Message-ID: <20230214183745.3958175-1-kamatam@amazon.com>
Date:   Tue, 14 Feb 2023 10:37:45 -0800
From:   Munehisa Kamata <kamatam@...zon.com>
To:     <surenb@...gle.com>
CC:     <ebiggers@...nel.org>, <hannes@...xchg.org>, <hdanton@...a.com>,
        <kamatam@...zon.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <mengcc@...zon.com>
Subject: Re: [PATCH v2] sched/psi: fix use-after-free in ep_remove_wait_queue()

On Tue, 2023-02-14 17:10:58 +0000, Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> Thanks!
> Overall LGTM, just a couple of nits (simplifications):
> 
> On Mon, Feb 13, 2023 at 11:04 PM Munehisa Kamata <kamatam@...zon.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.
 
Applied in v3.

> >
> >  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:

Applied in v3.

> >
> >  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.

Applied in v3.

> >
> > 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.

Applied in v3 as well.

> > 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@...r.kernel.org
> > Signed-off-by: Munehisa Kamata <kamatam@...zon.com>
> > Signed-off-by: Mengchi Cheng <mengcc@...zon.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" ?
 
Removed it in v3. The subject of the original sentense was a bit unclear to
me and I think I tried to have a simplified version of  "[Otherwise an
unexpected free of the queue] can happen if ... " there.

Thanks for all the suggestions.


Regards,
Munehisa

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

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