On Thu, 25 Jul 2024 21:45:03 +0530 Ajay Kaher <ajay.ka...@broadcom.com> wrote:
> On Mon, Jul 22, 2024 at 5:38 PM Mathias Krause <mini...@grsecurity.net> wrote: > > > > On 22.07.24 13:13, Ajay Kaher wrote: > > > On Sat, Jul 20, 2024 at 2:17 AM Mathias Krause <mini...@grsecurity.net> > > > wrote: > > >> > > >> I noticed, the user events ftrace selftest is crashing every now and > > >> then in our automated tests. Digging into, I found that the following > > >> is triggering the issue very reliable: > > >> > > >> - in one shell, as root: > > >> # while true; do ./kselftest/user_events/ftrace_test; done > > >> > > >> - in a second shell, again as root: > > >> # cd /sys/kernel/tracing > > >> # while true; do cat events/user_events/__test_event/format; done > > >> 2>/dev/null > > >> > > > > > > Tried to reproduced on 6.10.0-rc7-100.ph5+, only getting repeated output > > > as: > > < sending again after correcting alignments > > > Mathias, thanks for reporting. I am able to reproduce the 'KASAN: > slab-use-after-free'. > > Steve, let me know if anything wrong in my investigation: Hi Ajay, Thanks for analyzing this. > > [ 6264.339882] > ================================================================== > [ 6264.339970] BUG: KASAN: slab-use-after-free in f_start+0x2b5/0x370 > > This belongs to f_start() -> f_next() -> trace_get_fields(): > > trace_get_fields(struct trace_event_call *event_call) > { > if (!event_call->class->get_fields) > return &event_call->class->fields; > return event_call->class->get_fields(event_call); > } > > This happens while reading 'events/user_events/__test_event/format'. > > > Allocation: > [ 6264.347212] Allocated by task 3287: > [ 6264.348247] kasan_save_stack+0x26/0x50 > [ 6264.348256] kasan_save_track+0x14/0x40 > [ 6264.348260] kasan_save_alloc_info+0x37/0x50 > [ 6264.348265] __kasan_kmalloc+0xb3/0xc0 > [ 6264.348268] kmalloc_trace_noprof+0x168/0x330 > [ 6264.348280] user_event_parse_cmd+0x57b/0x26c0 > [ 6264.348286] user_events_ioctl+0xa92/0x1850 > [ 6264.348290] __x64_sys_ioctl+0x138/0x1b0 > [ 6264.348295] x64_sys_call+0x9a4/0x1f20 > [ 6264.348299] do_syscall_64+0x4b/0x110 > > user_event_parse_cmd() -> user_event_parse() { > . > user = kzalloc(sizeof(*user), GFP_KERNEL_ACCOUNT); > > Link: > https://elixir.bootlin.com/linux/v6.10/source/kernel/trace/trace_events_user.c#L2118 > > > Freed: > [ 6264.350333] kfree+0xd1/0x2b0 > [ 6264.350337] destroy_user_event.part.0+0x313/0x450 > [ 6264.350341] destroy_user_event+0x129/0x1a0 > [ 6264.350344] delayed_destroy_user_event+0x62/0xd0 > [ 6264.350347] process_one_work+0x621/0xf60 > [ 6264.350359] worker_thread+0x760/0x14f0 > > static int destroy_user_event(struct user_event *user) { > . > kfree(user->call.print_fmt); > kfree(EVENT_NAME(user)); > kfree(user); <-- > > Link: > https://elixir.bootlin.com/linux/v6.10/source/kernel/trace/trace_events_user.c#L1510 > > > Race condition: > > Thread A i.e. event reader able to reach the f_start() as the path is > valid. Thread A waiting for lock. At the sametime, Thread B has > acquired lock and removing events entry followed by free the > user_event object. Later once Thread A got the lock it tried to read > address which belongs to struct trace_event_call (struct > trace_event_call is member of struct user_event) > > Thread A (read event) Thread B (remove event) > > . worker_thread() > . > delayed_destroy_user_event() > . -> > acquire event_mutex > . destroy_user_event() > vfs_read() . > seq_read() . > f_start() -> acquire event_mutex eventfs_remove_dir() > . (waiting) kfree(user) > . (waiting) -> released event_mutex > acquired event_mutex > f_next() > trace_get_fields(): What really bothers me is that refcnt logic. I'm not sure if this is an issue, but the fact that you can inc the refcnt without holding the event_mutex looks wrong to me. I would guess it would WARN if that refcnt was incremented when zero, but there is a window where it gets set to 1 again. Too bad there's not a way to do a refcnt_set_if_zero() or something to atomically set the value but warn if it's not zero. But then again, if it did get incremented when zero, there should have been a warning then too. But I don't think that's causing this. Will look further. -- Steve > > I think you have added the following check in f_start() to prevent > this race condition, but somehow with eventfs still some gap to race > condition. > > static void *f_start(struct seq_file *m, loff_t *pos) { > mutex_lock(&event_mutex); > if (!event_file_data(m->private)) <-- > return ERR_PTR(-ENODEV); > > -Ajay