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diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt new file mode 100644 index 000000000..6c43f6ebe --- /dev/null +++ b/Documentation/livepatch/livepatch.txt @@ -0,0 +1,394 @@ +========= +Livepatch +========= + +This document outlines basic information about kernel livepatching. + +Table of Contents: + +1. Motivation +2. Kprobes, Ftrace, Livepatching +3. Consistency model +4. Livepatch module + 4.1. New functions + 4.2. Metadata + 4.3. Livepatch module handling +5. Livepatch life-cycle + 5.1. Registration + 5.2. Enabling + 5.3. Disabling + 5.4. Unregistration +6. Sysfs +7. Limitations + + +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. The theory is not yet finished. See the discussion at +http://thread.gmane.org/gmane.linux.kernel/1823033/focus=1828189 + +The current consistency model is very simple. It guarantees that either +the old or the new function is called. But various functions get redirected +one by one without any synchronization. + +In other words, the current implementation _never_ modifies the behavior +in the middle of the call. It is because it does _not_ rewrite the entire +function in the memory. Instead, the function gets redirected at the +very beginning. But this redirection is used immediately even when +some other functions from the same patch have not been redirected yet. + +See also the section "Limitations" below. + + +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.txt 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. Also if a more complex + consistency model is supported then a selected unit (thread, + kernel as a whole) will see the new code from the entire patch + only when it is in a safe state. + + +4.3. Livepatch module handling +------------------------------ + +The usual behavior is that the new functions will get used when +the livepatch module is loaded. For this, the module init() function +has to register the patch (struct klp_patch) and enable it. See the +section "Livepatch life-cycle" below for more details about these +two operations. + +Module removal is only safe when there are no users of the underlying +functions. The immediate consistency model is not able to detect this; +therefore livepatch modules cannot be removed. See "Limitations" below. + +5. Livepatch life-cycle +======================= + +Livepatching defines four basic operations that define the life cycle of each +live patch: registration, enabling, disabling and unregistration. There are +several reasons why it is done this way. + +First, the patch is applied only when all patched symbols for already +loaded objects are found. The error handling is much easier if this +check is done before particular functions get redirected. + +Second, the immediate consistency model does not guarantee that anyone is not +sleeping in the new code after the patch is reverted. This means that the new +code needs to stay around "forever". If the code is there, one could apply it +again. Therefore it makes sense to separate the operations that might be done +once and those that need to be repeated when the patch is enabled (applied) +again. + +Third, it might take some time until the entire system is migrated +when a more complex consistency model is used. The patch revert might +block the livepatch module removal for too long. Therefore it is useful +to revert the patch using a separate operation that might be called +explicitly. But it does not make sense to remove all information +until the livepatch module is really removed. + + +5.1. Registration +----------------- + +Each patch first has to be registered using klp_register_patch(). This makes +the patch known to the livepatch framework. Also it does some preliminary +computing and checks. + +In particular, the patch is added into the list of known patches. 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 operation +fails. + + +5.2. Enabling +------------- + +Registered patches might be enabled either by calling klp_enable_patch() or +by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will +start using the new implementation of the patched functions at this stage. + +In particular, 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. + +Functions might be patched multiple times. The ftrace handler is registered +only once for the given function. Further patches just add an entry to the +list (see field `func_stack`) of the struct klp_ops. The last added +entry is chosen by the ftrace handler and becomes the active function +replacement. + +Note that the patches might be enabled in a different order than they were +registered. + + +5.3. Disabling +-------------- + +Enabled patches might get disabled either by calling klp_disable_patch() or +by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage +either the code from the previously enabled patch or even the original +code gets used. + +Here 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. + +Patches must be disabled in exactly the reverse order in which they were +enabled. It makes the problem and the implementation much easier. + + +5.4. Unregistration +------------------- + +Disabled patches might be unregistered by calling klp_unregister_patch(). +This can be done only when the patch is disabled and the code is no longer +used. It must be called before the livepatch module gets unloaded. + +At this stage, all the relevant sys-fs entries are removed and the patch +is removed from the list of known patches. + + +6. Sysfs +======== + +Information about the registered patches can be found under +/sys/kernel/livepatch. The patches could be enabled and disabled +by writing there. + +See Documentation/ABI/testing/sysfs-kernel-livepatch for more details. + + +7. Limitations +============== + +The current Livepatch implementation has several limitations: + + + + The patch must not change the semantic of the patched functions. + + The current implementation guarantees only that either the old + or the new function is called. The functions are patched one + by one. It means that the patch must _not_ change the semantic + of the function. + + + + Data structures can not be patched. + + There is no support to version data structures or anyhow migrate + one structure into another. Also the simple consistency model does + not allow to switch more functions atomically. + + Once there is more complex consistency mode, it will be possible to + use some workarounds. For example, it will be possible to use a hole + for a new member because the data structure is aligned. Or it will + be possible to use an existing member for something else. + + There are no plans to add more generic support for modified structures + at the moment. + + + + 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". + + + + Anything inlined into __schedule() can not be patched. + + The switch_to macro is inlined into __schedule(). It switches the + context between two processes in the middle of the macro. It does + not save RIP in x86_64 version (contrary to 32-bit version). Instead, + the currently used __schedule()/switch_to() handles both processes. + + Now, let's have two different tasks. One calls the original + __schedule(), its registers are stored in a defined order and it + goes to sleep in the switch_to macro and some other task is restored + using the original __schedule(). Then there is the second task which + calls patched__schedule(), it goes to sleep there and the first task + is picked by the patched__schedule(). Its RSP is restored and now + the registers should be restored as well. But the order is different + in the new patched__schedule(), so... + + There is work in progress to remove this limitation. + + + + Livepatch modules can not be removed. + + The current implementation just redirects the functions at the very + beginning. It does not check if the functions are in use. In other + words, it knows when the functions get called but it does not + know when the functions return. Therefore it can not decide when + the livepatch module can be safely removed. + + This will get most likely solved once a more complex consistency model + is supported. The idea is that a safe state for patching should also + mean a safe state for removing the patch. + + Note that the patch itself might get disabled by writing zero + to /sys/kernel/livepatch/<patch>/enabled. It causes that the new + code will not longer get called. But it does not guarantee + that anyone is not sleeping anywhere in the new code. + + + + 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 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. |