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>

2 files changed, 29 insertions, 0 deletions

diff --git a/tools/testing/selftests/locking/Makefile b/tools/testing/selftests/locking/Makefile
new file mode 100644
index 000000000..6e7761ab3
--- /dev/null
+++ b/tools/testing/selftests/locking/Makefile
@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Makefile for locking/ww_mutx selftests
+
+# No binaries, but make sure arg-less "make" doesn't trigger "run_tests"
+all:
+
+TEST_PROGS := ww_mutex.sh
+
+include ../lib.mk
diff --git a/tools/testing/selftests/locking/ww_mutex.sh b/tools/testing/selftests/locking/ww_mutex.sh
new file mode 100755
index 000000000..91e4ac756
--- /dev/null
+++ b/tools/testing/selftests/locking/ww_mutex.sh
@@ -0,0 +1,19 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+
+# Runs API tests for struct ww_mutex (Wait/Wound mutexes)
+if ! /sbin/modprobe -q -n test-ww_mutex; then
+	echo "ww_mutex: module test-ww_mutex is not found [SKIP]"
+	exit $ksft_skip
+fi
+
+if /sbin/modprobe -q test-ww_mutex; then
+       /sbin/modprobe -q -r test-ww_mutex
+       echo "locking/ww_mutex: ok"
+else
+       echo "locking/ww_mutex: [FAIL]"
+       exit 1
+fi