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:
>
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:
[ 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():
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
