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 =========
 Livepatch
 =========
 
 This document outlines basic information about kernel livepatching.
 
 .. Table of Contents:
 
 .. contents:: :local:
 
 
 1. Motivation
 =============
 
 There are many situations where users are reluctant to reboot a system. It may
 be because their system is performing complex scientific computations or under
 heavy load during peak usage. In addition to keeping systems up and running,
 users want to also have a stable and secure system. Livepatching gives users
 both by allowing for function calls to be redirected; thus, fixing critical
 functions without a system reboot.
 
 
 2. Kprobes, Ftrace, Livepatching
 ================================
 
 There are multiple mechanisms in the Linux kernel that are directly related
 to redirection of code execution; namely: kernel probes, function tracing,
 and livepatching:
 
   - The kernel probes are the most generic. The code can be redirected by
     putting a breakpoint instruction instead of any instruction.
 
   - The function tracer calls the code from a predefined location that is
     close to the function entry point. This location is generated by the
     compiler using the '-pg' gcc option.
 
   - Livepatching typically needs to redirect the code at the very beginning
     of the function entry before the function parameters or the stack
     are in any way modified.
 
 All three approaches need to modify the existing code at runtime. Therefore
 they need to be aware of each other and not step over each other's toes.
 Most of these problems are solved by using the dynamic ftrace framework as
 a base. A Kprobe is registered as a ftrace handler when the function entry
 is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
 a live patch is called with the help of a custom ftrace handler. But there are
 some limitations, see below.
 
 
 3. Consistency model
 ====================
 
 Functions are there for a reason. They take some input parameters, get or
 release locks, read, process, and even write some data in a defined way,
 have return values. In other words, each function has a defined semantic.
 
 Many fixes do not change the semantic of the modified functions. For
 example, they add a NULL pointer or a boundary check, fix a race by adding
 a missing memory barrier, or add some locking around a critical section.
 Most of these changes are self contained and the function presents itself
 the same way to the rest of the system. In this case, the functions might
 be updated independently one by one.
 
 But there are more complex fixes. For example, a patch might change
 ordering of locking in multiple functions at the same time. Or a patch
 might exchange meaning of some temporary structures and update
 all the relevant functions. In this case, the affected unit
 (thread, whole kernel) need to start using all new versions of
 the functions at the same time. Also the switch must happen only
 when it is safe to do so, e.g. when the affected locks are released
 or no data are stored in the modified structures at the moment.
 
 The theory about how to apply functions a safe way is rather complex.
 The aim is to define a so-called consistency model. It attempts to define
 conditions when the new implementation could be used so that the system
 stays consistent.
 
 Livepatch has a consistency model which is a hybrid of kGraft and
 kpatch:  it uses kGraft's per-task consistency and syscall barrier
 switching combined with kpatch's stack trace switching.  There are also
 a number of fallback options which make it quite flexible.
 
 Patches are applied on a per-task basis, when the task is deemed safe to
 switch over.  When a patch is enabled, livepatch enters into a
 transition state where tasks are converging to the patched state.
 Usually this transition state can complete in a few seconds.  The same
 sequence occurs when a patch is disabled, except the tasks converge from
 the patched state to the unpatched state.
 
 An interrupt handler inherits the patched state of the task it
 interrupts.  The same is true for forked tasks: the child inherits the
 patched state of the parent.
 
 Livepatch uses several complementary approaches to determine when it's
 safe to patch tasks:
 
 1. The first and most effective approach is stack checking of sleeping
    tasks.  If no affected functions are on the stack of a given task,
    the task is patched.  In most cases this will patch most or all of
    the tasks on the first try.  Otherwise it'll keep trying
    periodically.  This option is only available if the architecture has
    reliable stacks (HAVE_RELIABLE_STACKTRACE).
 
 2. The second approach, if needed, is kernel exit switching.  A
    task is switched when it returns to user space from a system call, a
    user space IRQ, or a signal.  It's useful in the following cases:
 
    a) Patching I/O-bound user tasks which are sleeping on an affected
       function.  In this case you have to send SIGSTOP and SIGCONT to
       force it to exit the kernel and be patched.
    b) Patching CPU-bound user tasks.  If the task is highly CPU-bound
       then it will get patched the next time it gets interrupted by an
       IRQ.
 
 3. For idle "swapper" tasks, since they don't ever exit the kernel, they
    instead have a klp_update_patch_state() call in the idle loop which
    allows them to be patched before the CPU enters the idle state.
 
    (Note there's not yet such an approach for kthreads.)
 
 Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on
 the second approach. It's highly likely that some tasks may still be
 running with an old version of the function, until that function
 returns. In this case you would have to signal the tasks. This
 especially applies to kthreads. They may not be woken up and would need
 to be forced. See below for more information.
 
 Unless we can come up with another way to patch kthreads, architectures
 without HAVE_RELIABLE_STACKTRACE are not considered fully supported by
 the kernel livepatching.
 
 The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
 is in transition.  Only a single patch can be in transition at a given
 time.  A patch can remain in transition indefinitely, if any of the tasks
 are stuck in the initial patch state.
 
 A transition can be reversed and effectively canceled by writing the
 opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
 the transition is in progress.  Then all the tasks will attempt to
 converge back to the original patch state.
 
 There's also a /proc/<pid>/patch_state file which can be used to
 determine which tasks are blocking completion of a patching operation.
 If a patch is in transition, this file shows 0 to indicate the task is
 unpatched and 1 to indicate it's patched.  Otherwise, if no patch is in
 transition, it shows -1.  Any tasks which are blocking the transition
 can be signaled with SIGSTOP and SIGCONT to force them to change their
 patched state. This may be harmful to the system though. Sending a fake signal
 to all remaining blocking tasks is a better alternative. No proper signal is
 actually delivered (there is no data in signal pending structures). Tasks are
 interrupted or woken up, and forced to change their patched state. The fake
 signal is automatically sent every 15 seconds.
 
 Administrator can also affect a transition through
 /sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears
 TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched
 state. Important note! The force attribute is intended for cases when the
 transition gets stuck for a long time because of a blocking task. Administrator
 is expected to collect all necessary data (namely stack traces of such blocking
 tasks) and request a clearance from a patch distributor to force the transition.
 Unauthorized usage may cause harm to the system. It depends on the nature of the
 patch, which functions are (un)patched, and which functions the blocking tasks
 are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch
 modules is permanently disabled when the force feature is used. It cannot be
 guaranteed there is no task sleeping in such module. It implies unbounded
 reference count if a patch module is disabled and enabled in a loop.
 
 Moreover, the usage of force may also affect future applications of live
 patches and cause even more harm to the system. Administrator should first
 consider to simply cancel a transition (see above). If force is used, reboot
 should be planned and no more live patches applied.
 
 3.1 Adding consistency model support to new architectures
 ---------------------------------------------------------
 
 For adding consistency model support to new architectures, there are a
 few options:
 
 1) Add CONFIG_HAVE_RELIABLE_STACKTRACE.  This means porting objtool, and
    for non-DWARF unwinders, also making sure there's a way for the stack
    tracing code to detect interrupts on the stack.
 
 2) Alternatively, ensure that every kthread has a call to
    klp_update_patch_state() in a safe location.  Kthreads are typically
    in an infinite loop which does some action repeatedly.  The safe
    location to switch the kthread's patch state would be at a designated
    point in the loop where there are no locks taken and all data
    structures are in a well-defined state.
 
    The location is clear when using workqueues or the kthread worker
    API.  These kthreads process independent actions in a generic loop.
 
    It's much more complicated with kthreads which have a custom loop.
    There the safe location must be carefully selected on a case-by-case
    basis.
 
    In that case, arches without HAVE_RELIABLE_STACKTRACE would still be
    able to use the non-stack-checking parts of the consistency model:
 
    a) patching user tasks when they cross the kernel/user space
       boundary; and
 
    b) patching kthreads and idle tasks at their designated patch points.
 
    This option isn't as good as option 1 because it requires signaling
    user tasks and waking kthreads to patch them.  But it could still be
    a good backup option for those architectures which don't have
    reliable stack traces yet.
 
 
 4. Livepatch module
 ===================
 
 Livepatches are distributed using kernel modules, see
 samples/livepatch/livepatch-sample.c.
 
 The module includes a new implementation of functions that we want
 to replace. In addition, it defines some structures describing the
 relation between the original and the new implementation. Then there
 is code that makes the kernel start using the new code when the livepatch
 module is loaded. Also there is code that cleans up before the
 livepatch module is removed. All this is explained in more details in
 the next sections.
 
 
 4.1. New functions
 ------------------
 
 New versions of functions are typically just copied from the original
 sources. A good practice is to add a prefix to the names so that they
 can be distinguished from the original ones, e.g. in a backtrace. Also
 they can be declared as static because they are not called directly
 and do not need the global visibility.
 
 The patch contains only functions that are really modified. But they
 might want to access functions or data from the original source file
 that may only be locally accessible. This can be solved by a special
 relocation section in the generated livepatch module, see
 Documentation/livepatch/module-elf-format.rst for more details.
 
 
 4.2. Metadata
 -------------
 
 The patch is described by several structures that split the information
 into three levels:
 
   - struct klp_func is defined for each patched function. It describes
     the relation between the original and the new implementation of a
     particular function.
 
     The structure includes the name, as a string, of the original function.
     The function address is found via kallsyms at runtime.
 
     Then it includes the address of the new function. It is defined
     directly by assigning the function pointer. Note that the new
     function is typically defined in the same source file.
 
     As an optional parameter, the symbol position in the kallsyms database can
     be used to disambiguate functions of the same name. This is not the
     absolute position in the database, but rather the order it has been found
     only for a particular object ( vmlinux or a kernel module ). Note that
     kallsyms allows for searching symbols according to the object name.
 
   - struct klp_object defines an array of patched functions (struct
     klp_func) in the same object. Where the object is either vmlinux
     (NULL) or a module name.
 
     The structure helps to group and handle functions for each object
     together. Note that patched modules might be loaded later than
     the patch itself and the relevant functions might be patched
     only when they are available.
 
 
   - struct klp_patch defines an array of patched objects (struct
     klp_object).
 
     This structure handles all patched functions consistently and eventually,
     synchronously. The whole patch is applied only when all patched
     symbols are found. The only exception are symbols from objects
     (kernel modules) that have not been loaded yet.
 
     For more details on how the patch is applied on a per-task basis,
     see the "Consistency model" section.
 
 
 5. Livepatch life-cycle
 =======================
 
 Livepatching can be described by five basic operations:
 loading, enabling, replacing, disabling, removing.
 
 Where the replacing and the disabling operations are mutually
 exclusive. They have the same result for the given patch but
 not for the system.
 
 
 5.1. Loading
 ------------
 
 The only reasonable way is to enable the patch when the livepatch kernel
 module is being loaded. For this, klp_enable_patch() has to be called
 in the module_init() callback. There are two main reasons:
 
 First, only the module has an easy access to the related struct klp_patch.
 
 Second, the error code might be used to refuse loading the module when
 the patch cannot get enabled.
 
 
 5.2. Enabling
 -------------
 
 The livepatch gets enabled by calling klp_enable_patch() from
 the module_init() callback. The system will start using the new
 implementation of the patched functions at this stage.
 
 First, the addresses of the patched functions are found according to their
 names. The special relocations, mentioned in the section "New functions",
 are applied. The relevant entries are created under
 /sys/kernel/livepatch/<name>. The patch is rejected when any above
 operation fails.
 
 Second, livepatch enters into a transition state where tasks are converging
 to the patched state. If an original function is patched for the first
 time, a function specific struct klp_ops is created and an universal
 ftrace handler is registered\ [#]_. This stage is indicated by a value of '1'
 in /sys/kernel/livepatch/<name>/transition. For more information about
 this process, see the "Consistency model" section.
 
 Finally, once all tasks have been patched, the 'transition' value changes
 to '0'.
 
 .. [#]
 
     Note that functions might be patched multiple times. The ftrace handler
     is registered only once for a given function. Further patches just add
     an entry to the list (see field `func_stack`) of the struct klp_ops.
     The right implementation is selected by the ftrace handler, see
     the "Consistency model" section.
 
     That said, it is highly recommended to use cumulative livepatches
     because they help keeping the consistency of all changes. In this case,
     functions might be patched two times only during the transition period.
 
 
 5.3. Replacing
 --------------
 
 All enabled patches might get replaced by a cumulative patch that
 has the .replace flag set.
 
 Once the new patch is enabled and the 'transition' finishes then
 all the functions (struct klp_func) associated with the replaced
 patches are removed from the corresponding struct klp_ops. Also
 the ftrace handler is unregistered and the struct klp_ops is
 freed when the related function is not modified by the new patch
 and func_stack list becomes empty.
 
 See Documentation/livepatch/cumulative-patches.rst for more details.
 
 
 5.4. Disabling
 --------------
 
 Enabled patches might get disabled by writing '0' to
 /sys/kernel/livepatch/<name>/enabled.
 
 First, livepatch enters into a transition state where tasks are converging
 to the unpatched state. The system starts using either the code from
 the previously enabled patch or even the original one. This stage is
 indicated by a value of '1' in /sys/kernel/livepatch/<name>/transition.
 For more information about this process, see the "Consistency model"
 section.
 
 Second, once all tasks have been unpatched, the 'transition' value changes
 to '0'. All the functions (struct klp_func) associated with the to-be-disabled
 patch are removed from the corresponding struct klp_ops. The ftrace handler
 is unregistered and the struct klp_ops is freed when the func_stack list
 becomes empty.
 
 Third, the sysfs interface is destroyed.
 
 
 5.5. Removing
 -------------
 
 Module removal is only safe when there are no users of functions provided
 by the module. This is the reason why the force feature permanently
 disables the removal. Only when the system is successfully transitioned
 to a new patch state (patched/unpatched) without being forced it is
 guaranteed that no task sleeps or runs in the old code.
 
 
 6. Sysfs
 ========
 
 Information about the registered patches can be found under
 /sys/kernel/livepatch. The patches could be enabled and disabled
 by writing there.
 
 /sys/kernel/livepatch/<patch>/force attributes allow administrator to affect a
 patching operation.
 
 See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
 
 
 7. Limitations
 ==============
 
 The current Livepatch implementation has several limitations:
 
   - Only functions that can be traced could be patched.
 
     Livepatch is based on the dynamic ftrace. In particular, functions
     implementing ftrace or the livepatch ftrace handler could not be
     patched. Otherwise, the code would end up in an infinite loop. A
     potential mistake is prevented by marking the problematic functions
     by "notrace".
 
 
 
   - Livepatch works reliably only when the dynamic ftrace is located at
     the very beginning of the function.
 
     The function need to be redirected before the stack or the function
     parameters are modified in any way. For example, livepatch requires
     using -fentry gcc compiler option on x86_64.
 
     One exception is the PPC port. It uses relative addressing and TOC.
     Each function has to handle TOC and save LR before it could call
     the ftrace handler. This operation has to be reverted on return.
     Fortunately, the generic ftrace code has the same problem and all
     this is handled on the ftrace level.
 
 
   - Kretprobes using the ftrace framework conflict with the patched
     functions.
 
     Both kretprobes and livepatches use a ftrace handler that modifies
     the return address. The first user wins. Either the probe or the patch
     is rejected when the handler is already in use by the other.
 
 
   - Kprobes in the original function are ignored when the code is
     redirected to the new implementation.
 
     There is a work in progress to add warnings about this situation.

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