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, 51 insertions, 0 deletions

diff --git a/Documentation/userspace-api/liveupdate.rst b/Documentation/userspace-api/liveupdate.rst
new file mode 100644
index 000000000..cdd74627a
--- /dev/null
+++ b/Documentation/userspace-api/liveupdate.rst
@@ -0,0 +1,51 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================
+Live Update uAPI
+================
+:Author: Pasha Tatashin <pasha.tatashin@soleen.com>
+
+ioctl interface
+===============
+.. kernel-doc:: kernel/liveupdate/luo_core.c
+   :doc: LUO ioctl Interface
+
+ioctl uAPI
+===========
+.. kernel-doc:: include/uapi/linux/liveupdate.h
+
+Note on Compatibility
+=====================
+
+Note that the Live Update feature is still under development and subject to
+change, so compatibility is not guaranteed when kexec rebooting between
+different kernel versions. This is expected to change and stabilize in a future
+version.
+
+Userspace Integration
+=====================
+
+systemd (since version v261) uses LUO to preserve its per-service file
+descriptor store across a kexec-based live update. Services opt in by setting
+``FileDescriptorStoreMax=`` and ``FileDescriptorStorePreserve=`` in their unit,
+and push file descriptors with a name into the store via
+``sd_pid_notify_with_fds(... "FDSTORE=1\nFDNAME=foo")``.
+
+Services may also create their own LUO sessions (via ``/dev/liveupdate``) and
+push the resulting session fds into their file descriptor store like any other
+fd. systemd detects such session fds and handles them accordingly, and
+hands the re-retrieved session fd back to the service after kexec, using the
+existing file descriptor store service interface.
+
+For details, see:
+
+- `File Descriptor Store <https://systemd.io/FILE_DESCRIPTOR_STORE/>`_
+- `systemd.service(5) FileDescriptorStorePreserve=
+  <https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#FileDescriptorStorePreserve=>`_
+- `sd_pid_notify_with_fds(3)
+  <https://www.freedesktop.org/software/systemd/man/latest/sd_pid_notify_with_fds.html>`_
+
+See Also
+========
+
+- :doc:`Live Update Orchestrator </core-api/liveupdate>`