net: wan: farsync: Fix use-after-free bugs caused by unfinished tasklets

In the Linux kernel, the following vulnerability has been resolved:

net: wan: farsync: Fix use-after-free bugs caused by unfinished tasklets

When the FarSync T-series card is being detached, the fst_card_info is
deallocated in fst_remove_one(). However, the fst_tx_task or fst_int_task
may still be running or pending, leading to use-after-free bugs when the
already freed fst_card_info is accessed in fst_process_tx_work_q() or
fst_process_int_work_q().

A typical race condition is depicted below:

CPU 0 (cleanup) | CPU 1 (tasklet)
| fst_start_xmit()
fst_remove_one() | tasklet_schedule()
unregister_hdlc_device()|
| fst_process_tx_work_q() //handler
kfree(card) //free | do_bottom_half_tx()
| card-> //use

The following KASAN trace was captured:

==================================================================
BUG: KASAN: slab-use-after-free in do_bottom_half_tx+0xb88/0xd00
Read of size 4 at addr ffff88800aad101c by task ksoftirqd/3/32
...
Call Trace:
<IRQ>
dump_stack_lvl+0x55/0x70
print_report+0xcb/0x5d0
? do_bottom_half_tx+0xb88/0xd00
kasan_report+0xb8/0xf0
? do_bottom_half_tx+0xb88/0xd00
do_bottom_half_tx+0xb88/0xd00
? _raw_spin_lock_irqsave+0x85/0xe0
? __pfx__raw_spin_lock_irqsave+0x10/0x10
? __pfx___hrtimer_run_queues+0x10/0x10
fst_process_tx_work_q+0x67/0x90
tasklet_action_common+0x1fa/0x720
? hrtimer_interrupt+0x31f/0x780
handle_softirqs+0x176/0x530
__irq_exit_rcu+0xab/0xe0
sysvec_apic_timer_interrupt+0x70/0x80
...

Allocated by task 41 on cpu 3 at 72.330843s:
kasan_save_stack+0x24/0x50
kasan_save_track+0x17/0x60
__kasan_kmalloc+0x7f/0x90
fst_add_one+0x1a5/0x1cd0
local_pci_probe+0xdd/0x190
pci_device_probe+0x341/0x480
really_probe+0x1c6/0x6a0
__driver_probe_device+0x248/0x310
driver_probe_device+0x48/0x210
__device_attach_driver+0x160/0x320
bus_for_each_drv+0x101/0x190
__device_attach+0x198/0x3a0
device_initial_probe+0x78/0xa0
pci_bus_add_device+0x81/0xc0
pci_bus_add_devices+0x7e/0x190
enable_slot+0x9b9/0x1130
acpiphp_check_bridge.part.0+0x2e1/0x460
acpiphp_hotplug_notify+0x36c/0x3c0
acpi_device_hotplug+0x203/0xb10
acpi_hotplug_work_fn+0x59/0x80
...

Freed by task 41 on cpu 1 at 75.138639s:
kasan_save_stack+0x24/0x50
kasan_save_track+0x17/0x60
kasan_save_free_info+0x3b/0x60
__kasan_slab_free+0x43/0x70
kfree+0x135/0x410
fst_remove_one+0x2ca/0x540
pci_device_remove+0xa6/0x1d0
device_release_driver_internal+0x364/0x530
pci_stop_bus_device+0x105/0x150
pci_stop_and_remove_bus_device+0xd/0x20
disable_slot+0x116/0x260
acpiphp_disable_and_eject_slot+0x4b/0x190
acpiphp_hotplug_notify+0x230/0x3c0
acpi_device_hotplug+0x203/0xb10
acpi_hotplug_work_fn+0x59/0x80
...

The buggy address belongs to the object at ffff88800aad1000
which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 28 bytes inside of
freed 1024-byte region
The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0xaad0
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0x100000000000040(head|node=0|zone=1)
page_type: f5(slab)
raw: 0100000000000040 ffff888007042dc0 dead000000000122 0000000000000000
raw: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
head: 0100000000000040 ffff888007042dc0 dead000000000122 0000000000000000
head: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
head: 0100000000000003 ffffea00002ab401 00000000ffffffff 00000000ffffffff
head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff88800aad0f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff88800aad0f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff88800aad1000: fa fb
---truncated---