Re: [PATCH v2] poll: Fix use-after-free in poll_freewait()

From: Suren Baghdasaryan
Date: Mon Jun 26 2023 - 16:23:06 EST

On Fri, Jun 23, 2023 at 6:58 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Tue, Jun 20, 2023 at 5:09 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >
> > On Sun, Jun 18, 2023 at 6:28 AM lujialin (A) <lujialin4@xxxxxxxxxx> wrote:
> > >
> > > Hi Suren:
> > >
> > > kernel config:
> > > x86_64_defconfig
> > > CONFIG_PSI=y
> > > CONFIG_SLUB_DEBUG=y
> > > CONFIG_SLUB_DEBUG_ON=y
> > > CONFIG_KASAN=y
> > > CONFIG_KASAN_INLINE=y
> > >
> > > I make some change in code, in order to increase the recurrence probability.
> > > diff --git a/fs/select.c b/fs/select.c
> > > index 5edffee1162c..5ee5b74a8386 100644
> > > --- a/fs/select.c
> > > +++ b/fs/select.c
> > > @@ -139,6 +139,7 @@ void poll_freewait(struct poll_wqueues *pwq)
> > > {
> > > struct poll_table_page * p = pwq->table;
> > > int i;
> > > + mdelay(50);
> > > for (i = 0; i < pwq->inline_index; i++)
> > > free_poll_entry(pwq->inline_entries + i);
> > > while (p) {
> > >
> > > Here is the simple repo test.sh:
> > > #!/bin/bash
> > >
> > > RESOURCE_TYPES=("cpu" "memory" "io" "irq")
> > > #RESOURCE_TYPES=("cpu")
> > > cgroup_num=50
> > > test_dir=/sys/fs/cgroup/test
> > >
> > > function restart_cgroup() {
> > > num=$(expr $RANDOM % $cgroup_num + 1)
> > > rmdir $test_dir/test_$num
> > > mkdir $test_dir/test_$num
> > > }
> > >
> > > function create_triggers() {
> > > num=$(expr $RANDOM % $cgroup_num + 1)
> > > random=$(expr $RANDOM % "${#RESOURCE_TYPES[@]}")
> > > psi_type="${RESOURCE_TYPES[${random}]}"
> > > ./psi_monitor $test_dir/test_$num $psi_type &
> > > }
> > >
> > > mkdir $test_dir
> > > for i in $(seq 1 $cgroup_num)
> > > do
> > > mkdir $test_dir/test_$i
> > > done
> > > for j in $(seq 1 100)
> > > do
> > > restart_cgroup &
> > > create_triggers &
> > > done
> > >
> > > psi_monitor.c:
> > > #include <errno.h>
> > > #include <fcntl.h>
> > > #include <stdio.h>
> > > #include <poll.h>
> > > #include <string.h>
> > > #include <unistd.h>
> > >
> > > int main(int argc, char *argv[]) {
> > > const char trig[] = "full 1000000 1000000";
> > > struct pollfd fds;
> > > char filename[100];
> > >
> > > sprintf(filename, "%s/%s.pressure", argv[1], argv[2]);
> > >
> > > fds.fd = open(filename, O_RDWR | O_NONBLOCK);
> > > if (fds.fd < 0) {
> > > printf("%s open error: %s\n", filename,strerror(errno));
> > > return 1;
> > > }
> > > fds.events = POLLPRI;
> > > if (write(fds.fd, trig, strlen(trig) + 1) < 0) {
> > > printf("%s write error: %s\n",filename,strerror(errno));
> > > return 1;
> > > }
> > > while (1) {
> > > poll(&fds, 1, -1);
> > > }
> > > close(fds.fd);
> > > return 0;
> > > }
> >
> > Thanks a lot!
> > I'll try to get this reproduced and fixed by the end of this week.
>
> Ok, I was able to reproduce the issue and I think the ultimate problem
> here is that kernfs_release_file() can be called from both
> kernfs_fop_release() and kernfs_drain_open_files(). While
> kernfs_fop_release() is called when the FD's refcount is 0 and there
> are no users, kernfs_drain_open_files() can be called while there are
> still other users. In our scenario, kn->attr.ops->release points to
> cgroup_pressure_release() which destroys the psi trigger thinking that
> (since the file is "released") there could be no other users. So,
> shell process which issues the rmdir command will destroy the trigger
> once cgroup_pressure_release() is called and the reproducer will step
> on the freed wait_queue_head. The way kernfs_release_file() is
> implemented, it ensures that kn->attr.ops->release(of) is called only
> once (the first time), therefore cgroup_pressure_release() is never
> called in reproducer's context. That prevents me from implementing
> some kind of refcounting schema for psi triggers because we are never
> notified when the last user is gone.
> I think to fix this I would need to modify kernfs_release_file() and
> maybe add another operation in kernfs_ops to indicate that the last
> user is gone (smth like final_release()). It's not pretty but so far I
> did not find a better way. I'll think some more over the weekend and
> will try to post a patch implementing the fix on Monday.

Posted 2 patches to fix this at:
https://lore.kernel.org/all/20230626201713.1204982-1-surenb@xxxxxxxxxx/
Thanks,
Suren.

> Thanks,
> Suren.
>
>
> > Suren.
> >
> > > Thanks,
> > > Lu
> > > 在 2023/6/16 7:13, Suren Baghdasaryan 写道:
> > > > On Wed, Jun 14, 2023 at 11:19 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > > >>
> > > >> On Wed, Jun 14, 2023 at 10:40 AM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> > > >>>
> > > >>> On Wed, Jun 14, 2023 at 03:07:33PM +0800, Lu Jialin wrote:
> > > >>>> We found a UAF bug in remove_wait_queue as follows:
> > > >>>>
> > > >>>> ==================================================================
> > > >>>> BUG: KASAN: use-after-free in _raw_spin_lock_irqsave+0x71/0xe0
> > > >>>> Write of size 4 at addr ffff8881150d7b28 by task psi_trigger/15306
> > > >>>> Call Trace:
> > > >>>> dump_stack+0x9c/0xd3
> > > >>>> print_address_description.constprop.0+0x19/0x170
> > > >>>> __kasan_report.cold+0x6c/0x84
> > > >>>> kasan_report+0x3a/0x50
> > > >>>> check_memory_region+0xfd/0x1f0
> > > >>>> _raw_spin_lock_irqsave+0x71/0xe0
> > > >>>> remove_wait_queue+0x26/0xc0
> > > >>>> poll_freewait+0x6b/0x120
> > > >>>> do_sys_poll+0x305/0x400
> > > >>>> do_syscall_64+0x33/0x40
> > > >>>> entry_SYSCALL_64_after_hwframe+0x61/0xc6
> > > >>>>
> > > >>>> Allocated by task 15306:
> > > >>>> kasan_save_stack+0x1b/0x40
> > > >>>> __kasan_kmalloc.constprop.0+0xb5/0xe0
> > > >>>> psi_trigger_create.part.0+0xfc/0x450
> > > >>>> cgroup_pressure_write+0xfc/0x3b0
> > > >>>> cgroup_file_write+0x1b3/0x390
> > > >>>> kernfs_fop_write_iter+0x224/0x2e0
> > > >>>> new_sync_write+0x2ac/0x3a0
> > > >>>> vfs_write+0x365/0x430
> > > >>>> ksys_write+0xd5/0x1b0
> > > >>>> do_syscall_64+0x33/0x40
> > > >>>> entry_SYSCALL_64_after_hwframe+0x61/0xc6
> > > >>>>
> > > >>>> Freed by task 15850:
> > > >>>> kasan_save_stack+0x1b/0x40
> > > >>>> kasan_set_track+0x1c/0x30
> > > >>>> kasan_set_free_info+0x20/0x40
> > > >>>> __kasan_slab_free+0x151/0x180
> > > >>>> kfree+0xba/0x680
> > > >>>> cgroup_file_release+0x5c/0xe0
> > > >>>> kernfs_drain_open_files+0x122/0x1e0
> > > >>>> kernfs_drain+0xff/0x1e0
> > > >>>> __kernfs_remove.part.0+0x1d1/0x3b0
> > > >>>> kernfs_remove_by_name_ns+0x89/0xf0
> > > >>>> cgroup_addrm_files+0x393/0x3d0
> > > >>>> css_clear_dir+0x8f/0x120
> > > >>>> kill_css+0x41/0xd0
> > > >>>> cgroup_destroy_locked+0x166/0x300
> > > >>>> cgroup_rmdir+0x37/0x140
> > > >>>> kernfs_iop_rmdir+0xbb/0xf0
> > > >>>> vfs_rmdir.part.0+0xa5/0x230
> > > >>>> do_rmdir+0x2e0/0x320
> > > >>>> __x64_sys_unlinkat+0x99/0xc0
> > > >>>> do_syscall_64+0x33/0x40
> > > >>>> entry_SYSCALL_64_after_hwframe+0x61/0xc6
> > > >>>> ==================================================================
> > > >>>>
> > > >>>> If using epoll(), wake_up_pollfree will empty waitqueue and set
> > > >>>> wait_queue_head is NULL before free waitqueue of psi trigger. But is
> > > >>>> doesn't work when using poll(), which will lead a UAF problem in
> > > >>>> poll_freewait coms as following:
> > > >>>>
> > > >>>> (cgroup_rmdir) |
> > > >>>> psi_trigger_destroy |
> > > >>>> wake_up_pollfree(&t->event_wait) |
> > > >>>> synchronize_rcu(); |
> > > >>>> kfree(t) |
> > > >>>> | (poll_freewait)
> > > >>>> | free_poll_entry(pwq->inline_entries + i)
> > > >>>> | remove_wait_queue(entry->wait_address)
> > > >>>> | spin_lock_irqsave(&wq_head->lock)
> > > >>>>
> > > >>>> entry->wait_address in poll_freewait() is t->event_wait in cgroup_rmdir().
> > > >>>> t->event_wait is free in psi_trigger_destroy before call poll_freewait(),
> > > >>>> therefore wq_head in poll_freewait() has been already freed, which would
> > > >>>> lead to a UAF.
> > > >
> > > > Hi Lu,
> > > > Could you please share your reproducer along with the kernel config
> > > > you used? I'm trying to reproduce this UAF but every time I delete the
> > > > cgroup being polled, poll() simply returns POLLERR.
> > > > Thanks,
> > > > Suren.
> > > >
> > > >>>>
> > > >>>> similar problem for epoll() has been fixed commit c2dbe32d5db5
> > > >>>> ("sched/psi: Fix use-after-free in ep_remove_wait_queue()").
> > > >>>> epoll wakeup function ep_poll_callback() will empty waitqueue and set
> > > >>>> wait_queue_head is NULL when pollflags is POLLFREE and judge pwq->whead
> > > >>>> is NULL or not before remove_wait_queue in ep_remove_wait_queue(),
> > > >>>> which will fix the UAF bug in ep_remove_wait_queue.
> > > >>>>
> > > >>>> But poll wakeup function pollwake() doesn't do that. To fix the
> > > >>>> problem, we empty waitqueue and set wait_address is NULL in pollwake() when
> > > >>>> key is POLLFREE. otherwise in remove_wait_queue, which is similar to
> > > >>>> epoll().
> > > >>>>
> > > >>>> Fixes: 0e94682b73bf ("psi: introduce psi monitor")
> > > >>>> Suggested-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > >>>> Link: https://lore.kernel.org/all/CAJuCfpEoCRHkJF-=1Go9E94wchB4BzwQ1E3vHGWxNe+tEmSJoA@xxxxxxxxxxxxxx/#t
> > > >>>> Signed-off-by: Lu Jialin <lujialin4@xxxxxxxxxx>
> > > >>>> ---
> > > >>>> v2: correct commit msg and title suggested by Suren Baghdasaryan
> > > >>>> ---
> > > >>>> fs/select.c | 20 +++++++++++++++++++-
> > > >>>> 1 file changed, 19 insertions(+), 1 deletion(-)
> > > >>>>
> > > >>>> diff --git a/fs/select.c b/fs/select.c
> > > >>>> index 0ee55af1a55c..e64c7b4e9959 100644
> > > >>>> --- a/fs/select.c
> > > >>>> +++ b/fs/select.c
> > > >>>> @@ -132,7 +132,17 @@ EXPORT_SYMBOL(poll_initwait);
> > > >>>>
> > > >>>> static void free_poll_entry(struct poll_table_entry *entry)
> > > >>>> {
> > > >>>> - remove_wait_queue(entry->wait_address, &entry->wait);
> > > >>>> + wait_queue_head_t *whead;
> > > >>>> +
> > > >>>> + rcu_read_lock();
> > > >>>> + /* If it is cleared by POLLFREE, it should be rcu-safe.
> > > >>>> + * If we read NULL we need a barrier paired with smp_store_release()
> > > >>>> + * in pollwake().
> > > >>>> + */
> > > >>>> + whead = smp_load_acquire(&entry->wait_address);
> > > >>>> + if (whead)
> > > >>>> + remove_wait_queue(whead, &entry->wait);
> > > >>>> + rcu_read_unlock();
> > > >>>> fput(entry->filp);
> > > >>>> }
> > > >>>>
> > > >>>> @@ -215,6 +225,14 @@ static int pollwake(wait_queue_entry_t *wait, unsigned mode, int sync, void *key
> > > >>>> entry = container_of(wait, struct poll_table_entry, wait);
> > > >>>> if (key && !(key_to_poll(key) & entry->key))
> > > >>>> return 0;
> > > >>>> + if (key_to_poll(key) & POLLFREE) {
> > > >>>> + list_del_init(&wait->entry);
> > > >>>> + /* wait_address !=NULL protects us from the race with
> > > >>>> + * poll_freewait().
> > > >>>> + */
> > > >>>> + smp_store_release(&entry->wait_address, NULL);
> > > >>>> + return 0;
> > > >>>> + }
> > > >>>> return __pollwake(wait, mode, sync, key);
> > > >>>
> > > >>> I don't understand why this patch is needed.
> > > >>>
> > > >>> The last time I looked at POLLFREE, it is only needed because of asynchronous
> > > >>> polls. See my explanation in the commit message of commit 50252e4b5e989ce6.
> > > >>
> > > >> Ah, I missed that. Thanks for the correction.
> > > >>
> > > >>>
> > > >>> In summary, POLLFREE solves the problem of polled waitqueues whose lifetime is
> > > >>> tied to the current task rather than to the file being polled. Also refer to
> > > >>> the comment above wake_up_pollfree(), which mentions this.
> > > >>>
> > > >>> fs/select.c is synchronous polling, not asynchronous. Therefore, it should not
> > > >>> need to handle POLLFREE.
> > > >>>
> > > >>> If there's actually a bug here, most likely it's a bug in psi_trigger_poll()
> > > >>> where it is using a waitqueue whose lifetime is tied to neither the current task
> > > >>> nor the file being polled. That needs to be fixed.
> > > >>
> > > >> Yeah. We discussed this issue in
> > > >> https://lore.kernel.org/all/CAJuCfpFb0J5ZwO6kncjRG0_4jQLXUy-_dicpH5uGiWP8aKYEJQ@xxxxxxxxxxxxxx
> > > >> and the root cause is that cgroup_file_release() where
> > > >> psi_trigger_destroy() is called is not tied to the cgroup file's real
> > > >> lifetime (see my analysis here:
> > > >> https://lore.kernel.org/all/CAJuCfpFZ3B4530TgsSHqp5F_gwfrDujwRYewKReJru==MdEHQg@xxxxxxxxxxxxxx/#t).
> > > >> I guess it's time to do a deeper surgery and figure out a way to call
> > > >> psi_trigger_destroy() when the polled cgroup file is actually being
> > > >> destroyed. I'll take a closer look into this later today.
> > > >> A fix will likely require some cgroup or kernfs code changes, so
> > > >> CC'ing Tejun for visibility.
> > > >> Thanks,
> > > >> Suren.
> > > >>
> > > >>>
> > > >>> - Eric
> > > > .
> > > >