Merge branch 'fixes for concurrent htab updates'

Hou Tao says:

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

From: Hou Tao <houtao1@huawei.com>

Hi,

The patchset aims to fix the issues found during investigating the
syzkaller problem reported in [0]. It seems that the concurrent updates
to the same hash-table bucket may fail as shown in patch 1.

Patch 1 uses preempt_disable() to fix the problem for
htab_use_raw_lock() case. For !htab_use_raw_lock() case, the problem is
left to "BPF specific memory allocator" patchset [1] in which
!htab_use_raw_lock() will be removed.

Patch 2 fixes the out-of-bound memory read problem reported in [0]. The
problem has the root cause as patch 1 and it is fixed by handling -EBUSY
from htab_lock_bucket() correctly.

Patch 3 add two cases for hash-table update: one for the reentrancy of
bpf_map_update_elem(), and another one for concurrent updates of the
same hash-table bucket.

Comments are always welcome.

Regards,
Tao

[0]: https://lore.kernel.org/bpf/CACkBjsbuxaR6cv0kXJoVnBfL9ZJXjjoUcMpw_Ogc313jSrg14A@mail.gmail.com/
[1]: https://lore.kernel.org/bpf/20220819214232.18784-1-alexei.starovoitov@gmail.com/

Change Log:

v4:
 * rebased on bpf-next
 * add htab_update to DENYLIST.s390x

v3: https://lore.kernel.org/bpf/20220829023709.1958204-1-houtao@huaweicloud.com/
 * patch 1: update commit message and add Fixes tag
 * patch 2: add Fixes tag
 * patch 3: elaborate the description of test cases

v2: https://lore.kernel.org/bpf/bd60ef93-1c6a-2db2-557d-b09b92ad22bd@huaweicloud.com/
 * Note the fix is for CONFIG_PREEMPT case in commit message and add
   Reviewed-by tag for patch 1
 * Drop patch "bpf: Allow normally concurrent map updates for !htab_use_raw_lock() case"

v1: https://lore.kernel.org/bpf/20220821033223.2598791-1-houtao@huaweicloud.com/
====================
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/hashtab.c

@@ -162,17 +162,25 @@ static inline int htab_lock_bucket(const struct bpf_htab *htab,
 				   unsigned long *pflags)
 {
 	unsigned long flags;
+	bool use_raw_lock;
 
 	hash = hash & HASHTAB_MAP_LOCK_MASK;
 
+	use_raw_lock = htab_use_raw_lock(htab);
+	if (use_raw_lock)
+		preempt_disable();
+	else
 		migrate_disable();
 	if (unlikely(__this_cpu_inc_return(*(htab->map_locked[hash])) != 1)) {
 		__this_cpu_dec(*(htab->map_locked[hash]));
+		if (use_raw_lock)
+			preempt_enable();
+		else
 			migrate_enable();
 		return -EBUSY;
 	}
 
-	if (htab_use_raw_lock(htab))
+	if (use_raw_lock)
 		raw_spin_lock_irqsave(&b->raw_lock, flags);
 	else
 		spin_lock_irqsave(&b->lock, flags);
@@ -185,12 +193,17 @@ static inline void htab_unlock_bucket(const struct bpf_htab *htab,
 				      struct bucket *b, u32 hash,
 				      unsigned long flags)
 {
+	bool use_raw_lock = htab_use_raw_lock(htab);
+
 	hash = hash & HASHTAB_MAP_LOCK_MASK;
-	if (htab_use_raw_lock(htab))
+	if (use_raw_lock)
 		raw_spin_unlock_irqrestore(&b->raw_lock, flags);
 	else
 		spin_unlock_irqrestore(&b->lock, flags);
 	__this_cpu_dec(*(htab->map_locked[hash]));
+	if (use_raw_lock)
+		preempt_enable();
+	else
 		migrate_enable();
 }
 
@@ -1691,8 +1704,11 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	/* do not grab the lock unless need it (bucket_cnt > 0). */
 	if (locked) {
 		ret = htab_lock_bucket(htab, b, batch, &flags);
-		if (ret)
-			goto next_batch;
+		if (ret) {
+			rcu_read_unlock();
+			bpf_enable_instrumentation();
+			goto after_loop;
+		}
 	}
 
 	bucket_cnt = 0;

tools/testing/selftests/bpf/DENYLIST.s390x

@@ -68,3 +68,4 @@ unpriv_bpf_disabled                      # fentry
 setget_sockopt                           # attach unexpected error: -524                                               (trampoline)
 cb_refs                                  # expected error message unexpected error: -524                               (trampoline)
 cgroup_hierarchical_stats                # JIT does not support calling kernel function                                (kfunc)
+htab_update                              # failed to attach: ERROR: strerror_r(-524)=22                                (trampoline)

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2022. Huawei Technologies Co., Ltd */
+#define _GNU_SOURCE
+#include <sched.h>
+#include <stdbool.h>
+#include <test_progs.h>
+#include "htab_update.skel.h"
+
+struct htab_update_ctx {
+	int fd;
+	int loop;
+	bool stop;
+};
+
+static void test_reenter_update(void)
+{
+	struct htab_update *skel;
+	unsigned int key, value;
+	int err;
+
+	skel = htab_update__open();
+	if (!ASSERT_OK_PTR(skel, "htab_update__open"))
+		return;
+
+	/* lookup_elem_raw() may be inlined and find_kernel_btf_id() will return -ESRCH */
+	bpf_program__set_autoload(skel->progs.lookup_elem_raw, true);
+	err = htab_update__load(skel);
+	if (!ASSERT_TRUE(!err || err == -ESRCH, "htab_update__load") || err)
+		goto out;
+
+	skel->bss->pid = getpid();
+	err = htab_update__attach(skel);
+	if (!ASSERT_OK(err, "htab_update__attach"))
+		goto out;
+
+	/* Will trigger the reentrancy of bpf_map_update_elem() */
+	key = 0;
+	value = 0;
+	err = bpf_map_update_elem(bpf_map__fd(skel->maps.htab), &key, &value, 0);
+	if (!ASSERT_OK(err, "add element"))
+		goto out;
+
+	ASSERT_EQ(skel->bss->update_err, -EBUSY, "no reentrancy");
+out:
+	htab_update__destroy(skel);
+}
+
+static void *htab_update_thread(void *arg)
+{
+	struct htab_update_ctx *ctx = arg;
+	cpu_set_t cpus;
+	int i;
+
+	/* Pinned on CPU 0 */
+	CPU_ZERO(&cpus);
+	CPU_SET(0, &cpus);
+	pthread_setaffinity_np(pthread_self(), sizeof(cpus), &cpus);
+
+	i = 0;
+	while (i++ < ctx->loop && !ctx->stop) {
+		unsigned int key = 0, value = 0;
+		int err;
+
+		err = bpf_map_update_elem(ctx->fd, &key, &value, 0);
+		if (err) {
+			ctx->stop = true;
+			return (void *)(long)err;
+		}
+	}
+
+	return NULL;
+}
+
+static void test_concurrent_update(void)
+{
+	struct htab_update_ctx ctx;
+	struct htab_update *skel;
+	unsigned int i, nr;
+	pthread_t *tids;
+	int err;
+
+	skel = htab_update__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "htab_update__open_and_load"))
+		return;
+
+	ctx.fd = bpf_map__fd(skel->maps.htab);
+	ctx.loop = 1000;
+	ctx.stop = false;
+
+	nr = 4;
+	tids = calloc(nr, sizeof(*tids));
+	if (!ASSERT_NEQ(tids, NULL, "no mem"))
+		goto out;
+
+	for (i = 0; i < nr; i++) {
+		err = pthread_create(&tids[i], NULL, htab_update_thread, &ctx);
+		if (!ASSERT_OK(err, "pthread_create")) {
+			unsigned int j;
+
+			ctx.stop = true;
+			for (j = 0; j < i; j++)
+				pthread_join(tids[j], NULL);
+			goto out;
+		}
+	}
+
+	for (i = 0; i < nr; i++) {
+		void *thread_err = NULL;
+
+		pthread_join(tids[i], &thread_err);
+		ASSERT_EQ(thread_err, NULL, "update error");
+	}
+
+out:
+	if (tids)
+		free(tids);
+	htab_update__destroy(skel);
+}
+
+void test_htab_update(void)
+{
+	if (test__start_subtest("reenter_update"))
+		test_reenter_update();
+	if (test__start_subtest("concurrent_update"))
+		test_concurrent_update();
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2022. Huawei Technologies Co., Ltd */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 1);
+	__uint(key_size, sizeof(__u32));
+	__uint(value_size, sizeof(__u32));
+} htab SEC(".maps");
+
+int pid = 0;
+int update_err = 0;
+
+SEC("?fentry/lookup_elem_raw")
+int lookup_elem_raw(void *ctx)
+{
+	__u32 key = 0, value = 1;
+
+	if ((bpf_get_current_pid_tgid() >> 32) != pid)
+		return 0;
+
+	update_err = bpf_map_update_elem(&htab, &key, &value, 0);
+	return 0;
+}