ASoC: SDCA: fix NULL pointer dereference in sdca_dev_unregister_functions

Summary

In the Linux kernel, the following vulnerability has been resolved: ASoC: SDCA: fix NULL pointer dereference in sdca_dev_unregister_functions sdca_dev_unregister_functions() iterates over all SDCA function descriptors and calls sdca_dev_unregister() on each func_dev without checking for NULL. When a function registration has failed partway through, or the device cleanup races with probe deferral, func_dev entries may be NULL, leading to a kernel oops: BUG: kernel NULL pointer dereference, address: 0000000000000040 RIP: 0010:device_del+0x1e/0x3e0 Call Trace: sdca_dev_unregister_functions+0x37/0x60 [snd_soc_sdca] release_nodes+0x35/0xb0 devres_release_all+0x90/0x100 device_unbind_cleanup+0xe/0x80 device_release_driver_internal+0x1c1/0x200 bus_remove_device+0xc6/0x130 device_del+0x161/0x3e0 device_unregister+0x17/0x60 sdw_delete_slave+0xb6/0xd0 [soundwire_bus] sdw_bus_master_delete+0x1e/0x50 [soundwire_bus] ... sof_probe_work+0x19/0x30 [snd_sof] This was observed on a Lenovo ThinkPad X1 Carbon G14 (Panther Lake) with the SOF audio driver probe failing due to missing Panther Lake firmware, causing the subsequent cleanup of SoundWire devices to trigger the crash. Fix this with three changes: 1) Add a NULL guard in sdca_dev_unregister() so that callers do not need to pre-validate the pointer (defense in depth). 2) In sdca_dev_unregister_functions(), skip NULL func_dev entries and clear func_dev to NULL after unregistration, making the function idempotent and safe against double-invocation. 3) In sdca_dev_register_functions(), roll back all previously registered functions when a later one fails, so the function array is never left in a partially-populated state.