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) |
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.
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")
Signed-off-by: Lu Jialin <[email protected]>
---
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);
}
--
2.17.1
On Mon, Jun 12, 2023 at 11:24 PM Lu Jialin <[email protected]> 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) |
It's important to note that psi_trigger_destroy() calls
synchronize_rcu() before doing kfree(t), therefore the usage I think
is valid.
> 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.
>
> 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().
Thanks for the patch!
This seems similar to what ep_poll_callback/ep_remove_wait_queue do,
which I think makes sense. CC'ing Oleg Nesterov who implemented
ep_poll_callback/ep_remove_wait_queue logic and Eric Biggers who
worked on wake_up_pollfree() - both much more knowledgeable in this
area than me.
One issue I see with this patch is that the title says "sched/psi:
..." while it's fixing polling functionality. The patch is fixing the
mechanism used by psi triggers, not psi triggers themselves (well it
does but indirectly). Therefore I suggest changing that prefix to
something like "select: Fix use-after-free in poll_freewait()"
Thanks,
Suren.
>
> Fixes: 0e94682b73bf ("psi: introduce psi monitor")
> Signed-off-by: Lu Jialin <[email protected]>
> ---
> 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);
> }
>
> --
> 2.17.1
>
在 2023/6/14 7:42, Suren Baghdasaryan 写道:
> On Mon, Jun 12, 2023 at 11:24 PM Lu Jialin <[email protected]> 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) |
>
> It's important to note that psi_trigger_destroy() calls
> synchronize_rcu() before doing kfree(t), therefore the usage I think
> is valid.
>
>
>> 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.
>>
>> 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().
>
> Thanks for the patch!
> This seems similar to what ep_poll_callback/ep_remove_wait_queue do,
> which I think makes sense. CC'ing Oleg Nesterov who implemented
> ep_poll_callback/ep_remove_wait_queue logic and Eric Biggers who
> worked on wake_up_pollfree() - both much more knowledgeable in this
> area than me.
>
> One issue I see with this patch is that the title says "sched/psi:
> ..." while it's fixing polling functionality. The patch is fixing the
> mechanism used by psi triggers, not psi triggers themselves (well it
> does but indirectly). Therefore I suggest changing that prefix to
> something like "select: Fix use-after-free in poll_freewait()"
>
> Thanks,
> Suren.
>
>>
>> Fixes: 0e94682b73bf ("psi: introduce psi monitor")
>> Signed-off-by: Lu Jialin <[email protected]>
>> ---
>> 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);
>> }
>>
>> --
>> 2.17.1
>>
> .
>
Thanks for your suggestion.I will correct the commit msg and title in
v2.And cc to Oleg Nesterov and Eric Biggers.