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, 133 insertions, 0 deletions

diff --git a/Documentation/livepatch/callbacks.rst b/Documentation/livepatch/callbacks.rst
new file mode 100644
index 000000000..914445784
--- /dev/null
+++ b/Documentation/livepatch/callbacks.rst
@@ -0,0 +1,133 @@
+======================
+(Un)patching Callbacks
+======================
+
+Livepatch (un)patch-callbacks provide a mechanism for livepatch modules
+to execute callback functions when a kernel object is (un)patched.  They
+can be considered a **power feature** that **extends livepatching abilities**
+to include:
+
+  - Safe updates to global data
+
+  - "Patches" to init and probe functions
+
+  - Patching otherwise unpatchable code (i.e. assembly)
+
+In most cases, (un)patch callbacks will need to be used in conjunction
+with memory barriers and kernel synchronization primitives, like
+mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues.
+
+1. Motivation
+=============
+
+Callbacks differ from existing kernel facilities:
+
+  - Module init/exit code doesn't run when disabling and re-enabling a
+    patch.
+
+  - A module notifier can't stop a to-be-patched module from loading.
+
+Callbacks are part of the klp_object structure and their implementation
+is specific to that klp_object.  Other livepatch objects may or may not
+be patched, irrespective of the target klp_object's current state.
+
+2. Callback types
+=================
+
+Callbacks can be registered for the following livepatch actions:
+
+  * Pre-patch
+                 - before a klp_object is patched
+
+  * Post-patch
+                 - after a klp_object has been patched and is active
+                   across all tasks
+
+  * Pre-unpatch
+                 - before a klp_object is unpatched (ie, patched code is
+                   active), used to clean up post-patch callback
+                   resources
+
+  * Post-unpatch
+                 - after a klp_object has been patched, all code has
+                   been restored and no tasks are running patched code,
+                   used to cleanup pre-patch callback resources
+
+3. How it works
+===============
+
+Each callback is optional, omitting one does not preclude specifying any
+other.  However, the livepatching core executes the handlers in
+symmetry: pre-patch callbacks have a post-unpatch counterpart and
+post-patch callbacks have a pre-unpatch counterpart.  An unpatch
+callback will only be executed if its corresponding patch callback was
+executed.  Typical use cases pair a patch handler that acquires and
+configures resources with an unpatch handler tears down and releases
+those same resources.
+
+A callback is only executed if its host klp_object is loaded.  For
+in-kernel vmlinux targets, this means that callbacks will always execute
+when a livepatch is enabled/disabled.  For patch target kernel modules,
+callbacks will only execute if the target module is loaded.  When a
+module target is (un)loaded, its callbacks will execute only if the
+livepatch module is enabled.
+
+The pre-patch callback, if specified, is expected to return a status
+code (0 for success, -ERRNO on error).  An error status code indicates
+to the livepatching core that patching of the current klp_object is not
+safe and to stop the current patching request.  (When no pre-patch
+callback is provided, the transition is assumed to be safe.)  If a
+pre-patch callback returns failure, the kernel's module loader will:
+
+  - Refuse to load a livepatch, if the livepatch is loaded after
+    targeted code.
+
+    or:
+
+  - Refuse to load a module, if the livepatch was already successfully
+    loaded.
+
+No post-patch, pre-unpatch, or post-unpatch callbacks will be executed
+for a given klp_object if the object failed to patch, due to a failed
+pre_patch callback or for any other reason.
+
+If a patch transition is reversed, no pre-unpatch handlers will be run
+(this follows the previously mentioned symmetry -- pre-unpatch callbacks
+will only occur if their corresponding post-patch callback executed).
+
+If the object did successfully patch, but the patch transition never
+started for some reason (e.g., if another object failed to patch),
+only the post-unpatch callback will be called.
+
+4. Use cases
+============
+
+Sample livepatch modules demonstrating the callback API can be found in
+samples/livepatch/ directory.  These samples were modified for use in
+kselftests and can be found in the lib/livepatch directory.
+
+Global data update
+------------------
+
+A pre-patch callback can be useful to update a global variable.  For
+example, commit 75ff39ccc1bd ("tcp: make challenge acks less predictable")
+changes a global sysctl, as well as patches the tcp_send_challenge_ack()
+function.
+
+In this case, if we're being super paranoid, it might make sense to
+patch the data *after* patching is complete with a post-patch callback,
+so that tcp_send_challenge_ack() could first be changed to read
+sysctl_tcp_challenge_ack_limit with READ_ONCE.
+
+__init and probe function patches support
+-----------------------------------------
+
+Although __init and probe functions are not directly livepatch-able, it
+may be possible to implement similar updates via pre/post-patch
+callbacks.
+
+The commit 48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST") change the way that
+virtnet_probe() initialized its driver's net_device features.  A
+pre/post-patch callback could iterate over all such devices, making a
+similar change to their hw_features value.  (Client functions of the
+value may need to be updated accordingly.)