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========= 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 5. Livepatch life-cycle 5.1. Loading 5.2. Enabling 5.3. Replacing 5.4. Disabling 5.5. Removing 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. 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/

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