bpf: add program side {rd, wr}only support for maps

This work adds two new map creation flags BPF_F_RDONLY_PROG
and BPF_F_WRONLY_PROG in order to allow for read-only or
write-only BPF maps from a BPF program side.

Today we have BPF_F_RDONLY and BPF_F_WRONLY, but this only
applies to system call side, meaning the BPF program has full
read/write access to the map as usual while bpf(2) calls with
map fd can either only read or write into the map depending
on the flags. BPF_F_RDONLY_PROG and BPF_F_WRONLY_PROG allows
for the exact opposite such that verifier is going to reject
program loads if write into a read-only map or a read into a
write-only map is detected. For read-only map case also some
helpers are forbidden for programs that would alter the map
state such as map deletion, update, etc. As opposed to the two
BPF_F_RDONLY / BPF_F_WRONLY flags, BPF_F_RDONLY_PROG as well
as BPF_F_WRONLY_PROG really do correspond to the map lifetime.

We've enabled this generic map extension to various non-special
maps holding normal user data: array, hash, lru, lpm, local
storage, queue and stack. Further generic map types could be
followed up in future depending on use-case. Main use case
here is to forbid writes into .rodata map values from verifier
side.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -430,6 +430,35 @@ struct bpf_array {
 #define BPF_COMPLEXITY_LIMIT_INSNS      1000000 /* yes. 1M insns */
 #define MAX_TAIL_CALL_CNT 32
 
+#define BPF_F_ACCESS_MASK	(BPF_F_RDONLY |		\
+				 BPF_F_RDONLY_PROG |	\
+				 BPF_F_WRONLY |		\
+				 BPF_F_WRONLY_PROG)
+
+#define BPF_MAP_CAN_READ	BIT(0)
+#define BPF_MAP_CAN_WRITE	BIT(1)
+
+static inline u32 bpf_map_flags_to_cap(struct bpf_map *map)
+{
+	u32 access_flags = map->map_flags & (BPF_F_RDONLY_PROG | BPF_F_WRONLY_PROG);
+
+	/* Combination of BPF_F_RDONLY_PROG | BPF_F_WRONLY_PROG is
+	 * not possible.
+	 */
+	if (access_flags & BPF_F_RDONLY_PROG)
+		return BPF_MAP_CAN_READ;
+	else if (access_flags & BPF_F_WRONLY_PROG)
+		return BPF_MAP_CAN_WRITE;
+	else
+		return BPF_MAP_CAN_READ | BPF_MAP_CAN_WRITE;
+}
+
+static inline bool bpf_map_flags_access_ok(u32 access_flags)
+{
+	return (access_flags & (BPF_F_RDONLY_PROG | BPF_F_WRONLY_PROG)) !=
+	       (BPF_F_RDONLY_PROG | BPF_F_WRONLY_PROG);
+}
+
 struct bpf_event_entry {
 	struct perf_event *event;
 	struct file *perf_file;

include/uapi/linux/bpf.h

@@ -294,7 +294,7 @@ enum bpf_attach_type {
 
 #define BPF_OBJ_NAME_LEN 16U
 
-/* Flags for accessing BPF object */
+/* Flags for accessing BPF object from syscall side. */
 #define BPF_F_RDONLY		(1U << 3)
 #define BPF_F_WRONLY		(1U << 4)
 
@@ -304,6 +304,10 @@ enum bpf_attach_type {
 /* Zero-initialize hash function seed. This should only be used for testing. */
 #define BPF_F_ZERO_SEED		(1U << 6)
 
+/* Flags for accessing BPF object from program side. */
+#define BPF_F_RDONLY_PROG	(1U << 7)
+#define BPF_F_WRONLY_PROG	(1U << 8)
+
 /* flags for BPF_PROG_QUERY */
 #define BPF_F_QUERY_EFFECTIVE	(1U << 0)
 

kernel/bpf/arraymap.c

@@ -22,7 +22,7 @@
 #include "map_in_map.h"
 
 #define ARRAY_CREATE_FLAG_MASK \
-	(BPF_F_NUMA_NODE | BPF_F_RDONLY | BPF_F_WRONLY)
+	(BPF_F_NUMA_NODE | BPF_F_ACCESS_MASK)
 
 static void bpf_array_free_percpu(struct bpf_array *array)
 {
@@ -63,6 +63,7 @@ int array_map_alloc_check(union bpf_attr *attr)
 	if (attr->max_entries == 0 || attr->key_size != 4 ||
 	    attr->value_size == 0 ||
 	    attr->map_flags & ~ARRAY_CREATE_FLAG_MASK ||
+	    !bpf_map_flags_access_ok(attr->map_flags) ||
 	    (percpu && numa_node != NUMA_NO_NODE))
 		return -EINVAL;
 
@@ -472,6 +473,9 @@ static int fd_array_map_alloc_check(union bpf_attr *attr)
 	/* only file descriptors can be stored in this type of map */
 	if (attr->value_size != sizeof(u32))
 		return -EINVAL;
+	/* Program read-only/write-only not supported for special maps yet. */
+	if (attr->map_flags & (BPF_F_RDONLY_PROG | BPF_F_WRONLY_PROG))
+		return -EINVAL;
 	return array_map_alloc_check(attr);
 }
 

kernel/bpf/hashtab.c

@@ -23,7 +23,7 @@
 
 #define HTAB_CREATE_FLAG_MASK						\
 	(BPF_F_NO_PREALLOC | BPF_F_NO_COMMON_LRU | BPF_F_NUMA_NODE |	\
-	 BPF_F_RDONLY | BPF_F_WRONLY | BPF_F_ZERO_SEED)
+	 BPF_F_ACCESS_MASK | BPF_F_ZERO_SEED)
 
 struct bucket {
 	struct hlist_nulls_head head;
@@ -262,8 +262,8 @@ static int htab_map_alloc_check(union bpf_attr *attr)
 		/* Guard against local DoS, and discourage production use. */
 		return -EPERM;
 
-	if (attr->map_flags & ~HTAB_CREATE_FLAG_MASK)
-		/* reserved bits should not be used */
+	if (attr->map_flags & ~HTAB_CREATE_FLAG_MASK ||
+	    !bpf_map_flags_access_ok(attr->map_flags))
 		return -EINVAL;
 
 	if (!lru && percpu_lru)

kernel/bpf/local_storage.c

@@ -14,7 +14,7 @@ DEFINE_PER_CPU(struct bpf_cgroup_storage*, bpf_cgroup_storage[MAX_BPF_CGROUP_STO
 #ifdef CONFIG_CGROUP_BPF
 
 #define LOCAL_STORAGE_CREATE_FLAG_MASK					\
-	(BPF_F_NUMA_NODE | BPF_F_RDONLY | BPF_F_WRONLY)
+	(BPF_F_NUMA_NODE | BPF_F_ACCESS_MASK)
 
 struct bpf_cgroup_storage_map {
 	struct bpf_map map;
@@ -282,8 +282,8 @@ static struct bpf_map *cgroup_storage_map_alloc(union bpf_attr *attr)
 	if (attr->value_size > PAGE_SIZE)
 		return ERR_PTR(-E2BIG);
 
-	if (attr->map_flags & ~LOCAL_STORAGE_CREATE_FLAG_MASK)
-		/* reserved bits should not be used */
+	if (attr->map_flags & ~LOCAL_STORAGE_CREATE_FLAG_MASK ||
+	    !bpf_map_flags_access_ok(attr->map_flags))
 		return ERR_PTR(-EINVAL);
 
 	if (attr->max_entries)

kernel/bpf/lpm_trie.c

@@ -538,7 +538,7 @@ static int trie_delete_elem(struct bpf_map *map, void *_key)
 #define LPM_KEY_SIZE_MIN	LPM_KEY_SIZE(LPM_DATA_SIZE_MIN)
 
 #define LPM_CREATE_FLAG_MASK	(BPF_F_NO_PREALLOC | BPF_F_NUMA_NODE |	\
-				 BPF_F_RDONLY | BPF_F_WRONLY)
+				 BPF_F_ACCESS_MASK)
 
 static struct bpf_map *trie_alloc(union bpf_attr *attr)
 {
@@ -553,6 +553,7 @@ static struct bpf_map *trie_alloc(union bpf_attr *attr)
 	if (attr->max_entries == 0 ||
 	    !(attr->map_flags & BPF_F_NO_PREALLOC) ||
 	    attr->map_flags & ~LPM_CREATE_FLAG_MASK ||
+	    !bpf_map_flags_access_ok(attr->map_flags) ||
 	    attr->key_size < LPM_KEY_SIZE_MIN ||
 	    attr->key_size > LPM_KEY_SIZE_MAX ||
 	    attr->value_size < LPM_VAL_SIZE_MIN ||

kernel/bpf/queue_stack_maps.c

@@ -11,8 +11,7 @@
 #include "percpu_freelist.h"
 
 #define QUEUE_STACK_CREATE_FLAG_MASK \
-	(BPF_F_NUMA_NODE | BPF_F_RDONLY | BPF_F_WRONLY)
-
+	(BPF_F_NUMA_NODE | BPF_F_ACCESS_MASK)
 
 struct bpf_queue_stack {
 	struct bpf_map map;
@@ -52,7 +51,8 @@ static int queue_stack_map_alloc_check(union bpf_attr *attr)
 	/* check sanity of attributes */
 	if (attr->max_entries == 0 || attr->key_size != 0 ||
 	    attr->value_size == 0 ||
-	    attr->map_flags & ~QUEUE_STACK_CREATE_FLAG_MASK)
+	    attr->map_flags & ~QUEUE_STACK_CREATE_FLAG_MASK ||
+	    !bpf_map_flags_access_ok(attr->map_flags))
 		return -EINVAL;
 
 	if (attr->value_size > KMALLOC_MAX_SIZE)

kernel/bpf/syscall.c

@@ -501,6 +501,8 @@ static int map_check_btf(struct bpf_map *map, const struct btf *btf,
 	map->spin_lock_off = btf_find_spin_lock(btf, value_type);
 
 	if (map_value_has_spin_lock(map)) {
+		if (map->map_flags & BPF_F_RDONLY_PROG)
+			return -EACCES;
 		if (map->map_type != BPF_MAP_TYPE_HASH &&
 		    map->map_type != BPF_MAP_TYPE_ARRAY &&
 		    map->map_type != BPF_MAP_TYPE_CGROUP_STORAGE)

kernel/bpf/verifier.c

@@ -1439,6 +1439,28 @@ static int check_stack_access(struct bpf_verifier_env *env,
 	return 0;
 }
 
+static int check_map_access_type(struct bpf_verifier_env *env, u32 regno,
+				 int off, int size, enum bpf_access_type type)
+{
+	struct bpf_reg_state *regs = cur_regs(env);
+	struct bpf_map *map = regs[regno].map_ptr;
+	u32 cap = bpf_map_flags_to_cap(map);
+
+	if (type == BPF_WRITE && !(cap & BPF_MAP_CAN_WRITE)) {
+		verbose(env, "write into map forbidden, value_size=%d off=%d size=%d\n",
+			map->value_size, off, size);
+		return -EACCES;
+	}
+
+	if (type == BPF_READ && !(cap & BPF_MAP_CAN_READ)) {
+		verbose(env, "read from map forbidden, value_size=%d off=%d size=%d\n",
+			map->value_size, off, size);
+		return -EACCES;
+	}
+
+	return 0;
+}
+
 /* check read/write into map element returned by bpf_map_lookup_elem() */
 static int __check_map_access(struct bpf_verifier_env *env, u32 regno, int off,
 			      int size, bool zero_size_allowed)
@@ -2024,7 +2046,9 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 			verbose(env, "R%d leaks addr into map\n", value_regno);
 			return -EACCES;
 		}
-
+		err = check_map_access_type(env, regno, off, size, t);
+		if (err)
+			return err;
 		err = check_map_access(env, regno, off, size, false);
 		if (!err && t == BPF_READ && value_regno >= 0)
 			mark_reg_unknown(env, regs, value_regno);
@@ -2327,6 +2351,10 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
 		return check_packet_access(env, regno, reg->off, access_size,
 					   zero_size_allowed);
 	case PTR_TO_MAP_VALUE:
+		if (check_map_access_type(env, regno, reg->off, access_size,
+					  meta && meta->raw_mode ? BPF_WRITE :
+					  BPF_READ))
+			return -EACCES;
 		return check_map_access(env, regno, reg->off, access_size,
 					zero_size_allowed);
 	default: /* scalar_value|ptr_to_stack or invalid ptr */
@@ -3059,6 +3087,7 @@ record_func_map(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
 		int func_id, int insn_idx)
 {
 	struct bpf_insn_aux_data *aux = &env->insn_aux_data[insn_idx];
+	struct bpf_map *map = meta->map_ptr;
 
 	if (func_id != BPF_FUNC_tail_call &&
 	    func_id != BPF_FUNC_map_lookup_elem &&
@@ -3069,11 +3098,24 @@ record_func_map(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
 	    func_id != BPF_FUNC_map_peek_elem)
 		return 0;
 
-	if (meta->map_ptr == NULL) {
+	if (map == NULL) {
 		verbose(env, "kernel subsystem misconfigured verifier\n");
 		return -EINVAL;
 	}
 
+	/* In case of read-only, some additional restrictions
+	 * need to be applied in order to prevent altering the
+	 * state of the map from program side.
+	 */
+	if ((map->map_flags & BPF_F_RDONLY_PROG) &&
+	    (func_id == BPF_FUNC_map_delete_elem ||
+	     func_id == BPF_FUNC_map_update_elem ||
+	     func_id == BPF_FUNC_map_push_elem ||
+	     func_id == BPF_FUNC_map_pop_elem)) {
+		verbose(env, "write into map forbidden\n");
+		return -EACCES;
+	}
+
 	if (!BPF_MAP_PTR(aux->map_state))
 		bpf_map_ptr_store(aux, meta->map_ptr,
 				  meta->map_ptr->unpriv_array);