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, 35 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/debugfs-moxtet b/Documentation/ABI/testing/debugfs-moxtet
new file mode 100644
index 000000000..637d8587d
--- /dev/null
+++ b/Documentation/ABI/testing/debugfs-moxtet
@@ -0,0 +1,35 @@
+What:		/sys/kernel/debug/moxtet/input
+Date:		March 2019
+KernelVersion:	5.3
+Contact:	Marek Behún <kabel@kernel.org>
+Description:	(Read) Read input from the shift registers, in hexadecimal.
+		Returns N+1 bytes, where N is the number of Moxtet connected
+		modules. The first byte is from the CPU board itself.
+
+		Example::
+
+			101214
+
+		==  =======================================
+		10  CPU board with SD card
+		12  2 = PCIe module, 1 = IRQ not active
+		14  4 = Peridot module, 1 = IRQ not active
+		==  =======================================
+
+What:		/sys/kernel/debug/moxtet/output
+Date:		March 2019
+KernelVersion:	5.3
+Contact:	Marek Behún <kabel@kernel.org>
+Description:	(RW) Read last written value to the shift registers, in
+		hexadecimal, or write values to the shift registers, also
+		in hexadecimal.
+
+		Example::
+
+		    0102
+
+		==  ================================================
+		01  01 was last written, or is to be written, to the
+		    first module's shift register
+		02  the same for second module
+		==  ================================================