serial: amba-pl011: avoid SBSA UART accessing DMACR register

Summary

In the Linux kernel, the following vulnerability has been resolved: serial: amba-pl011: avoid SBSA UART accessing DMACR register Chapter "B Generic UART" in "ARM Server Base System Architecture" [1] documentation describes a generic UART interface. Such generic UART does not support DMA. In current code, sbsa_uart_pops and amba_pl011_pops share the same stop_rx operation, which will invoke pl011_dma_rx_stop, leading to an access of the DMACR register. This commit adds a using_rx_dma check in pl011_dma_rx_stop to avoid the access to DMACR register for SBSA UARTs which does not support DMA. When the kernel enables DMA engine with "CONFIG_DMA_ENGINE=y", Linux SBSA PL011 driver will access PL011 DMACR register in some functions. For most real SBSA Pl011 hardware implementations, the DMACR write behaviour will be ignored. So these DMACR operations will not cause obvious problems. But for some virtual SBSA PL011 hardware, like Xen virtual SBSA PL011 (vpl011) device, the behaviour might be different. Xen vpl011 emulation will inject a data abort to guest, when guest is accessing an unimplemented UART register. As Xen VPL011 is SBSA compatible, it will not implement DMACR register. So when Linux SBSA PL011 driver access DMACR register, it will get an unhandled data abort fault and the application will get a segmentation fault: Unhandled fault at 0xffffffc00944d048 Mem abort info: ESR = 0x96000000 EC = 0x25: DABT (current EL), IL = 32 bits SET = 0, FnV = 0 EA = 0, S1PTW = 0 FSC = 0x00: ttbr address size fault Data abort info: ISV = 0, ISS = 0x00000000 CM = 0, WnR = 0 swapper pgtable: 4k pages, 39-bit VAs, pgdp=0000000020e2e000 [ffffffc00944d048] pgd=100000003ffff803, p4d=100000003ffff803, pud=100000003ffff803, pmd=100000003fffa803, pte=006800009c090f13 Internal error: ttbr address size fault: 96000000 [#1] PREEMPT SMP ... Call trace: pl011_stop_rx+0x70/0x80 tty_port_shutdown+0x7c/0xb4 tty_port_close+0x60/0xcc uart_close+0x34/0x8c tty_release+0x144/0x4c0 __fput+0x78/0x220 ____fput+0x1c/0x30 task_work_run+0x88/0xc0 do_notify_resume+0x8d0/0x123c el0_svc+0xa8/0xc0 el0t_64_sync_handler+0xa4/0x130 el0t_64_sync+0x1a0/0x1a4 Code: b9000083 b901f001 794038a0 8b000042 (b9000041) ---[ end trace 83dd93df15c3216f ]--- note: bootlogd[132] exited with preempt_count 1 /etc/rcS.d/S07bootlogd: line 47: 132 Segmentation fault start-stop-daemon This has been discussed in the Xen community, and we think it should fix this in Linux. See [2] for more information. [1] https://developer.arm.com/documentation/den0094/c/?lang=en [2] https://lists.xenproject.org/archives/html/xen-devel/2022-11/msg00543.html