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, 202 insertions, 0 deletions

diff --git a/Documentation/process/kernel-driver-statement.rst b/Documentation/process/kernel-driver-statement.rst
new file mode 100644
index 000000000..a849790a6
--- /dev/null
+++ b/Documentation/process/kernel-driver-statement.rst
@@ -0,0 +1,202 @@
+.. _process_statement_driver:
+
+Kernel Driver Statement
+-----------------------
+
+Position Statement on Linux Kernel Modules
+==========================================
+
+
+We, the undersigned Linux kernel developers, consider any closed-source
+Linux kernel module or driver to be harmful and undesirable. We have
+repeatedly found them to be detrimental to Linux users, businesses, and
+the greater Linux ecosystem. Such modules negate the openness,
+stability, flexibility, and maintainability of the Linux development
+model and shut their users off from the expertise of the Linux
+community. Vendors that provide closed-source kernel modules force their
+customers to give up key Linux advantages or choose new vendors.
+Therefore, in order to take full advantage of the cost savings and
+shared support benefits open source has to offer, we urge vendors to
+adopt a policy of supporting their customers on Linux with open-source
+kernel code.
+
+We speak only for ourselves, and not for any company we might work for
+today, have in the past, or will in the future.
+
+ - Dave Airlie
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