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, 167 insertions, 0 deletions

diff --git a/Documentation/livepatch/system-state.rst b/Documentation/livepatch/system-state.rst
new file mode 100644
index 000000000..7a3935fd8
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+++ b/Documentation/livepatch/system-state.rst
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+====================
+System State Changes
+====================
+
+Some users are really reluctant to reboot a system. This brings the need
+to provide more livepatches and maintain some compatibility between them.
+
+Maintaining more livepatches is much easier with cumulative livepatches.
+Each new livepatch completely replaces any older one. It can keep,
+add, and even remove fixes. And it is typically safe to replace any version
+of the livepatch with any other one thanks to the atomic replace feature.
+
+The problems might come with shadow variables and callbacks. They might
+change the system behavior or state so that it is no longer safe to
+go back and use an older livepatch or the original kernel code. Also
+any new livepatch must be able to detect what changes have already been
+done by the already installed livepatches.
+
+This is where the livepatch system state tracking gets useful. It
+allows to:
+
+  - store data needed to manipulate and restore the system state
+
+  - define compatibility between livepatches using a change id
+    and version
+
+
+1. Livepatch system state API
+=============================
+
+The state of the system might get modified either by several livepatch callbacks
+or by the newly used code. Also it must be possible to find changes done by
+already installed livepatches.
+
+Each modified state is described by struct klp_state, see
+include/linux/livepatch.h.
+
+Each livepatch defines an array of struct klp_states. They mention
+all states that the livepatch modifies.
+
+The livepatch author must define the following two fields for each
+struct klp_state:
+
+  - *id*
+
+    - Non-zero number used to identify the affected system state.
+
+  - *version*
+
+    - Number describing the variant of the system state change that
+      is supported by the given livepatch.
+
+The state can be manipulated using two functions:
+
+  - klp_get_state()
+
+    - Get struct klp_state associated with the given livepatch
+      and state id.
+
+  - klp_get_prev_state()
+
+    - Get struct klp_state associated with the given feature id and
+      already installed livepatches.
+
+2. Livepatch compatibility
+==========================
+
+The system state version is used to prevent loading incompatible livepatches.
+The check is done when the livepatch is enabled. The rules are:
+
+  - Any completely new system state modification is allowed.
+
+  - System state modifications with the same or higher version are allowed
+    for already modified system states.
+
+  - Cumulative livepatches must handle all system state modifications from
+    already installed livepatches.
+
+  - Non-cumulative livepatches are allowed to touch already modified
+    system states.
+
+3. Supported scenarios
+======================
+
+Livepatches have their life-cycle and the same is true for the system
+state changes. Every compatible livepatch has to support the following
+scenarios:
+
+  - Modify the system state when the livepatch gets enabled and the state
+    has not been already modified by a livepatches that are being
+    replaced.
+
+  - Take over or update the system state modification when is has already
+    been done by a livepatch that is being replaced.
+
+  - Restore the original state when the livepatch is disabled.
+
+  - Restore the previous state when the transition is reverted.
+    It might be the original system state or the state modification
+    done by livepatches that were being replaced.
+
+  - Remove any already made changes when error occurs and the livepatch
+    cannot get enabled.
+
+4. Expected usage
+=================
+
+System states are usually modified by livepatch callbacks. The expected
+role of each callback is as follows:
+
+*pre_patch()*
+
+  - Allocate *state->data* when necessary. The allocation might fail
+    and *pre_patch()* is the only callback that could stop loading
+    of the livepatch. The allocation is not needed when the data
+    are already provided by previously installed livepatches.
+
+  - Do any other preparatory action that is needed by
+    the new code even before the transition gets finished.
+    For example, initialize *state->data*.
+
+    The system state itself is typically modified in *post_patch()*
+    when the entire system is able to handle it.
+
+  - Clean up its own mess in case of error. It might be done by a custom
+    code or by calling *post_unpatch()* explicitly.
+
+*post_patch()*
+
+  - Copy *state->data* from the previous livepatch when they are
+    compatible.
+
+  - Do the actual system state modification. Eventually allow
+    the new code to use it.
+
+  - Make sure that *state->data* has all necessary information.
+
+  - Free *state->data* from replaces livepatches when they are
+    not longer needed.
+
+*pre_unpatch()*
+
+  - Prevent the code, added by the livepatch, relying on the system
+    state change.
+
+  - Revert the system state modification..
+
+*post_unpatch()*
+
+  - Distinguish transition reverse and livepatch disabling by
+    checking *klp_get_prev_state()*.
+
+  - In case of transition reverse, restore the previous system
+    state. It might mean doing nothing.
+
+  - Remove any not longer needed setting or data.
+
+.. note::
+
+   *pre_unpatch()* typically does symmetric operations to *post_patch()*.
+   Except that it is called only when the livepatch is being disabled.
+   Therefore it does not need to care about any previously installed
+   livepatch.
+
+   *post_unpatch()* typically does symmetric operations to *pre_patch()*.
+   It might be called also during the transition reverse. Therefore it
+   has to handle the state of the previously installed livepatches.