media: cx23885: Fix a null-ptr-deref bug in buffer_prepare() and buffer_finish()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: media: cx23885: Fix a null-ptr-deref bug in buffer_prepare() and buffer_finish() When the driver calls cx23885_risc_buffer() to prepare the buffer, the function call dma_alloc_coherent may fail, resulting in a empty buffer risc->cpu. Later when we free the buffer or access the buffer, null ptr deref is triggered. This bug is similar to the following one: https://git.linuxtv.org/media_stage.git/commit/?id=2b064d91440b33fba5b452f2d1b31f13ae911d71. We believe the bug can be also dynamically triggered from user side. Similarly, we fix this by checking the return value of cx23885_risc_buffer() and the value of risc->cpu before buffer free.

References

• https://git.kernel.org/stable/c/f0a06203f2fe63f04311467200c99c4ee1926578
• https://git.kernel.org/stable/c/6738841f6fcf23e9fc30e2449f32fc84ee19c6f1
• https://git.kernel.org/stable/c/5b8e5e28e85a546dfccc3895befe0e823fdd7c89
• https://git.kernel.org/stable/c/47e8b73bc35d7c54642f78e498697692f6358996