ARM: 9485/1: mm: acquire mmap write lock around show_pte() for user faultsgrafted

When CONFIG_DEBUG_USER=y, and cmdline "user_debug=31" is set,
a user fault may trigger show_pte() without any lock.
If another thread in the same process concurrently calls munmap(),
the page table pages may be freed while show_pte() is still traversing
them, causing a use-after-free in show_pte().

If CONFIG_ARM_LPAE=y, this may cause a kernel panic if the pages table
of PMD are freed when show_pte() is running.

Acquire mmap_write_lock() around show_pte() for user faults to fix the
contention.

For user faults, additionally restrict that show_pte() is called only
when the addr is a user-space address (addr < TASK_SIZE). This is because
the lock of tsk->mm only protects the virtual memory of user address space,
furthermore, dumping the page tables of a kernel-space address for user
faults is unnecessary and may have security implications.

Keep everything unchanged for kernel faults, because the kernel is
already in the "oops" state, acquiring a lock may risk a deadlock.

Co-developed-by: Qi Xi <xiqi2@huawei.com>

Fixes: 6d021b724481 ("ARM: dump pgd, pmd and pte states on unhandled data abort faults")
Link: https://lore.kernel.org/20260716014022.2823-1-xieyuanbin1@huawei.com
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Signed-off-by: Qi Xi <xiqi2@huawei.com>
Signed-off-by: Xie Yuanbin <xieyuanbin1@huawei.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

1 files changed, 105 insertions, 0 deletions

diff --git a/Documentation/driver-api/serial/driver.rst b/Documentation/driver-api/serial/driver.rst
new file mode 100644
index 000000000..c1db6a1a6
--- /dev/null
+++ b/Documentation/driver-api/serial/driver.rst
@@ -0,0 +1,105 @@
+====================
+Low Level Serial API
+====================
+
+
+This document is meant as a brief overview of some aspects of the new serial
+driver.  It is not complete, any questions you have should be directed to
+<rmk@arm.linux.org.uk>
+
+The reference implementation is contained within amba-pl011.c.
+
+
+
+Low Level Serial Hardware Driver
+--------------------------------
+
+The low level serial hardware driver is responsible for supplying port
+information (defined by uart_port) and a set of control methods (defined
+by uart_ops) to the core serial driver.  The low level driver is also
+responsible for handling interrupts for the port, and providing any
+console support.
+
+
+Console Support
+---------------
+
+The serial core provides a few helper functions.  This includes
+decoding command line arguments (uart_parse_options()).
+
+There is also a helper function (uart_console_write()) which performs a
+character by character write, translating newlines to CRLF sequences.
+Driver writers are recommended to use this function rather than implementing
+their own version.
+
+
+Locking
+-------
+
+It is the responsibility of the low level hardware driver to perform the
+necessary locking using port->lock.  There are some exceptions (which
+are described in the struct uart_ops listing below.)
+
+There are two locks.  A per-port spinlock, and an overall semaphore.
+
+From the core driver perspective, the port->lock locks the following
+data::
+
+	port->mctrl
+	port->icount
+	port->state->xmit.head (circ_buf->head)
+	port->state->xmit.tail (circ_buf->tail)
+
+The low level driver is free to use this lock to provide any additional
+locking.
+
+The port_sem semaphore is used to protect against ports being added/
+removed or reconfigured at inappropriate times. Since v2.6.27, this
+semaphore has been the 'mutex' member of the tty_port struct, and
+commonly referred to as the port mutex.
+
+
+uart_ops
+--------
+
+.. kernel-doc:: include/linux/serial_core.h
+   :identifiers: uart_ops
+
+Other functions
+---------------
+
+.. kernel-doc:: drivers/tty/serial/serial_core.c
+   :identifiers: uart_update_timeout uart_get_baud_rate uart_get_divisor
+           uart_match_port uart_write_wakeup uart_register_driver
+           uart_unregister_driver uart_suspend_port uart_resume_port
+           uart_add_one_port uart_remove_one_port uart_console_write
+           uart_parse_earlycon uart_parse_options uart_set_options
+           uart_get_lsr_info uart_handle_dcd_change uart_handle_cts_change
+           uart_try_toggle_sysrq
+
+.. kernel-doc:: include/linux/serial_core.h
+   :identifiers: uart_port_tx_limited uart_port_tx
+
+Other notes
+-----------
+
+It is intended some day to drop the 'unused' entries from uart_port, and
+allow low level drivers to register their own individual uart_port's with
+the core.  This will allow drivers to use uart_port as a pointer to a
+structure containing both the uart_port entry with their own extensions,
+thus::
+
+	struct my_port {
+		struct uart_port	port;
+		int			my_stuff;
+	};
+
+Modem control lines via GPIO
+----------------------------
+
+Some helpers are provided in order to set/get modem control lines via GPIO.
+
+.. kernel-doc:: drivers/tty/serial/serial_mctrl_gpio.c
+   :identifiers: mctrl_gpio_init mctrl_gpio_to_gpiod
+           mctrl_gpio_set mctrl_gpio_get mctrl_gpio_enable_ms
+           mctrl_gpio_disable_ms_sync mctrl_gpio_disable_ms_no_sync