bpf-for-netdev

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Merge tag 'for-netdev' of https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf

Daniel Borkmann says:

====================
pull-request: bpf 2024-07-11

The following pull-request contains BPF updates for your *net* tree.

We've added 4 non-merge commits during the last 2 day(s) which contain
a total of 4 files changed, 262 insertions(+), 19 deletions(-).

The main changes are:

1) Fixes for a BPF timer lockup and a use-after-free scenario when timers
   are used concurrently, from Kumar Kartikeya Dwivedi.

2) Fix the argument order in the call to bpf_map_kvcalloc() which could
   otherwise lead to a compilation error, from Mohammad Shehar Yaar Tausif.

bpf-for-netdev

* tag 'for-netdev' of https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf:
  selftests/bpf: Add timer lockup selftest
  bpf: Defer work in bpf_timer_cancel_and_free
  bpf: Fail bpf_timer_cancel when callback is being cancelled
  bpf: fix order of args in call to bpf_map_kvcalloc
====================

Link: https://patch.msgid.link/20240711084016.25757-1-daniel@iogearbox.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

kernel/bpf/bpf_local_storage.c

@@ -782,8 +782,8 @@ bpf_local_storage_map_alloc(union bpf_attr *attr,
 	nbuckets = max_t(u32, 2, nbuckets);
 	smap->bucket_log = ilog2(nbuckets);
 
-	smap->buckets = bpf_map_kvcalloc(&smap->map, sizeof(*smap->buckets),
-					 nbuckets, GFP_USER | __GFP_NOWARN);
+	smap->buckets = bpf_map_kvcalloc(&smap->map, nbuckets,
+					 sizeof(*smap->buckets), GFP_USER | __GFP_NOWARN);
 	if (!smap->buckets) {
 		err = -ENOMEM;
 		goto free_smap;

kernel/bpf/helpers.c

@@ -1084,7 +1084,10 @@ struct bpf_async_cb {
 	struct bpf_prog *prog;
 	void __rcu *callback_fn;
 	void *value;
-	struct rcu_head rcu;
+	union {
+		struct rcu_head rcu;
+		struct work_struct delete_work;
+	};
 	u64 flags;
 };
 
@@ -1107,6 +1110,7 @@ struct bpf_async_cb {
 struct bpf_hrtimer {
 	struct bpf_async_cb cb;
 	struct hrtimer timer;
+	atomic_t cancelling;
 };
 
 struct bpf_work {
@@ -1219,6 +1223,21 @@ static void bpf_wq_delete_work(struct work_struct *work)
 	kfree_rcu(w, cb.rcu);
 }
 
+static void bpf_timer_delete_work(struct work_struct *work)
+{
+	struct bpf_hrtimer *t = container_of(work, struct bpf_hrtimer, cb.delete_work);
+
+	/* Cancel the timer and wait for callback to complete if it was running.
+	 * If hrtimer_cancel() can be safely called it's safe to call
+	 * kfree_rcu(t) right after for both preallocated and non-preallocated
+	 * maps.  The async->cb = NULL was already done and no code path can see
+	 * address 't' anymore. Timer if armed for existing bpf_hrtimer before
+	 * bpf_timer_cancel_and_free will have been cancelled.
+	 */
+	hrtimer_cancel(&t->timer);
+	kfree_rcu(t, cb.rcu);
+}
+
 static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u64 flags,
 			    enum bpf_async_type type)
 {
@@ -1262,6 +1281,8 @@ static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u
 		clockid = flags & (MAX_CLOCKS - 1);
 		t = (struct bpf_hrtimer *)cb;
 
+		atomic_set(&t->cancelling, 0);
+		INIT_WORK(&t->cb.delete_work, bpf_timer_delete_work);
 		hrtimer_init(&t->timer, clockid, HRTIMER_MODE_REL_SOFT);
 		t->timer.function = bpf_timer_cb;
 		cb->value = (void *)async - map->record->timer_off;
@@ -1440,7 +1461,8 @@ static void drop_prog_refcnt(struct bpf_async_cb *async)
 
 BPF_CALL_1(bpf_timer_cancel, struct bpf_async_kern *, timer)
 {
-	struct bpf_hrtimer *t;
+	struct bpf_hrtimer *t, *cur_t;
+	bool inc = false;
 	int ret = 0;
 
 	if (in_nmi())
@@ -1452,14 +1474,41 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_async_kern *, timer)
 		ret = -EINVAL;
 		goto out;
 	}
-	if (this_cpu_read(hrtimer_running) == t) {
+
+	cur_t = this_cpu_read(hrtimer_running);
+	if (cur_t == t) {
 		/* If bpf callback_fn is trying to bpf_timer_cancel()
 		 * its own timer the hrtimer_cancel() will deadlock
-		 * since it waits for callback_fn to finish
+		 * since it waits for callback_fn to finish.
 		 */
 		ret = -EDEADLK;
 		goto out;
 	}
+
+	/* Only account in-flight cancellations when invoked from a timer
+	 * callback, since we want to avoid waiting only if other _callbacks_
+	 * are waiting on us, to avoid introducing lockups. Non-callback paths
+	 * are ok, since nobody would synchronously wait for their completion.
+	 */
+	if (!cur_t)
+		goto drop;
+	atomic_inc(&t->cancelling);
+	/* Need full barrier after relaxed atomic_inc */
+	smp_mb__after_atomic();
+	inc = true;
+	if (atomic_read(&cur_t->cancelling)) {
+		/* We're cancelling timer t, while some other timer callback is
+		 * attempting to cancel us. In such a case, it might be possible
+		 * that timer t belongs to the other callback, or some other
+		 * callback waiting upon it (creating transitive dependencies
+		 * upon us), and we will enter a deadlock if we continue
+		 * cancelling and waiting for it synchronously, since it might
+		 * do the same. Bail!
+		 */
+		ret = -EDEADLK;
+		goto out;
+	}
+drop:
 	drop_prog_refcnt(&t->cb);
 out:
 	__bpf_spin_unlock_irqrestore(&timer->lock);
@@ -1467,6 +1516,8 @@ out:
 	 * if it was running.
 	 */
 	ret = ret ?: hrtimer_cancel(&t->timer);
+	if (inc)
+		atomic_dec(&t->cancelling);
 	rcu_read_unlock();
 	return ret;
 }
@@ -1512,25 +1563,39 @@ void bpf_timer_cancel_and_free(void *val)
 
 	if (!t)
 		return;
-	/* Cancel the timer and wait for callback to complete if it was running.
-	 * If hrtimer_cancel() can be safely called it's safe to call kfree(t)
-	 * right after for both preallocated and non-preallocated maps.
-	 * The async->cb = NULL was already done and no code path can
-	 * see address 't' anymore.
-	 *
-	 * Check that bpf_map_delete/update_elem() wasn't called from timer
-	 * callback_fn. In such case don't call hrtimer_cancel() (since it will
-	 * deadlock) and don't call hrtimer_try_to_cancel() (since it will just
-	 * return -1). Though callback_fn is still running on this cpu it's
+	/* We check that bpf_map_delete/update_elem() was called from timer
+	 * callback_fn. In such case we don't call hrtimer_cancel() (since it
+	 * will deadlock) and don't call hrtimer_try_to_cancel() (since it will
+	 * just return -1). Though callback_fn is still running on this cpu it's
 	 * safe to do kfree(t) because bpf_timer_cb() read everything it needed
 	 * from 't'. The bpf subprog callback_fn won't be able to access 't',
 	 * since async->cb = NULL was already done. The timer will be
 	 * effectively cancelled because bpf_timer_cb() will return
 	 * HRTIMER_NORESTART.
+	 *
+	 * However, it is possible the timer callback_fn calling us armed the
+	 * timer _before_ calling us, such that failing to cancel it here will
+	 * cause it to possibly use struct hrtimer after freeing bpf_hrtimer.
+	 * Therefore, we _need_ to cancel any outstanding timers before we do
+	 * kfree_rcu, even though no more timers can be armed.
+	 *
+	 * Moreover, we need to schedule work even if timer does not belong to
+	 * the calling callback_fn, as on two different CPUs, we can end up in a
+	 * situation where both sides run in parallel, try to cancel one
+	 * another, and we end up waiting on both sides in hrtimer_cancel
+	 * without making forward progress, since timer1 depends on time2
+	 * callback to finish, and vice versa.
+	 *
+	 *  CPU 1 (timer1_cb)			CPU 2 (timer2_cb)
+	 *  bpf_timer_cancel_and_free(timer2)	bpf_timer_cancel_and_free(timer1)
+	 *
+	 * To avoid these issues, punt to workqueue context when we are in a
+	 * timer callback.
 	 */
-	if (this_cpu_read(hrtimer_running) != t)
-		hrtimer_cancel(&t->timer);
-	kfree_rcu(t, cb.rcu);
+	if (this_cpu_read(hrtimer_running))
+		queue_work(system_unbound_wq, &t->cb.delete_work);
+	else
+		bpf_timer_delete_work(&t->cb.delete_work);
 }
 
 /* This function is called by map_delete/update_elem for individual element and

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

@@ -0,0 +1,91 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE
+#include <sched.h>
+#include <test_progs.h>
+#include <pthread.h>
+#include <network_helpers.h>
+
+#include "timer_lockup.skel.h"
+
+static long cpu;
+static int *timer1_err;
+static int *timer2_err;
+static bool skip;
+
+volatile int k = 0;
+
+static void *timer_lockup_thread(void *arg)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts,
+		.data_in	= &pkt_v4,
+		.data_size_in	= sizeof(pkt_v4),
+		.repeat		= 1000,
+	);
+	int i, prog_fd = *(int *)arg;
+	cpu_set_t cpuset;
+
+	CPU_ZERO(&cpuset);
+	CPU_SET(__sync_fetch_and_add(&cpu, 1), &cpuset);
+	ASSERT_OK(pthread_setaffinity_np(pthread_self(), sizeof(cpuset),
+				         &cpuset),
+		  "cpu affinity");
+
+	for (i = 0; !READ_ONCE(*timer1_err) && !READ_ONCE(*timer2_err); i++) {
+		bpf_prog_test_run_opts(prog_fd, &opts);
+		/* Skip the test if we can't reproduce the race in a reasonable
+		 * amount of time.
+		 */
+		if (i > 50) {
+			WRITE_ONCE(skip, true);
+			break;
+		}
+	}
+
+	return NULL;
+}
+
+void test_timer_lockup(void)
+{
+	int timer1_prog, timer2_prog;
+	struct timer_lockup *skel;
+	pthread_t thrds[2];
+	void *ret;
+
+	skel = timer_lockup__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "timer_lockup__open_and_load"))
+		return;
+
+	timer1_prog = bpf_program__fd(skel->progs.timer1_prog);
+	timer2_prog = bpf_program__fd(skel->progs.timer2_prog);
+
+	timer1_err = &skel->bss->timer1_err;
+	timer2_err = &skel->bss->timer2_err;
+
+	if (!ASSERT_OK(pthread_create(&thrds[0], NULL, timer_lockup_thread,
+				      &timer1_prog),
+		       "pthread_create thread1"))
+		goto out;
+	if (!ASSERT_OK(pthread_create(&thrds[1], NULL, timer_lockup_thread,
+				      &timer2_prog),
+		       "pthread_create thread2")) {
+		pthread_exit(&thrds[0]);
+		goto out;
+	}
+
+	pthread_join(thrds[1], &ret);
+	pthread_join(thrds[0], &ret);
+
+	if (skip) {
+		test__skip();
+		goto out;
+	}
+
+	if (*timer1_err != -EDEADLK && *timer1_err != 0)
+		ASSERT_FAIL("timer1_err bad value");
+	if (*timer2_err != -EDEADLK && *timer2_err != 0)
+		ASSERT_FAIL("timer2_err bad value");
+out:
+	timer_lockup__destroy(skel);
+	return;
+}

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

@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <time.h>
+#include <errno.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct elem {
+	struct bpf_timer t;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct elem);
+} timer1_map SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct elem);
+} timer2_map SEC(".maps");
+
+int timer1_err;
+int timer2_err;
+
+static int timer_cb1(void *map, int *k, struct elem *v)
+{
+	struct bpf_timer *timer;
+	int key = 0;
+
+	timer = bpf_map_lookup_elem(&timer2_map, &key);
+	if (timer)
+		timer2_err = bpf_timer_cancel(timer);
+
+	return 0;
+}
+
+static int timer_cb2(void *map, int *k, struct elem *v)
+{
+	struct bpf_timer *timer;
+	int key = 0;
+
+	timer = bpf_map_lookup_elem(&timer1_map, &key);
+	if (timer)
+		timer1_err = bpf_timer_cancel(timer);
+
+	return 0;
+}
+
+SEC("tc")
+int timer1_prog(void *ctx)
+{
+	struct bpf_timer *timer;
+	int key = 0;
+
+	timer = bpf_map_lookup_elem(&timer1_map, &key);
+	if (timer) {
+		bpf_timer_init(timer, &timer1_map, CLOCK_BOOTTIME);
+		bpf_timer_set_callback(timer, timer_cb1);
+		bpf_timer_start(timer, 1, BPF_F_TIMER_CPU_PIN);
+	}
+
+	return 0;
+}
+
+SEC("tc")
+int timer2_prog(void *ctx)
+{
+	struct bpf_timer *timer;
+	int key = 0;
+
+	timer = bpf_map_lookup_elem(&timer2_map, &key);
+	if (timer) {
+		bpf_timer_init(timer, &timer2_map, CLOCK_BOOTTIME);
+		bpf_timer_set_callback(timer, timer_cb2);
+		bpf_timer_start(timer, 1, BPF_F_TIMER_CPU_PIN);
+	}
+
+	return 0;
+}