Merge branch 'Use __GFP_ZERO in bpf memory allocator'

Hou Tao says:

====================

From: Hou Tao <houtao1@huawei.com>

Hi,

The patchset tries to fix the hard-up problem found when checking how htab
handles element reuse in bpf memory allocator. The immediate reuse of
freed elements will reinitialize special fields (e.g., bpf_spin_lock) in
htab map value and it may corrupt lookup procedure with BFP_F_LOCK flag
which acquires bpf-spin-lock during value copying, and lead to hard-lock
as shown in patch #2. Patch #1 fixes it by using __GFP_ZERO when allocating
the object from slab and the behavior is similar with the preallocated
hash-table case. Please see individual patches for more details. And comments
are always welcome.

Regards,

Change Log:
v1:
  * Use __GFP_ZERO instead of ctor to avoid retpoline overhead (from Alexei)
  * Add comments for check_and_init_map_value() (from Alexei)
  * split __GFP_ZERO patches out of the original patchset to unblock
    the development work of others.

RFC: https://lore.kernel.org/bpf/20221230041151.1231169-1-houtao@huaweicloud.com
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -363,6 +363,13 @@ static inline void bpf_obj_init(const struct btf_field_offs *foffs, void *obj)
 		memset(obj + foffs->field_off[i], 0, foffs->field_sz[i]);
 }
 
+/* 'dst' must be a temporary buffer and should not point to memory that is being
+ * used in parallel by a bpf program or bpf syscall, otherwise the access from
+ * the bpf program or bpf syscall may be corrupted by the reinitialization,
+ * leading to weird problems. Even 'dst' is newly-allocated from bpf memory
+ * allocator, it is still possible for 'dst' to be used in parallel by a bpf
+ * program or bpf syscall.
+ */
 static inline void check_and_init_map_value(struct bpf_map *map, void *dst)
 {
 	bpf_obj_init(map->field_offs, dst);

kernel/bpf/hashtab.c

@@ -1004,8 +1004,6 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 			l_new = ERR_PTR(-ENOMEM);
 			goto dec_count;
 		}
-		check_and_init_map_value(&htab->map,
-					 l_new->key + round_up(key_size, 8));
 	}
 
 	memcpy(l_new->key, key, key_size);
@@ -1592,6 +1590,7 @@ static int __htab_map_lookup_and_delete_elem(struct bpf_map *map, void *key,
 			else
 				copy_map_value(map, value, l->key +
 					       roundup_key_size);
+			/* Zeroing special fields in the temp buffer */
 			check_and_init_map_value(map, value);
 		}
 
@@ -1792,6 +1791,7 @@ again_nocopy:
 						      true);
 			else
 				copy_map_value(map, dst_val, value);
+			/* Zeroing special fields in the temp buffer */
 			check_and_init_map_value(map, dst_val);
 		}
 		if (do_delete) {

kernel/bpf/memalloc.c

@@ -143,7 +143,7 @@ static void *__alloc(struct bpf_mem_cache *c, int node)
 		return obj;
 	}
 
-	return kmalloc_node(c->unit_size, flags, node);
+	return kmalloc_node(c->unit_size, flags | __GFP_ZERO, node);
 }
 
 static struct mem_cgroup *get_memcg(const struct bpf_mem_cache *c)

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

@@ -0,0 +1,101 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2023. Huawei Technologies Co., Ltd */
+#define _GNU_SOURCE
+#include <sched.h>
+#include <stdbool.h>
+#include <test_progs.h>
+#include "htab_reuse.skel.h"
+
+struct htab_op_ctx {
+	int fd;
+	int loop;
+	bool stop;
+};
+
+struct htab_val {
+	unsigned int lock;
+	unsigned int data;
+};
+
+static void *htab_lookup_fn(void *arg)
+{
+	struct htab_op_ctx *ctx = arg;
+	int i = 0;
+
+	while (i++ < ctx->loop && !ctx->stop) {
+		struct htab_val value;
+		unsigned int key;
+
+		/* Use BPF_F_LOCK to use spin-lock in map value. */
+		key = 7;
+		bpf_map_lookup_elem_flags(ctx->fd, &key, &value, BPF_F_LOCK);
+	}
+
+	return NULL;
+}
+
+static void *htab_update_fn(void *arg)
+{
+	struct htab_op_ctx *ctx = arg;
+	int i = 0;
+
+	while (i++ < ctx->loop && !ctx->stop) {
+		struct htab_val value;
+		unsigned int key;
+
+		key = 7;
+		value.lock = 0;
+		value.data = key;
+		bpf_map_update_elem(ctx->fd, &key, &value, BPF_F_LOCK);
+		bpf_map_delete_elem(ctx->fd, &key);
+
+		key = 24;
+		value.lock = 0;
+		value.data = key;
+		bpf_map_update_elem(ctx->fd, &key, &value, BPF_F_LOCK);
+		bpf_map_delete_elem(ctx->fd, &key);
+	}
+
+	return NULL;
+}
+
+void test_htab_reuse(void)
+{
+	unsigned int i, wr_nr = 1, rd_nr = 4;
+	pthread_t tids[wr_nr + rd_nr];
+	struct htab_reuse *skel;
+	struct htab_op_ctx ctx;
+	int err;
+
+	skel = htab_reuse__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "htab_reuse__open_and_load"))
+		return;
+
+	ctx.fd = bpf_map__fd(skel->maps.htab);
+	ctx.loop = 500;
+	ctx.stop = false;
+
+	memset(tids, 0, sizeof(tids));
+	for (i = 0; i < wr_nr; i++) {
+		err = pthread_create(&tids[i], NULL, htab_update_fn, &ctx);
+		if (!ASSERT_OK(err, "pthread_create")) {
+			ctx.stop = true;
+			goto reap;
+		}
+	}
+	for (i = 0; i < rd_nr; i++) {
+		err = pthread_create(&tids[i + wr_nr], NULL, htab_lookup_fn, &ctx);
+		if (!ASSERT_OK(err, "pthread_create")) {
+			ctx.stop = true;
+			goto reap;
+		}
+	}
+
+reap:
+	for (i = 0; i < wr_nr + rd_nr; i++) {
+		if (!tids[i])
+			continue;
+		pthread_join(tids[i], NULL);
+	}
+	htab_reuse__destroy(skel);
+}

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

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2023. Huawei Technologies Co., Ltd */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct htab_val {
+	struct bpf_spin_lock lock;
+	unsigned int data;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 64);
+	__type(key, unsigned int);
+	__type(value, struct htab_val);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+} htab SEC(".maps");