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Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027115420.205279-1-vegard.nossum@oracle.com
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======================
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(Un)patching Callbacks
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======================
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Livepatch (un)patch-callbacks provide a mechanism for livepatch modules
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to execute callback functions when a kernel object is (un)patched. They
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can be considered a **power feature** that **extends livepatching abilities**
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to include:
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- Safe updates to global data
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- "Patches" to init and probe functions
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- Patching otherwise unpatchable code (i.e. assembly)
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In most cases, (un)patch callbacks will need to be used in conjunction
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with memory barriers and kernel synchronization primitives, like
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mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues.
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1. Motivation
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=============
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Callbacks differ from existing kernel facilities:
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- Module init/exit code doesn't run when disabling and re-enabling a
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patch.
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- A module notifier can't stop a to-be-patched module from loading.
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Callbacks are part of the klp_object structure and their implementation
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is specific to that klp_object. Other livepatch objects may or may not
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be patched, irrespective of the target klp_object's current state.
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2. Callback types
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=================
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Callbacks can be registered for the following livepatch actions:
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* Pre-patch
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- before a klp_object is patched
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* Post-patch
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- after a klp_object has been patched and is active
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across all tasks
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* Pre-unpatch
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- before a klp_object is unpatched (ie, patched code is
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active), used to clean up post-patch callback
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resources
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* Post-unpatch
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- after a klp_object has been patched, all code has
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been restored and no tasks are running patched code,
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used to cleanup pre-patch callback resources
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3. How it works
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===============
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Each callback is optional, omitting one does not preclude specifying any
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other. However, the livepatching core executes the handlers in
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symmetry: pre-patch callbacks have a post-unpatch counterpart and
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post-patch callbacks have a pre-unpatch counterpart. An unpatch
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callback will only be executed if its corresponding patch callback was
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executed. Typical use cases pair a patch handler that acquires and
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configures resources with an unpatch handler tears down and releases
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those same resources.
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A callback is only executed if its host klp_object is loaded. For
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in-kernel vmlinux targets, this means that callbacks will always execute
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when a livepatch is enabled/disabled. For patch target kernel modules,
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callbacks will only execute if the target module is loaded. When a
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module target is (un)loaded, its callbacks will execute only if the
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livepatch module is enabled.
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The pre-patch callback, if specified, is expected to return a status
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code (0 for success, -ERRNO on error). An error status code indicates
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to the livepatching core that patching of the current klp_object is not
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safe and to stop the current patching request. (When no pre-patch
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callback is provided, the transition is assumed to be safe.) If a
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pre-patch callback returns failure, the kernel's module loader will:
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- Refuse to load a livepatch, if the livepatch is loaded after
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targeted code.
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or:
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- Refuse to load a module, if the livepatch was already successfully
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loaded.
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No post-patch, pre-unpatch, or post-unpatch callbacks will be executed
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for a given klp_object if the object failed to patch, due to a failed
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pre_patch callback or for any other reason.
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If a patch transition is reversed, no pre-unpatch handlers will be run
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(this follows the previously mentioned symmetry -- pre-unpatch callbacks
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will only occur if their corresponding post-patch callback executed).
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If the object did successfully patch, but the patch transition never
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started for some reason (e.g., if another object failed to patch),
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only the post-unpatch callback will be called.
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4. Use cases
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============
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Sample livepatch modules demonstrating the callback API can be found in
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samples/livepatch/ directory. These samples were modified for use in
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kselftests and can be found in the lib/livepatch directory.
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Global data update
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------------------
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A pre-patch callback can be useful to update a global variable. For
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example, commit 75ff39ccc1bd ("tcp: make challenge acks less predictable")
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changes a global sysctl, as well as patches the tcp_send_challenge_ack()
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function.
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In this case, if we're being super paranoid, it might make sense to
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patch the data *after* patching is complete with a post-patch callback,
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so that tcp_send_challenge_ack() could first be changed to read
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sysctl_tcp_challenge_ack_limit with READ_ONCE.
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__init and probe function patches support
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-----------------------------------------
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Although __init and probe functions are not directly livepatch-able, it
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may be possible to implement similar updates via pre/post-patch
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callbacks.
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The commit 48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST") change the way that
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virtnet_probe() initialized its driver's net_device features. A
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pre/post-patch callback could iterate over all such devices, making a
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similar change to their hw_features value. (Client functions of the
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value may need to be updated accordingly.)
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