bpf: Return hashes of maps in BPF_OBJ_GET_INFO_BY_FD

Currently only array maps are supported, but the implementation can be
extended for other maps and objects. The hash is memoized only for
exclusive and frozen maps as their content is stable until the exclusive
program modifies the map.

This is required  for BPF signing, enabling a trusted loader program to
verify a map's integrity. The loader retrieves
the map's runtime hash from the kernel and compares it against an
expected hash computed at build time.

Signed-off-by: KP Singh <[email protected]>

Author: sinkap

include/linux/bpf.h

@@ -7,6 +7,7 @@
 #include <uapi/linux/bpf.h>
 #include <uapi/linux/filter.h>
 
+#include <crypto/sha2.h>
 #include <linux/workqueue.h>
 #include <linux/file.h>
 #include <linux/percpu.h>
@@ -110,6 +111,7 @@ struct bpf_map_ops {
 	long (*map_pop_elem)(struct bpf_map *map, void *value);
 	long (*map_peek_elem)(struct bpf_map *map, void *value);
 	void *(*map_lookup_percpu_elem)(struct bpf_map *map, void *key, u32 cpu);
+	int (*map_get_hash)(struct bpf_map *map, u32 hash_buf_size, void *hash_buf);
 
 	/* funcs called by prog_array and perf_event_array map */
 	void *(*map_fd_get_ptr)(struct bpf_map *map, struct file *map_file,
@@ -289,6 +291,7 @@ struct bpf_map_owner {
 };
 
 struct bpf_map {
+	u8 sha[SHA256_DIGEST_SIZE];
 	const struct bpf_map_ops *ops;
 	struct bpf_map *inner_map_meta;
 #ifdef CONFIG_SECURITY

include/uapi/linux/bpf.h

@@ -6668,6 +6668,8 @@ struct bpf_map_info {
 	__u32 btf_value_type_id;
 	__u32 btf_vmlinux_id;
 	__u64 map_extra;
+	__aligned_u64 hash;
+	__u32 hash_size;
 } __attribute__((aligned(8)));
 
 struct bpf_btf_info {

kernel/bpf/arraymap.c

@@ -12,6 +12,7 @@
 #include <uapi/linux/btf.h>
 #include <linux/rcupdate_trace.h>
 #include <linux/btf_ids.h>
+#include <crypto/sha2.h>
 
 #include "map_in_map.h"
 
@@ -174,6 +175,17 @@ static void *array_map_lookup_elem(struct bpf_map *map, void *key)
 	return array->value + (u64)array->elem_size * (index & array->index_mask);
 }
 
+static int array_map_get_hash(struct bpf_map *map, u32 hash_buf_size,
+			       void *hash_buf)
+{
+	struct bpf_array *array = container_of(map, struct bpf_array, map);
+
+	sha256(array->value, (u64)array->elem_size * array->map.max_entries,
+	       hash_buf);
+	memcpy(array->map.sha, hash_buf, sizeof(array->map.sha));
+	return 0;
+}
+
 static int array_map_direct_value_addr(const struct bpf_map *map, u64 *imm,
 				       u32 off)
 {
@@ -800,6 +812,7 @@ const struct bpf_map_ops array_map_ops = {
 	.map_mem_usage = array_map_mem_usage,
 	.map_btf_id = &array_map_btf_ids[0],
 	.iter_seq_info = &iter_seq_info,
+	.map_get_hash = &array_map_get_hash,
 };
 
 const struct bpf_map_ops percpu_array_map_ops = {

kernel/bpf/syscall.c

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-only
 /* Copyright (c) 2011-2014 PLUMgrid, http://plumgrid.com
  */
+#include <crypto/sha2.h>
 #include <linux/bpf.h>
 #include <linux/bpf-cgroup.h>
 #include <linux/bpf_trace.h>
@@ -5185,6 +5186,9 @@ static int bpf_map_get_info_by_fd(struct file *file,
 	info_len = min_t(u32, sizeof(info), info_len);
 
 	memset(&info, 0, sizeof(info));
+	if (copy_from_user(&info, uinfo, info_len))
+		return -EFAULT;
+
 	info.type = map->map_type;
 	info.id = map->id;
 	info.key_size = map->key_size;
@@ -5209,6 +5213,25 @@ static int bpf_map_get_info_by_fd(struct file *file,
 			return err;
 	}
 
+	if (info.hash) {
+		char __user *uhash = u64_to_user_ptr(info.hash);
+
+		if (!map->ops->map_get_hash)
+			return -EINVAL;
+
+		if (info.hash_size != SHA256_DIGEST_SIZE)
+			return -EINVAL;
+
+		err = map->ops->map_get_hash(map, SHA256_DIGEST_SIZE, map->sha);
+		if (err != 0)
+			return err;
+
+		if (copy_to_user(uhash, map->sha, SHA256_DIGEST_SIZE) != 0)
+			return -EFAULT;
+	} else if (info.hash_size) {
+		return -EINVAL;
+	}
+
 	if (copy_to_user(uinfo, &info, info_len) ||
 	    put_user(info_len, &uattr->info.info_len))
 		return -EFAULT;

tools/include/uapi/linux/bpf.h

@@ -6668,6 +6668,8 @@ struct bpf_map_info {
 	__u32 btf_value_type_id;
 	__u32 btf_vmlinux_id;
 	__u64 map_extra;
+	__aligned_u64 hash;
+	__u32 hash_size;
 } __attribute__((aligned(8)));
 
 struct bpf_btf_info {

tools/testing/selftests/bpf/progs/verifier_map_ptr.c

@@ -70,10 +70,13 @@ __naked void bpf_map_ptr_write_rejected(void)
 	: __clobber_all);
 }
 
+/* The first element of struct bpf_map is a SHA256 hash of 32 bytes, accessing
+ * into this array is valid. The opts field is now at offset 33.
+ */
 SEC("socket")
 __description("bpf_map_ptr: read non-existent field rejected")
 __failure
-__msg("cannot access ptr member ops with moff 0 in struct bpf_map with off 1 size 4")
+__msg("cannot access ptr member ops with moff 32 in struct bpf_map with off 33 size 4")
 __failure_unpriv
 __msg_unpriv("access is allowed only to CAP_PERFMON and CAP_SYS_ADMIN")
 __flag(BPF_F_ANY_ALIGNMENT)
@@ -82,7 +85,7 @@ __naked void read_non_existent_field_rejected(void)
 	asm volatile ("					\
 	r6 = 0;						\
 	r1 = %[map_array_48b] ll;			\
-	r6 = *(u32*)(r1 + 1);				\
+	r6 = *(u32*)(r1 + 33);				\
 	r0 = 1;						\
 	exit;						\
 "	: