Hello,
We are developing a tool to perform static analysis on the bpf
subsystem to detect locking violations. Our tool reported the
raw_spin_lock_irqsave() in bpf_percpu_lru_pop_free(). This function is
used by htab_percpu_lru_map_update_elem() which can be called from an
NMI. A deadlock can happen if a bpf program holding the lock is
interrupted by the same program in NMI. The report was generated for
kernel version 6.6-rc4, however, we believe this should still exist in
the latest kernel.
We tried to validate the report on v6.7 by running a PoC. Below is the
lockdep splat. The PoC is attached at the end.
Thanks,
Priya
[ 1051.101034] ================================
[ 1051.101037] WARNING: inconsistent lock state
[ 1051.101040] 6.7.0-dirty #14 Not tainted
[ 1051.101048] --------------------------------
[ 1051.101051] inconsistent {INITIAL USE} -> {IN-NMI} usage.
[ 1051.101056] lru_percpu_perf/1263 [HC1[1]:SC0[0]:HE0:SE1] takes:
[ 1051.101071] ffffe8fffc22cdd8 (&l->lock){....}-{2:2}, at:
bpf_lru_pop_free+0xc1/0x13a0
[ 1051.101129] {INITIAL USE} state was registered at:
[ 1051.101134] lock_acquire+0x193/0x4c0
[ 1051.101153] _raw_spin_lock_irqsave+0x3f/0x90
[ 1051.101167] bpf_lru_pop_free+0xc1/0x13a0
[ 1051.101179] __htab_lru_percpu_map_update_elem+0x177/0xa20
[ 1051.101197] bpf_prog_47d4157ca618f90f_lru_tp+0x61/0x8a
[ 1051.101215] trace_call_bpf+0x273/0x920
[ 1051.101229] perf_trace_run_bpf_submit+0x8f/0x1c0
[ 1051.101243] perf_trace_sched_switch+0x5c9/0x9c0
[ 1051.101255] __traceiter_sched_switch+0x6f/0xc0
[ 1051.101268] __schedule+0xae0/0x2ae0
[ 1051.101282] schedule+0xe6/0x270
[ 1051.101295] exit_to_user_mode_prepare+0x97/0x190
[ 1051.101314] irqentry_exit_to_user_mode+0xa/0x30
[ 1051.101331] asm_sysvec_apic_timer_interrupt+0x1a/0x20
[ 1051.101344] irq event stamp: 39528
[ 1051.101348] hardirqs last enabled at (39527): [<ffffffff8480144a>]
asm_sysvec_apic_timer_interrupt+0x1a/0x20
[ 1051.101365] hardirqs last disabled at (39528): [<ffffffff8478ca89>]
exc_nmi+0x159/0x200
[ 1051.101380] softirqs last enabled at (39526): [<ffffffff847b5541>]
__do_softirq+0x4e1/0x73e
[ 1051.101399] softirqs last disabled at (39519): [<ffffffff811ab473>]
irq_exit_rcu+0x93/0xc0
[ 1051.101415]
[ 1051.101415] other info that might help us debug this:
[ 1051.101418] Possible unsafe locking scenario:
[ 1051.101418]
[ 1051.101420] CPU0
[ 1051.101422] ----
[ 1051.101424] lock(&l->lock);
[ 1051.101430] <Interrupt>
[ 1051.101432] lock(&l->lock);
[ 1051.101438]
[ 1051.101438] *** DEADLOCK ***
[ 1051.101438]
[ 1051.101440] no locks held by lru_percpu_perf/1263.
[ 1051.101446]
[ 1051.101446] stack backtrace:
[ 1051.101452] CPU: 1 PID: 1263 Comm: lru_percpu_perf Not tainted
6.7.0-dirty #14
[ 1051.101466] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.13.0-1ubuntu1.1 04/01/2014
[ 1051.101474] Call Trace:
[ 1051.101482] <TASK>
[ 1051.101488] dump_stack_lvl+0x91/0xf0
[ 1051.101514] lock_acquire+0x35b/0x4c0
[ 1051.101534] ? __pfx_lock_acquire+0x10/0x10
[ 1051.101553] ? bpf_lru_pop_free+0xc1/0x13a0
[ 1051.101568] ? trace_event_raw_event_bpf_trace_printk+0x14a/0x210
[ 1051.101584] ? __pfx_trace_event_raw_event_bpf_trace_printk+0x10/0x10
[ 1051.101599] ? bstr_printf+0x348/0xf40
[ 1051.101621] _raw_spin_lock_irqsave+0x3f/0x90
[ 1051.101636] ? bpf_lru_pop_free+0xc1/0x13a0
[ 1051.101650] bpf_lru_pop_free+0xc1/0x13a0
[ 1051.101666] ? trace_bpf_trace_printk+0x11d/0x140
[ 1051.101679] ? bpf_bprintf_cleanup+0x66/0xd0
[ 1051.101693] ? htab_map_hash+0x18e/0x880
[ 1051.101709] __htab_lru_percpu_map_update_elem+0x177/0xa20
[ 1051.101734] bpf_prog_af2271334a1c4e36_lru_percpu_perf+0x5d/0x61
[ 1051.101750] bpf_overflow_handler+0x184/0x4a0
[ 1051.101765] ? __pfx_bpf_overflow_handler+0x10/0x10
[ 1051.101786] __perf_event_overflow+0x4c2/0x9e0
[ 1051.101806] handle_pmi_common+0x4d7/0x800
[ 1051.101823] ? __lock_acquire+0x150a/0x3b10
[ 1051.101845] ? __pfx_handle_pmi_common+0x10/0x10
[ 1051.101867] ? hlock_class+0x4e/0x140
[ 1051.101881] ? lock_release+0x587/0xaa0
[ 1051.101901] ? __pfx_lock_release+0x10/0x10
[ 1051.101920] ? lock_is_held_type+0xa1/0x120
[ 1051.101939] ? rcu_gpnum_ovf+0x12d/0x180
[ 1051.101958] ? lockdep_hardirqs_on_prepare+0x12d/0x400
[ 1051.101980] ? look_up_lock_class+0x56/0x140
[ 1051.101998] ? lock_acquire+0x272/0x4c0
[ 1051.102016] ? intel_bts_interrupt+0x115/0x3e0
[ 1051.102036] intel_pmu_handle_irq+0x246/0xd90
[ 1051.102058] perf_event_nmi_handler+0x4c/0x70
[ 1051.102073] nmi_handle+0x1a6/0x520
[ 1051.102096] default_do_nmi+0x64/0x1c0
[ 1051.102112] exc_nmi+0x187/0x200
[ 1051.102126] asm_exc_nmi+0xb6/0xff
[ 1051.102139] RIP: 0033:0x555fd93d960b
[ 1051.102150] Code: ff ff ff ff 48 8b 05 54 9a 04 00 48 89 c1 ba 2f
00 00 00 be 01 00 00 00 48 8d 05 f8 1b 03 00 48 89 c7 e8 d8 f6 ff ff
eb 10 90 <0f> b6 05 37 9a 04 00 83 f0 01 84 c0 75 f2 90 48 83 bd 08 fe
ff ff
[ 1051.102162] RSP: 002b:00007ffe81a6f6e0 EFLAGS: 00000202
[ 1051.102178] RAX: 0000000000000001 RBX: 0000555fdac9eee8 RCX: 0000000000000000
[ 1051.102187] RDX: 0000000555fdac9e RSI: 0000555fdac9d010 RDI: 0000000000000007
[ 1051.102196] RBP: 00007ffe81a6f910 R08: 0000555fdac9ee10 R09: 00000004dac9d2e0
[ 1051.102204] R10: 0000000000000000 R11: 08e02bbb6d91ca99 R12: 00007ffe81a6fa28
[ 1051.102213] R13: 0000555fd93d9084 R14: 0000555fd94209d8 R15: 00007fad2f48a040
[ 1051.102231] </TASK>
The lockdep warning can be triggered using the following user and bpf programs.
================================
#include <unistd.h>
#include <sys/syscall.h>
#include <linux/perf_event.h>
#include <bpf/libbpf.h>
#include <bpf/bpf.h>
#include <sys/resource.h>
#include <signal.h>
#include "lru_percpu_perf.skel.h"
static volatile bool exiting = false;
static void sig_handler(int sig)
{
exiting = true;
return;
}
extern int parse_cpu_mask_file(const char *fcpu, bool **mask, int *mask_sz);
static long perf_event_open(struct perf_event_attr *hw_event, pid_t
pid, int cpu, int group_fd,
unsigned long flags)
{
int ret;
ret = syscall(__NR_perf_event_open, hw_event, pid, cpu,
group_fd, flags);
return ret;
}
void bump_memlock_rlimit(void)
{
struct rlimit rlim_new = {
.rlim_cur = RLIM_INFINITY,
.rlim_max = RLIM_INFINITY,
};
if (setrlimit(RLIMIT_MEMLOCK, &rlim_new)) {
fprintf(stderr, "Failed to increase RLIMIT_MEMLOCK limit!\n");
exit(1);
}
return;
}
int main(int argc, char *const argv[])
{
const char *online_cpus_file = "/sys/devices/system/cpu/online";
int cpu;
struct lru_percpu_perf_bpf *skel = NULL;
struct perf_event_attr attr;
struct bpf_link **links = NULL;
int num_cpus, num_online_cpus;
int *pefds = NULL, pefd;
int i, err = 0;
bool *online_mask = NULL;
struct bpf_program *prog;
struct bpf_object *obj;
struct bpf_map *map;
char filename[256];
bump_memlock_rlimit();
signal(SIGINT, sig_handler);
signal(SIGTERM, sig_handler);
err = parse_cpu_mask_file(online_cpus_file, &online_mask,
&num_online_cpus);
if (err) {
fprintf(stderr, "Fail to get online CPU numbers: %d\n", err);
goto cleanup;
}
num_cpus = libbpf_num_possible_cpus();
if (num_cpus <= 0) {
fprintf(stderr, "Fail to get the number of processors\n");
err = -1;
goto cleanup;
}
snprintf(filename, sizeof(filename), ".output/lru_percpu_perf.bpf.o");
obj = bpf_object__open_file(filename, NULL);
if (libbpf_get_error(obj)) {
fprintf(stderr, "ERROR: opening BPF object file failed\n");
goto cleanup;
}
map = bpf_object__find_map_by_name(obj, "pb");
if (libbpf_get_error(map)) {
fprintf(stderr, "ERROR: finding a map in obj file failed\n");
goto cleanup;
}
if (bpf_object__load(obj)) {
fprintf(stderr, "ERROR: loading BPF object file failed\n");
goto cleanup;
}
pefds = malloc(num_cpus * sizeof(int));
for (i = 0; i < num_cpus; i++) {
pefds[i] = -1;
}
links = calloc(num_cpus, sizeof(struct bpf_link *));
memset(&attr, 0, sizeof(attr));
attr.type = PERF_TYPE_HARDWARE;
attr.config = PERF_COUNT_HW_CPU_CYCLES;
attr.sample_freq = 10;
attr.inherit = 1;
attr.freq = 1;
for (cpu = 0; cpu < 2; cpu++) {
//skip offline/not present CPUs
if (cpu >= num_online_cpus || !online_mask[cpu])
continue;
// Set up performance monitoring on a CPU/Core
pefd = perf_event_open(&attr, 0, -1, -1, 0);
if (pefd < 0) {
fprintf(stderr, "Fail to set up performance
monitor on a CPU/Core\n");
err = -1;
goto cleanup;
}
pefds[cpu] = pefd;
prog = bpf_object__find_program_by_name(obj, "lru_percpu_perf");
if (!prog) {
fprintf(stderr, "ERROR: finding a prog in obj
file failed\n");
goto cleanup;
}
links[cpu] = bpf_program__attach_perf_event(prog, pefds[cpu]);
if (!links[cpu]) {
err = -1;
fprintf(stderr, "ERROR: bpf_program__attach failed\n");
goto cleanup;
}
}
while(!exiting){
}
cleanup:
if (links) {
for (cpu = 0; cpu < num_cpus; cpu++)
bpf_link__destroy(links[cpu]);
free(links);
}
if (pefds) {
for (i = 0; i < num_cpus; i++) {
if (pefds[i] >= 0)
close(pefds[i]);
}
free(pefds);
}
lru_percpu_perf_bpf__destroy(skel);
free(online_mask);
return -err;
}
==============================
#include "vmlinux.h"
#include <linux/version.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>
#include <bpf/bpf_core_read.h>
struct {
__uint(type, BPF_MAP_TYPE_LRU_PERCPU_HASH);
__type(key, int);
__type(value,int);
__uint(max_entries, 255);
__uint(map_flags, 2);
} pb SEC(".maps");
SEC("perf_event")
int lru_percpu_perf(void *ctx)
{
int key = 2;
int init_val = 1;
long *value;
int i;
bpf_map_update_elem(&pb, &key, &init_val, BPF_ANY);
value = bpf_map_lookup_elem(&pb, &key);
int ret = bpf_map_delete_elem(&pb, &key);
return 0;
}
char _license[] SEC("license") = "GPL";
