CVE-2024-26895: Use After Free

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

wifi: wilc1000: prevent use-after-free on vif when cleaning up all interfaces

wilc_netdev_cleanup currently triggers a KASAN warning, which can be observed on interface registration error path, or simply by removing the module/unbinding device from driver:

echo spi0.1 > /sys/bus/spi/drivers/wilc1000_spi/unbind

================================================================== BUG: KASAN: slab-use-after-free in wilc_netdev_cleanup+0x508/0x5cc Read of size 4 at addr c54d1ce8 by task sh/86

CPU: 0 PID: 86 Comm: sh Not tainted 6.8.0-rc1+ #117 Hardware name: Atmel SAMA5 unwind_backtrace from show_stack+0x18/0x1c show_stack from dump_stack_lvl+0x34/0x58 dump_stack_lvl from print_report+0x154/0x500 print_report from kasan_report+0xac/0xd8 kasan_report from wilc_netdev_cleanup+0x508/0x5cc wilc_netdev_cleanup from wilc_bus_remove+0xc8/0xec wilc_bus_remove from spi_remove+0x8c/0xac spi_remove from device_release_driver_internal+0x434/0x5f8 device_release_driver_internal from unbind_store+0xbc/0x108 unbind_store from kernfs_fop_write_iter+0x398/0x584 kernfs_fop_write_iter from vfs_write+0x728/0xf88 vfs_write from ksys_write+0x110/0x1e4 ksys_write from ret_fast_syscall+0x0/0x1c

[...]

Allocated by task 1: kasan_save_track+0x30/0x5c __kasan_kmalloc+0x8c/0x94 __kmalloc_node+0x1cc/0x3e4 kvmalloc_node+0x48/0x180 alloc_netdev_mqs+0x68/0x11dc alloc_etherdev_mqs+0x28/0x34 wilc_netdev_ifc_init+0x34/0x8ec wilc_cfg80211_init+0x690/0x910 wilc_bus_probe+0xe0/0x4a0 spi_probe+0x158/0x1b0 really_probe+0x270/0xdf4 __driver_probe_device+0x1dc/0x580 driver_probe_device+0x60/0x140 __driver_attach+0x228/0x5d4 bus_for_each_dev+0x13c/0x1a8 bus_add_driver+0x2a0/0x608 driver_register+0x24c/0x578 do_one_initcall+0x180/0x310 kernel_init_freeable+0x424/0x484 kernel_init+0x20/0x148 ret_from_fork+0x14/0x28

Freed by task 86: kasan_save_track+0x30/0x5c kasan_save_free_info+0x38/0x58 __kasan_slab_free+0xe4/0x140 kfree+0xb0/0x238 device_release+0xc0/0x2a8 kobject_put+0x1d4/0x46c netdev_run_todo+0x8fc/0x11d0 wilc_netdev_cleanup+0x1e4/0x5cc wilc_bus_remove+0xc8/0xec spi_remove+0x8c/0xac device_release_driver_internal+0x434/0x5f8 unbind_store+0xbc/0x108 kernfs_fop_write_iter+0x398/0x584 vfs_write+0x728/0xf88 ksys_write+0x110/0x1e4 ret_fast_syscall+0x0/0x1c [...]

David Mosberger-Tan initial investigation [1] showed that this use-after-free is due to netdevice unregistration during vif list traversal. When unregistering a net device, since the needs_free_netdev has been set to true during registration, the netdevice object is also freed, and as a consequence, the corresponding vif object too, since it is attached to it as private netdevice data. The next occurrence of the loop then tries to access freed vif pointer to the list to move forward in the list.

Fix this use-after-free thanks to two mechanisms: - navigate in the list with list_for_each_entry_safe, which allows to safely modify the list as we go through each element. For each element, remove it from the list with list_del_rcu - make sure to wait for RCU grace period end after each vif removal to make sure it is safe to free the corresponding vif too (through unregister_netdev)

Since we are in a RCU "modifier" path (not a "reader" path), and because such path is expected not to be concurrent to any other modifier (we are using the vif_mutex lock), we do not need to use RCU list API, that's why we can benefit from list_for_each_entry_safe.

[1] https://lore.kernel.org/linux-wireless/[email protected]/