From af253aef183a31ce62d2e39fc520b0ebfb562bb9 Mon Sep 17 00:00:00 2001 From: Mohammad Shehar Yaar Tausif Date: Wed, 10 Jul 2024 12:05:22 +0200 Subject: [PATCH 1/4] bpf: fix order of args in call to bpf_map_kvcalloc The original function call passed size of smap->bucket before the number of buckets which raises the error 'calloc-transposed-args' on compilation. Vlastimil Babka added: The order of parameters can be traced back all the way to 6ac99e8f23d4 ("bpf: Introduce bpf sk local storage") accross several refactorings, and that's why the commit is used as a Fixes: tag. In v6.10-rc1, a different commit 2c321f3f70bc ("mm: change inlined allocation helpers to account at the call site") however exposed the order of args in a way that gcc-14 has enough visibility to start warning about it, because (in !CONFIG_MEMCG case) bpf_map_kvcalloc is then a macro alias for kvcalloc instead of a static inline wrapper. To sum up the warning happens when the following conditions are all met: - gcc-14 is used (didn't see it with gcc-13) - commit 2c321f3f70bc is present - CONFIG_MEMCG is not enabled in .config - CONFIG_WERROR turns this from a compiler warning to error Fixes: 6ac99e8f23d4 ("bpf: Introduce bpf sk local storage") Reviewed-by: Andrii Nakryiko Tested-by: Christian Kujau Signed-off-by: Mohammad Shehar Yaar Tausif Signed-off-by: Vlastimil Babka Link: https://lore.kernel.org/r/20240710100521.15061-2-vbabka@suse.cz Signed-off-by: Alexei Starovoitov --- kernel/bpf/bpf_local_storage.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c index 976cb258a0ed..c938dea5ddbf 100644 --- a/kernel/bpf/bpf_local_storage.c +++ b/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; From d4523831f07a267a943f0dde844bf8ead7495f13 Mon Sep 17 00:00:00 2001 From: Kumar Kartikeya Dwivedi Date: Tue, 9 Jul 2024 18:54:38 +0000 Subject: [PATCH 2/4] bpf: Fail bpf_timer_cancel when callback is being cancelled Given a schedule: timer1 cb timer2 cb bpf_timer_cancel(timer2); bpf_timer_cancel(timer1); Both bpf_timer_cancel calls would wait for the other callback to finish executing, introducing a lockup. Add an atomic_t count named 'cancelling' in bpf_hrtimer. This keeps track of all in-flight cancellation requests for a given BPF timer. Whenever cancelling a BPF timer, we must check if we have outstanding cancellation requests, and if so, we must fail the operation with an error (-EDEADLK) since cancellation is synchronous and waits for the callback to finish executing. This implies that we can enter a deadlock situation involving two or more timer callbacks executing in parallel and attempting to cancel one another. Note that we avoid incrementing the cancelling counter for the target timer (the one being cancelled) if bpf_timer_cancel is not invoked from a callback, to avoid spurious errors. The whole point of detecting cur->cancelling and returning -EDEADLK is to not enter a busy wait loop (which may or may not lead to a lockup). This does not apply in case the caller is in a non-callback context, the other side can continue to cancel as it sees fit without running into errors. Background on prior attempts: Earlier versions of this patch used a bool 'cancelling' bit and used the following pattern under timer->lock to publish cancellation status. lock(t->lock); t->cancelling = true; mb(); if (cur->cancelling) return -EDEADLK; unlock(t->lock); hrtimer_cancel(t->timer); t->cancelling = false; The store outside the critical section could overwrite a parallel requests t->cancelling assignment to true, to ensure the parallely executing callback observes its cancellation status. It would be necessary to clear this cancelling bit once hrtimer_cancel is done, but lack of serialization introduced races. Another option was explored where bpf_timer_start would clear the bit when (re)starting the timer under timer->lock. This would ensure serialized access to the cancelling bit, but may allow it to be cleared before in-flight hrtimer_cancel has finished executing, such that lockups can occur again. Thus, we choose an atomic counter to keep track of all outstanding cancellation requests and use it to prevent lockups in case callbacks attempt to cancel each other while executing in parallel. Reported-by: Dohyun Kim Reported-by: Neel Natu Fixes: b00628b1c7d5 ("bpf: Introduce bpf timers.") Signed-off-by: Kumar Kartikeya Dwivedi Link: https://lore.kernel.org/r/20240709185440.1104957-2-memxor@gmail.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/helpers.c | 38 +++++++++++++++++++++++++++++++++++--- 1 file changed, 35 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 2a69a9a36c0f..22e779ca50d5 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -1107,6 +1107,7 @@ struct bpf_async_cb { struct bpf_hrtimer { struct bpf_async_cb cb; struct hrtimer timer; + atomic_t cancelling; }; struct bpf_work { @@ -1262,6 +1263,7 @@ 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); 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 +1442,8 @@ 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 +1455,41 @@ 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 +1497,8 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_async_kern *, timer) * if it was running. */ ret = ret ?: hrtimer_cancel(&t->timer); + if (inc) + atomic_dec(&t->cancelling); rcu_read_unlock(); return ret; } From a6fcd19d7eac1335eb76bc16b6a66b7f574d1d69 Mon Sep 17 00:00:00 2001 From: Kumar Kartikeya Dwivedi Date: Tue, 9 Jul 2024 18:54:39 +0000 Subject: [PATCH 3/4] bpf: Defer work in bpf_timer_cancel_and_free Currently, the same case as previous patch (two timer callbacks trying to cancel each other) can be invoked through bpf_map_update_elem as well, or more precisely, freeing map elements containing timers. Since this relies on hrtimer_cancel as well, it is prone to the same deadlock situation as the previous patch. It would be sufficient to use hrtimer_try_to_cancel to fix this problem, as the timer cannot be enqueued after async_cancel_and_free. Once async_cancel_and_free has been done, the timer must be reinitialized before it can be armed again. The callback running in parallel trying to arm the timer will fail, and freeing bpf_hrtimer without waiting is sufficient (given kfree_rcu), and bpf_timer_cb will return HRTIMER_NORESTART, preventing the timer from being rearmed again. However, there exists a UAF scenario where the callback arms the timer before entering this function, such that if cancellation fails (due to timer callback invoking this routine, or the target timer callback running concurrently). In such a case, if the timer expiration is significantly far in the future, the RCU grace period expiration happening before it will free the bpf_hrtimer state and along with it the struct hrtimer, that is enqueued. Hence, it is clear cancellation needs to occur after async_cancel_and_free, and yet it cannot be done inline due to deadlock issues. We thus modify bpf_timer_cancel_and_free to defer work to the global workqueue, adding a work_struct alongside rcu_head (both used at _different_ points of time, so can share space). Update existing code comments to reflect the new state of affairs. Fixes: b00628b1c7d5 ("bpf: Introduce bpf timers.") Signed-off-by: Kumar Kartikeya Dwivedi Link: https://lore.kernel.org/r/20240709185440.1104957-3-memxor@gmail.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/helpers.c | 61 ++++++++++++++++++++++++++++++++++---------- 1 file changed, 47 insertions(+), 14 deletions(-) diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 22e779ca50d5..3243c83ef3e3 100644 --- a/kernel/bpf/helpers.c +++ b/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; }; @@ -1220,6 +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) { @@ -1264,6 +1282,7 @@ static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u 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; @@ -1544,25 +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 From 50bd5a0c658d132507673c4d59347c025dd149ed Mon Sep 17 00:00:00 2001 From: Kumar Kartikeya Dwivedi Date: Thu, 11 Jul 2024 05:27:09 +0000 Subject: [PATCH 4/4] selftests/bpf: Add timer lockup selftest Add a selftest that tries to trigger a situation where two timer callbacks are attempting to cancel each other's timer. By running them continuously, we hit a condition where both run in parallel and cancel each other. Without the fix in the previous patch, this would cause a lockup as hrtimer_cancel on either side will wait for forward progress from the callback. Ensure that this situation leads to a EDEADLK error. Signed-off-by: Kumar Kartikeya Dwivedi Signed-off-by: Daniel Borkmann Link: https://lore.kernel.org/bpf/20240711052709.2148616-1-memxor@gmail.com --- .../selftests/bpf/prog_tests/timer_lockup.c | 91 +++++++++++++++++++ .../selftests/bpf/progs/timer_lockup.c | 87 ++++++++++++++++++ 2 files changed, 178 insertions(+) create mode 100644 tools/testing/selftests/bpf/prog_tests/timer_lockup.c create mode 100644 tools/testing/selftests/bpf/progs/timer_lockup.c diff --git a/tools/testing/selftests/bpf/prog_tests/timer_lockup.c b/tools/testing/selftests/bpf/prog_tests/timer_lockup.c new file mode 100644 index 000000000000..871d16cb95cf --- /dev/null +++ b/tools/testing/selftests/bpf/prog_tests/timer_lockup.c @@ -0,0 +1,91 @@ +// SPDX-License-Identifier: GPL-2.0 + +#define _GNU_SOURCE +#include +#include +#include +#include + +#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; +} diff --git a/tools/testing/selftests/bpf/progs/timer_lockup.c b/tools/testing/selftests/bpf/progs/timer_lockup.c new file mode 100644 index 000000000000..3e520133281e --- /dev/null +++ b/tools/testing/selftests/bpf/progs/timer_lockup.c @@ -0,0 +1,87 @@ +// SPDX-License-Identifier: GPL-2.0 + +#include +#include +#include +#include +#include +#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; +}