serial: protect uart_port_dtr_rts() in uart_shutdown() too

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: serial: protect uart_port_dtr_rts() in uart_shutdown() too Commit af224ca2df29 (serial: core: Prevent unsafe uart port access, part 3) added few uport == NULL checks. It added one to uart_shutdown(), so the commit assumes, uport can be NULL in there. But right after that protection, there is an unprotected "uart_port_dtr_rts(uport, false);" call. That is invoked only if HUPCL is set, so I assume that is the reason why we do not see lots of these reports. Or it cannot be NULL at this point at all for some reason :P. Until the above is investigated, stay on the safe side and move this dereference to the if too. I got this inconsistency from Coverity under CID 1585130. Thanks.

References

• https://git.kernel.org/stable/c/2fe399bb8efd0d325ab1138cf8e3ecf23a39e96d
• https://git.kernel.org/stable/c/399927f0f875b93f3d5a0336d382ba48b8671eb2
• https://git.kernel.org/stable/c/d7b5876a6e74cdf8468a478be6b23f2f5464ac7a
• https://git.kernel.org/stable/c/e418d91195d29d5f9c9685ff309b92b04b41dc40
• https://git.kernel.org/stable/c/76ed24a34223bb2c6b6162e1d8389ec4e602a290
• https://git.kernel.org/stable/c/602babaa84d627923713acaf5f7e9a4369e77473