another use-after-free in ep_remove_wait_queue()

From: Munehisa Kamata
Date: Fri Jan 06 2023 - 17:49:38 EST

Hi,

We found the following use-after-free with userspace code polling on
a pressure file in a non-root cgroup using epoll.

[ 57.183661] BUG: KASAN: use-after-free in _raw_spin_lock_irqsave+0x76/0x130
[ 57.186662] Write of size 4 at addr ffff888114976428 by task a.out/2426

[ 57.190551] CPU: 0 PID: 2426 Comm: a.out Not tainted 6.2.0-rc2+ #16
[ 57.193384] Hardware name: Amazon EC2 c5a.large/, BIOS 1.0 10/16/2017
[ 57.196272] Call Trace:
[ 57.197565] <TASK>
[ 57.198714] dump_stack_lvl+0x8f/0xc0
[ 57.200494] print_report+0x16c/0x4e0
[ 57.202084] ? _raw_spin_lock_irqsave+0x76/0x130
[ 57.204077] kasan_report+0xc3/0xf0
[ 57.205587] ? entry_SYSCALL_64_after_hwframe+0x63/0xcd
[ 57.207760] ? _raw_spin_lock_irqsave+0x76/0x130
[ 57.209685] kasan_check_range+0x2d2/0x310
[ 57.211477] _raw_spin_lock_irqsave+0x76/0x130
[ 57.213355] remove_wait_queue+0x25/0x130
[ 57.215102] ep_free+0x12d/0x220
[ 57.216506] ep_eventpoll_release+0x3c/0x40
[ 57.218254] __fput+0x32b/0x700
[ 57.221486] task_work_run+0x1db/0x230
[ 57.224885] exit_to_user_mode_prepare+0xfd/0x100
[ 57.228662] syscall_exit_to_user_mode+0x20/0x40
[ 57.232360] do_syscall_64+0x52/0x90
[ 57.235691] entry_SYSCALL_64_after_hwframe+0x63/0xcd
[ 57.239572] RIP: 0033:0x7fadf96e1c44
[ 57.242865] Code: 00 00 b8 ff ff ff ff eb 9c b8 ff ff ff ff eb 95 e8 01 e2 01 00 90 8b 05 2a ac 2c 00 48 63 ff 85 c0 75 11 b8 03 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 3a f3 c3 48 83 ec 18 48 89 7c 24 08 e8 e4 a0
[ 57.255244] RSP: 002b:00007ffd1d1b7b98 EFLAGS: 00000246 ORIG_RAX: 0000000000000003
[ 57.261714] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007fadf96e1c44
[ 57.266293] RDX: 0000000000000000 RSI: 00007ffd1d1b7b60 RDI: 0000000000000004
[ 57.270979] RBP: 00007ffd1d1b7bf0 R08: 00000000004007e0 R09: 00007fadf9a0f240
[ 57.275856] R10: 00000000000006ba R11: 0000000000000246 R12: 00000000004005e0
[ 57.280478] R13: 00007ffd1d1b7cd0 R14: 0000000000000000 R15: 0000000000000000
[ 57.285059] </TASK>

[ 57.290402] Allocated by task 2426:
[ 57.293705] kasan_set_track+0x3d/0x60
[ 57.297102] __kasan_kmalloc+0x85/0x90
[ 57.300491] psi_trigger_create+0x155/0x850
[ 57.304040] pressure_write+0x200/0x510
[ 57.307508] cgroup_file_write+0x1de/0x3e0
[ 57.310949] kernfs_fop_write_iter+0x27d/0x380
[ 57.314601] vfs_write+0x7d7/0xaa0
[ 57.317891] ksys_write+0xd7/0x1a0
[ 57.321152] do_syscall_64+0x43/0x90
[ 57.324496] entry_SYSCALL_64_after_hwframe+0x63/0xcd

[ 57.330887] Freed by task 2429:
[ 57.334053] kasan_set_track+0x3d/0x60
[ 57.337448] kasan_save_free_info+0x27/0x40
[ 57.340977] ____kasan_slab_free+0x11d/0x170
[ 57.344570] slab_free_freelist_hook+0x87/0x150
[ 57.348236] __kmem_cache_free+0xcb/0x180
[ 57.351710] psi_trigger_destroy+0x458/0x550
[ 57.355274] cgroup_file_release+0x96/0x110
[ 57.358779] kernfs_drain_open_files+0x238/0x420
[ 57.362519] kernfs_drain+0x191/0x2a0
[ 57.365901] __kernfs_remove+0x3a6/0x600
[ 57.369363] kernfs_remove_by_name_ns+0xc2/0x120
[ 57.373073] cgroup_addrm_files+0x90f/0xcf0
[ 57.376610] cgroup_destroy_locked+0x48a/0x730
[ 57.380260] cgroup_rmdir+0x2b/0x130
[ 57.383650] kernfs_iop_rmdir+0x17a/0x230
[ 57.387201] vfs_rmdir+0x196/0x410
[ 57.390442] do_rmdir+0x1c7/0x3f0
[ 57.393651] __x64_sys_rmdir+0x45/0x50
[ 57.397000] do_syscall_64+0x43/0x90
[ 57.400340] entry_SYSCALL_64_after_hwframe+0x63/0xcd

[ 57.406689] The buggy address belongs to the object at ffff888114976400
which belongs to the cache kmalloc-128 of size 128
[ 57.414907] The buggy address is located 40 bytes inside of
128-byte region [ffff888114976400, ffff888114976480)

[ 57.425474] The buggy address belongs to the physical page:
[ 57.429541] page:000000008c5ecb31 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x114976
[ 57.436725] flags: 0x2fffff80000200(slab|node=0|zone=2|lastcpupid=0x1fffff)
[ 57.441235] raw: 002fffff80000200 ffff8881000418c0 dead000000000100 dead000000000122
[ 57.447793] raw: 0000000000000000 0000000000100010 00000001ffffffff 0000000000000000
[ 57.454274] page dumped because: kasan: bad access detected

[ 57.460990] Memory state around the buggy address:
[ 57.464744] ffff888114976300: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[ 57.471126] ffff888114976380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 57.477447] >ffff888114976400: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[ 57.483833] ^
[ 57.487541] ffff888114976480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 57.493976] ffff888114976500: 00 00 00 00 00 00 00 00 00 00 00 00 00 01 fc fc

Here is the simple repro.

#include <fcntl.h>
#include <sys/epoll.h>
#include <unistd.h>

int main(void)
{
const char trigger_str[] = "some 100000 1000000";
int fd, epfd;
struct epoll_event event;
struct epoll_event events[1];

fd = open("/cgroup2/test/cpu.pressure", O_RDWR);
write(fd, trigger_str, sizeof(trigger_str));
epfd = epoll_create(1);
event.events = EPOLLPRI | EPOLLET;
event.data.fd = fd;
epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &event);
epoll_wait(epfd, events, 1, -1); /* returns after rmdir */
sleep(5);
close(epfd);
close(fd);

return 0;
}

# mkdir /cgroup2
# mount -t cgroup2 none /cgroup2
# mkdir /cgroup2/test
# ./a.out &
# rmdir /cgroup2/test

Looks like calling wake_up_pollfree() in psi_trigger_destroy() can properly
clear the queue and then avoid this use-after-free, but POLLFREE wasn't
considered enough there for the past similar issue[1]. While
wake_up_pollfree() could *also* be called in psi_trigger_destroy(), it may
be awkward and there can be more appropriate solution. It would be great if
experts could have a look.

[1] https://lkml.org/lkml/2021/11/16/264


Regards,
Munehisa