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

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