bpf: Check bloom filter map value size

This patch adds a missing check to bloom filter creating, rejecting
values above KMALLOC_MAX_SIZE. This brings the bloom map in line with
many other map types.

The lack of this protection can cause kernel crashes for value sizes
that overflow int's. Such a crash was caught by syzkaller. The next
patch adds more guard-rails at a lower level.

Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240327024245.318299-2-andreimatei1@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: andreimatei

kernel/bpf/bloom_filter.c

@@ -80,6 +80,18 @@ static int bloom_map_get_next_key(struct bpf_map *map, void *key, void *next_key
 	return -EOPNOTSUPP;
 }
 
+/* Called from syscall */
+static int bloom_map_alloc_check(union bpf_attr *attr)
+{
+	if (attr->value_size > KMALLOC_MAX_SIZE)
+		/* if value_size is bigger, the user space won't be able to
+		 * access the elements.
+		 */
+		return -E2BIG;
+
+	return 0;
+}
+
 static struct bpf_map *bloom_map_alloc(union bpf_attr *attr)
 {
 	u32 bitset_bytes, bitset_mask, nr_hash_funcs, nr_bits;
@@ -191,6 +203,7 @@ static u64 bloom_map_mem_usage(const struct bpf_map *map)
 BTF_ID_LIST_SINGLE(bpf_bloom_map_btf_ids, struct, bpf_bloom_filter)
 const struct bpf_map_ops bloom_filter_map_ops = {
 	.map_meta_equal = bpf_map_meta_equal,
+	.map_alloc_check = bloom_map_alloc_check,
 	.map_alloc = bloom_map_alloc,
 	.map_free = bloom_map_free,
 	.map_get_next_key = bloom_map_get_next_key,

tools/testing/selftests/bpf/prog_tests/bloom_filter_map.c

@@ -2,6 +2,7 @@
 /* Copyright (c) 2021 Facebook */
 
 #include <sys/syscall.h>
+#include <limits.h>
 #include <test_progs.h>
 #include "bloom_filter_map.skel.h"
 
@@ -21,6 +22,11 @@ static void test_fail_cases(void)
 	if (!ASSERT_LT(fd, 0, "bpf_map_create bloom filter invalid value size 0"))
 		close(fd);
 
+	/* Invalid value size: too big */
+	fd = bpf_map_create(BPF_MAP_TYPE_BLOOM_FILTER, NULL, 0, INT32_MAX, 100, NULL);
+	if (!ASSERT_LT(fd, 0, "bpf_map_create bloom filter invalid value too large"))
+		close(fd);
+
 	/* Invalid max entries size */
 	fd = bpf_map_create(BPF_MAP_TYPE_BLOOM_FILTER, NULL, 0, sizeof(value), 0, NULL);
 	if (!ASSERT_LT(fd, 0, "bpf_map_create bloom filter invalid max entries size"))