struct_walk(log, btf, t, off, size, &id);
+		err = btf_struct_walk(log, btf, t, off, size, &id, &tmp_flag);
 
 		switch (err) {
 		case WALK_PTR:
@@ -5305,6 +5325,7 @@ int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,
 			 * we're done.
 			 */
 			*next_btf_id = id;
+			*flag = tmp_flag;
 			return PTR_TO_BTF_ID;
 		case WALK_SCALAR:
 			return SCALAR_VALUE;
@@ -5349,6 +5370,7 @@ bool btf_struct_ids_match(struct bpf_verifier_log *log,
 			  const struct btf *need_btf, u32 need_type_id)
 {
 	const struct btf_type *type;
+	enum bpf_type_flag flag;
 	int err;
 
 	/* Are we already done? */
@@ -5359,7 +5381,7 @@ again:
 	type = btf_type_by_id(btf, id);
 	if (!type)
 		return false;
-	err = btf_struct_walk(log, btf, type, off, 1, &id);
+	err = btf_struct_walk(log, btf, type, off, 1, &id, &flag);
 	if (err != WALK_STRUCT)
 		return false;
 

kernel/bpf/verifier.c

@@ -536,7 +536,7 @@ static bool is_cmpxchg_insn(const struct bpf_insn *insn)
 static const char *reg_type_str(struct bpf_verifier_env *env,
 				enum bpf_reg_type type)
 {
-	char postfix[16] = {0}, prefix[16] = {0};
+	char postfix[16] = {0}, prefix[32] = {0};
 	static const char * const str[] = {
 		[NOT_INIT]		= "?",
 		[SCALAR_VALUE]		= "inv",
@@ -570,9 +570,11 @@ static const char *reg_type_str(struct bpf_verifier_env *env,
 	}
 
 	if (type & MEM_RDONLY)
-		strncpy(prefix, "rdonly_", 16);
+		strncpy(prefix, "rdonly_", 32);
 	if (type & MEM_ALLOC)
-		strncpy(prefix, "alloc_", 16);
+		strncpy(prefix, "alloc_", 32);
+	if (type & MEM_USER)
+		strncpy(prefix, "user_", 32);
 
 	snprintf(env->type_str_buf, TYPE_STR_BUF_LEN, "%s%s%s",
 		 prefix, str[base_type(type)], postfix);
@@ -1547,14 +1549,15 @@ static void mark_reg_not_init(struct bpf_verifier_env *env,
 static void mark_btf_ld_reg(struct bpf_verifier_env *env,
 			    struct bpf_reg_state *regs, u32 regno,
 			    enum bpf_reg_type reg_type,
-			    struct btf *btf, u32 btf_id)
+			    struct btf *btf, u32 btf_id,
+			    enum bpf_type_flag flag)
 {
 	if (reg_type == SCALAR_VALUE) {
 		mark_reg_unknown(env, regs, regno);
 		return;
 	}
 	mark_reg_known_zero(env, regs, regno);
-	regs[regno].type = PTR_TO_BTF_ID;
+	regs[regno].type = PTR_TO_BTF_ID | flag;
 	regs[regno].btf = btf;
 	regs[regno].btf_id = btf_id;
 }
@@ -4152,6 +4155,7 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 	struct bpf_reg_state *reg = regs + regno;
 	const struct btf_type *t = btf_type_by_id(reg->btf, reg->btf_id);
 	const char *tname = btf_name_by_offset(reg->btf, t->name_off);
+	enum bpf_type_flag flag = 0;
 	u32 btf_id;
 	int ret;
 
@@ -4171,9 +4175,16 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 		return -EACCES;
 	}
 
+	if (reg->type & MEM_USER) {
+		verbose(env,
+			"R%d is ptr_%s access user memory: off=%d\n",
+			regno, tname, off);
+		return -EACCES;
+	}
+
 	if (env->ops->btf_struct_access) {
 		ret = env->ops->btf_struct_access(&env->log, reg->btf, t,
-						  off, size, atype, &btf_id);
+						  off, size, atype, &btf_id, &flag);
 	} else {
 		if (atype != BPF_READ) {
 			verbose(env, "only read is supported\n");
@@ -4181,14 +4192,14 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 		}
 
 		ret = btf_struct_access(&env->log, reg->btf, t, off, size,
-					atype, &btf_id);
+					atype, &btf_id, &flag);
 	}
 
 	if (ret < 0)
 		return ret;
 
 	if (atype == BPF_READ && value_regno >= 0)
-		mark_btf_ld_reg(env, regs, value_regno, ret, reg->btf, btf_id);
+		mark_btf_ld_reg(env, regs, value_regno, ret, reg->btf, btf_id, flag);
 
 	return 0;
 }
@@ -4201,6 +4212,7 @@ static int check_ptr_to_map_access(struct bpf_verifier_env *env,
 {
 	struct bpf_reg_state *reg = regs + regno;
 	struct bpf_map *map = reg->map_ptr;
+	enum bpf_type_flag flag = 0;
 	const struct btf_type *t;
 	const char *tname;
 	u32 btf_id;
@@ -4238,12 +4250,12 @@ static int check_ptr_to_map_access(struct bpf_verifier_env *env,
 		return -EACCES;
 	}
 
-	ret = btf_struct_access(&env->log, btf_vmlinux, t, off, size, atype, &btf_id);
+	ret = btf_struct_access(&env->log, btf_vmlinux, t, off, size, atype, &btf_id, &flag);
 	if (ret < 0)
 		return ret;
 
 	if (value_regno >= 0)
-		mark_btf_ld_reg(env, regs, value_regno, ret, btf_vmlinux, btf_id);
+		mark_btf_ld_reg(env, regs, value_regno, ret, btf_vmlinux, btf_id, flag);
 
 	return 0;
 }
@@ -4444,7 +4456,8 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 		if (err < 0)
 			return err;
 
-		err = check_ctx_access(env, insn_idx, off, size, t, &reg_type, &btf, &btf_id);
+		err = check_ctx_access(env, insn_idx, off, size, t, &reg_type, &btf,
+				       &btf_id);
 		if (err)
 			verbose_linfo(env, insn_idx, "; ");
 		if (!err && t == BPF_READ && value_regno >= 0) {

net/bpf/bpf_dummy_struct_ops.c

@@ -145,7 +145,8 @@ static int bpf_dummy_ops_btf_struct_access(struct bpf_verifier_log *log,
 					   const struct btf *btf,
 					   const struct btf_type *t, int off,
 					   int size, enum bpf_access_type atype,
-					   u32 *next_btf_id)
+					   u32 *next_btf_id,
+					   enum bpf_type_flag *flag)
 {
 	const struct btf_type *state;
 	s32 type_id;
@@ -162,7 +163,8 @@ static int bpf_dummy_ops_btf_struct_access(struct bpf_verifier_log *log,
 		return -EACCES;
 	}
 
-	err = btf_struct_access(log, btf, t, off, size, atype, next_btf_id);
+	err = btf_struct_access(log, btf, t, off, size, atype, next_btf_id,
+				flag);
 	if (err < 0)
 		return err;
 

net/ipv4/bpf_tcp_ca.c

@@ -96,12 +96,14 @@ static int bpf_tcp_ca_btf_struct_access(struct bpf_verifier_log *log,
 					const struct btf *btf,
 					const struct btf_type *t, int off,
 					int size, enum bpf_access_type atype,
-					u32 *next_btf_id)
+					u32 *next_btf_id,
+					enum bpf_type_flag *flag)
 {
 	size_t end;
 
 	if (atype == BPF_READ)
-		return btf_struct_access(log, btf, t, off, size, atype, next_btf_id);
+		return btf_struct_access(log, btf, t, off, size, atype, next_btf_id,
+					 flag);
 
 	if (t != tcp_sock_type) {
 		bpf_log(log, "only read is supported\n");