Re: [PATCH v2] sched/psi: fix use-after-free in ep_remove_wait_queue()

[Suren Baghdasaryan <surenb@google.com>]

Thanks!
Overall LGTM, just a couple of nits (simplifications):

On Mon, Feb 13, 2023 at 11:04 PM Munehisa Kamata <kamatam@amazon.com> wrote:
>
> If a non-root cgroup gets removed when there is a thread that registered
> trigger and is polling on a pressure file within the cgroup, the polling
> waitqueue gets freed without clearing the queue and reference in the
> following path.

Let's remove "without clearing the queue and reference" in the above
sentence. The next section explains why this is problematic, therefore
mentioning that here is unnecessary IMHO.

>
>  do_rmdir
>    cgroup_rmdir
>      kernfs_drain_open_files
>        cgroup_file_release
>          cgroup_pressure_release
>            psi_trigger_destroy
>
> However, the polling thread can keep having the last reference to the
> pressure file that is tied to the freed waitqueue until explicit close or
> exit later.

Suggest replacing: However, the polling thread still has a reference
to the pressure file it is polling and will access the freed waitqueue
when file is closed or upon exit:

>
>  fput
>    ep_eventpoll_release
>      ep_free
>        ep_remove_wait_queue
>          remove_wait_queue
>
> Then, the thread accesses to the already-freed waitqueue when dropping the
> reference and results in use-after-free as pasted below.

Suggest replacing: This results is use-after-free as pasted below.

>
> The fundamental problem here is that the lifetime of the waitqueue is not
> tied to the file's real lifetime as shown above.

The fundamental problem here is that cgroup_file_release() (and
consequently waitqueue's lifetime) is not tied to the file's real
lifetime.

> Using wake_up_pollfree()
> here might be less than ideal, but it also is not fully contradicting the
> comment at commit 42288cb44c4b ("wait: add wake_up_pollfree()") since the
> waitqueue's lifetime is not tied to file's one and can be considered as
> another special case. While this would be fixable by somehow making
> cgroup_file_release() be tied to the fput(), it would require sizable
> refactoring at cgroups or higher layer which might be more justifiable if
> we identify more cases like this.
>
>  BUG: KASAN: use-after-free in _raw_spin_lock_irqsave+0x60/0xc0
>  Write of size 4 at addr ffff88810e625328 by task a.out/4404
>
>  CPU: 19 PID: 4404 Comm: a.out Not tainted 6.2.0-rc6 #38
>  Hardware name: Amazon EC2 c5a.8xlarge/, BIOS 1.0 10/16/2017
>  Call Trace:
>  <TASK>
>  dump_stack_lvl+0x73/0xa0
>  print_report+0x16c/0x4e0
>  ? _printk+0x59/0x80
>  ? __virt_addr_valid+0xb8/0x130
>  ? _raw_spin_lock_irqsave+0x60/0xc0
>  kasan_report+0xc3/0xf0
>  ? _raw_spin_lock_irqsave+0x60/0xc0
>  kasan_check_range+0x2d2/0x310
>  _raw_spin_lock_irqsave+0x60/0xc0
>  remove_wait_queue+0x1a/0xa0
>  ep_free+0x12c/0x170
>  ep_eventpoll_release+0x26/0x30
>  __fput+0x202/0x400
>  task_work_run+0x11d/0x170
>  do_exit+0x495/0x1130
>  ? update_cfs_rq_load_avg+0x2c2/0x2e0
>  do_group_exit+0x100/0x100
>  get_signal+0xd67/0xde0
>  ? finish_task_switch+0x15f/0x3a0
>  arch_do_signal_or_restart+0x2a/0x2b0
>  exit_to_user_mode_prepare+0x94/0x100
>  syscall_exit_to_user_mode+0x20/0x40
>  do_syscall_64+0x52/0x90
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>  RIP: 0033:0x7f8e392bfb91
>  Code: Unable to access opcode bytes at 0x7f8e392bfb67.
>  RSP: 002b:00007fff261e08d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000022
>  RAX: fffffffffffffdfe RBX: 0000000000000000 RCX: 00007f8e392bfb91
>  RDX: 0000000000000001 RSI: 00007fff261e08e8 RDI: 0000000000000004
>  RBP: 00007fff261e0920 R08: 0000000000400780 R09: 00007f8e3960f240
>  R10: 00000000000003df R11: 0000000000000246 R12: 00000000004005a0
>  R13: 00007fff261e0a00 R14: 0000000000000000 R15: 0000000000000000
>  </TASK>
>
>  Allocated by task 4404:
>  kasan_set_track+0x3d/0x60
>  __kasan_kmalloc+0x85/0x90
>  psi_trigger_create+0x113/0x3e0
>  pressure_write+0x146/0x2e0
>  cgroup_file_write+0x11c/0x250
>  kernfs_fop_write_iter+0x186/0x220
>  vfs_write+0x3d8/0x5c0
>  ksys_write+0x90/0x110
>  do_syscall_64+0x43/0x90
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
>  Freed by task 4407:
>  kasan_set_track+0x3d/0x60
>  kasan_save_free_info+0x27/0x40
>  ____kasan_slab_free+0x11d/0x170
>  slab_free_freelist_hook+0x87/0x150
>  __kmem_cache_free+0xcb/0x180
>  psi_trigger_destroy+0x2e8/0x310
>  cgroup_file_release+0x4f/0xb0
>  kernfs_drain_open_files+0x165/0x1f0
>  kernfs_drain+0x162/0x1a0
>  __kernfs_remove+0x1fb/0x310
>  kernfs_remove_by_name_ns+0x95/0xe0
>  cgroup_addrm_files+0x67f/0x700
>  cgroup_destroy_locked+0x283/0x3c0
>  cgroup_rmdir+0x29/0x100
>  kernfs_iop_rmdir+0xd1/0x140
>  vfs_rmdir+0xfe/0x240
>  do_rmdir+0x13d/0x280
>  __x64_sys_rmdir+0x2c/0x30
>  do_syscall_64+0x43/0x90
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> v2: updated commit message
>
> Link: https://lore.kernel.org/lkml/20230106224859.4123476-1-kamatam@amazon.com/
> Fixes: 0e94682b73bf ("psi: introduce psi monitor")
> Cc: stable@vger.kernel.org
> Signed-off-by: Munehisa Kamata <kamatam@amazon.com>
> Signed-off-by: Mengchi Cheng <mengcc@amazon.com>
> ---
>  kernel/sched/psi.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 8ac8b81bfee6..6e66c15f6450 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -1343,10 +1343,11 @@ void psi_trigger_destroy(struct psi_trigger *t)
>
>         group = t->group;
>         /*
> -        * Wakeup waiters to stop polling. Can happen if cgroup is deleted
> -        * from under a polling process.
> +        * Wakeup waiters to stop polling and clear the queue to prevent it from
> +        * being accessed later. Can happen if cgroup is deleted from under a
> +        * polling process otherwise.

This "otherwise" at the end seems extra. Was there a continuation of
this comment which was removed without removing this "otherwise" ?

>          */
> -       wake_up_interruptible(&t->event_wait);
> +       wake_up_pollfree(&t->event_wait);
>
>         mutex_lock(&group->trigger_lock);
>
> --
> 2.38.1
>