On Wed Sep 17 17:59:26 2025 +0800, Duoming Zhou wrote:
> The original code uses cancel_delayed_work() in flexcop_pci_remove(), which
> does not guarantee that the delayed work item irq_check_work has fully
> completed if it was already running. This leads to use-after-free scenarios
> where flexcop_pci_remove() may free the flexcop_device while irq_check_work
> is still active and attempts to dereference the device.
>
> A typical race condition is illustrated below:
>
> CPU 0 (remove) | CPU 1 (delayed work callback)
> flexcop_pci_remove() | flexcop_pci_irq_check_work()
> cancel_delayed_work() |
> flexcop_device_kfree(fc_pci->fc_dev) |
> | fc = fc_pci->fc_dev; // UAF
>
> This is confirmed by a KASAN report:
>
> ==================================================================
> BUG: KASAN: slab-use-after-free in __run_timer_base.part.0+0x7d7/0x8c0
> Write of size 8 at addr ffff8880093aa8c8 by task bash/135
> ...
> Call Trace:
> <IRQ>
> dump_stack_lvl+0x55/0x70
> print_report+0xcf/0x610
> ? __run_timer_base.part.0+0x7d7/0x8c0
> kasan_report+0xb8/0xf0
> ? __run_timer_base.part.0+0x7d7/0x8c0
> __run_timer_base.part.0+0x7d7/0x8c0
> ? __pfx___run_timer_base.part.0+0x10/0x10
> ? __pfx_read_tsc+0x10/0x10
> ? ktime_get+0x60/0x140
> ? lapic_next_event+0x11/0x20
> ? clockevents_program_event+0x1d4/0x2a0
> run_timer_softirq+0xd1/0x190
> handle_softirqs+0x16a/0x550
> irq_exit_rcu+0xaf/0xe0
> sysvec_apic_timer_interrupt+0x70/0x80
> </IRQ>
> ...
>
> Allocated by task 1:
> kasan_save_stack+0x24/0x50
> kasan_save_track+0x14/0x30
> __kasan_kmalloc+0x7f/0x90
> __kmalloc_noprof+0x1be/0x460
> flexcop_device_kmalloc+0x54/0xe0
> flexcop_pci_probe+0x1f/0x9d0
> local_pci_probe+0xdc/0x190
> pci_device_probe+0x2fe/0x470
> really_probe+0x1ca/0x5c0
> __driver_probe_device+0x248/0x310
> driver_probe_device+0x44/0x120
> __driver_attach+0xd2/0x310
> bus_for_each_dev+0xed/0x170
> bus_add_driver+0x208/0x500
> driver_register+0x132/0x460
> do_one_initcall+0x89/0x300
> kernel_init_freeable+0x40d/0x720
> kernel_init+0x1a/0x150
> ret_from_fork+0x10c/0x1a0
> ret_from_fork_asm+0x1a/0x30
>
> Freed by task 135:
> kasan_save_stack+0x24/0x50
> kasan_save_track+0x14/0x30
> kasan_save_free_info+0x3a/0x60
> __kasan_slab_free+0x3f/0x50
> kfree+0x137/0x370
> flexcop_device_kfree+0x32/0x50
> pci_device_remove+0xa6/0x1d0
> device_release_driver_internal+0xf8/0x210
> pci_stop_bus_device+0x105/0x150
> pci_stop_and_remove_bus_device_locked+0x15/0x30
> remove_store+0xcc/0xe0
> kernfs_fop_write_iter+0x2c3/0x440
> vfs_write+0x871/0xd70
> ksys_write+0xee/0x1c0
> do_syscall_64+0xac/0x280
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> ...
>
> Replace cancel_delayed_work() with cancel_delayed_work_sync() to ensure
> that the delayed work item is properly canceled and any executing delayed
> work has finished before the device memory is deallocated.
>
> This bug was initially identified through static analysis. To reproduce
> and test it, I simulated the B2C2 FlexCop PCI device in QEMU and introduced
> artificial delays within the flexcop_pci_irq_check_work() function to
> increase the likelihood of triggering the bug.
>
> Fixes: 382c5546d618 ("V4L/DVB (10694): [PATCH] software IRQ watchdog for
> Flexcop B2C2 DVB PCI cards")
> Cc: [email protected]
> Signed-off-by: Duoming Zhou <[email protected]>
> Signed-off-by: Hans Verkuil <[email protected]>
Patch committed.
Thanks,
Hans Verkuil
drivers/media/pci/b2c2/flexcop-pci.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/drivers/media/pci/b2c2/flexcop-pci.c
b/drivers/media/pci/b2c2/flexcop-pci.c
index 486c8ec0fa60..ab53c5b02c48 100644
--- a/drivers/media/pci/b2c2/flexcop-pci.c
+++ b/drivers/media/pci/b2c2/flexcop-pci.c
@@ -411,7 +411,7 @@ static void flexcop_pci_remove(struct pci_dev *pdev)
struct flexcop_pci *fc_pci = pci_get_drvdata(pdev);
if (irq_chk_intv > 0)
- cancel_delayed_work(&fc_pci->irq_check_work);
+ cancel_delayed_work_sync(&fc_pci->irq_check_work);
flexcop_pci_dma_exit(fc_pci);
flexcop_device_exit(fc_pci->fc_dev);
