Merge branches 'for-4.12/upstream' and 'for-4.12/klp-hybrid-consistency-model' into for-linus

		code is currently applied.  Writing 0 will disable the patch

		while writing 1 will re-enable the patch.

What:		/sys/kernel/livepatch/<patch>/transition

Date:		Feb 2017

KernelVersion:	4.12.0

Contact:	live-patching@vger.kernel.org

Description:

		An attribute which indicates whether the patch is currently in

		transition.

What:		/sys/kernel/livepatch/<patch>/<object>

Date:		Nov 2014

KernelVersion:	3.19.0

  3.8   /proc/<pid>/fdinfo/<fd> - Information about opened file

  3.9   /proc/<pid>/map_files - Information about memory mapped files

  3.10  /proc/<pid>/timerslack_ns - Task timerslack value

  3.11	/proc/<pid>/patch_state - Livepatch patch operation state

  4	Configuring procfs

  4.1	Mount options

An application setting the value must have PTRACE_MODE_ATTACH_FSCREDS level

permissions on the task specified to change its timerslack_ns value.

3.11	/proc/<pid>/patch_state - Livepatch patch operation state

-----------------------------------------------------------------

When CONFIG_LIVEPATCH is enabled, this file displays the value of the

patch state for the task.

A value of '-1' indicates that no patch is in transition.

A value of '0' indicates that a patch is in transition and the task is

unpatched.  If the patch is being enabled, then the task hasn't been

patched yet.  If the patch is being disabled, then the task has already

been unpatched.

A value of '1' indicates that a patch is in transition and the task is

patched.  If the patch is being enabled, then the task has already been

patched.  If the patch is being disabled, then the task hasn't been

unpatched yet.

------------------------------------------------------------------------------

Configuring procfs

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.

be updated independently one by one.  (This can be done by setting the

'immediate' flag in the klp_patch struct.)

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

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

https://lkml.kernel.org/r/20141107140458.GA21774@suse.cz

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.

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.

   c) In the future it could be useful for applying patches for

      architectures which don't yet have HAVE_RELIABLE_STACKTRACE.  In

      this case you would have to signal most of the tasks on the

      system.  However this isn't supported yet because there's

      currently no way to patch kthreads without

      HAVE_RELIABLE_STACKTRACE.

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.)

All the above approaches may be skipped by setting the 'immediate' flag

in the 'klp_patch' struct, which will disable per-task consistency and

patch all tasks immediately.  This can be useful if the patch doesn't

change any function or data semantics.  Note that, even with this flag

set, it's possible that some tasks may still be running with an old

version of the function, until that function returns.

There's also an 'immediate' flag in the 'klp_func' struct which allows

you to specify that certain functions in the patch can be applied

without per-task consistency.  This might be useful if you want to patch

a common function like schedule(), and the function change doesn't need

consistency but the rest of the patch does.

For architectures which don't have HAVE_RELIABLE_STACKTRACE, the user

must set patch->immediate which causes all tasks to be patched

immediately.  This option should be used with care, only when the patch

doesn't change any function or data semantics.

In the future, architectures which don't have HAVE_RELIABLE_STACKTRACE

may be allowed to use per-task consistency if we can come up with

another way to patch kthreads.

The /sys/kernel/livepatch/<patch>/transition file shows whether a patch

is in transition.  Only a single patch (the topmost patch on the stack)

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.

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.

In the meantime, patches for such architectures can bypass the

consistency model by setting klp_patch.immediate to true.  This option

is perfectly fine for patches which don't change the semantics of the

patched functions.  In practice, this is usable for ~90% of security

fixes.  Use of this option also means the patch can't be unloaded after

it has been disabled.

4. Livepatch module

4.2. Metadata

------------

-------------

The patch is described by several structures that split the information

into three levels:

    only for a particular object ( vmlinux or a kernel module ). Note that

    kallsyms allows for searching symbols according to the object name.

    There's also an 'immediate' flag which, when set, patches the

    function immediately, bypassing the consistency model safety checks.

  + 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.

    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.

    (kernel modules) that have not been loaded yet.

    Setting the 'immediate' flag applies the patch to all tasks

    immediately, bypassing the consistency model safety checks.

    For more details on how the patch is applied on a per-task basis,

    see the "Consistency model" section.

4.3. Livepatch module handling

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.

functions. The immediate consistency model is not able to detect this. The

code just redirects the functions at the very beginning and 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 cannot be decided when the livepatch module can be safely

removed. This is solved by a hybrid consistency model. When the system is

transitioned to a new patch state (patched/unpatched) it is guaranteed that

no task sleeps or runs in the old code.

5. Livepatch life-cycle

=======================

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.

When a patch is enabled, livepatch enters into a transition state where

tasks are converging to the patched state.  This is indicated by a value

of '1' in /sys/kernel/livepatch/<name>/transition.  Once all tasks have

been patched, the 'transition' value changes to '0'.  For more

information about this process, see the "Consistency model" section.

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

either the code from the previously enabled patch or even the original

code gets used.

When a patch is disabled, livepatch enters into a transition state where

tasks are converging to the unpatched state.  This is indicated by a

value of '1' in /sys/kernel/livepatch/<name>/transition.  Once all tasks

have been unpatched, the 'transition' value changes to '0'.  For more

information about this process, see the "Consistency model" section.

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

    by "notrace".

  + 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.

	  Architecture supports the 'objtool check' host tool command, which

	  performs compile-time stack metadata validation.

config HAVE_RELIABLE_STACKTRACE

	bool

	help

	  Architecture has a save_stack_trace_tsk_reliable() function which

	  only returns a stack trace if it can guarantee the trace is reliable.

config HAVE_ARCH_HASH

	bool

	default n

					   TIF_NEED_RESCHED */

#define TIF_32BIT		4	/* 32 bit binary */

#define TIF_RESTORE_TM		5	/* need to restore TM FP/VEC/VSX */

#define TIF_PATCH_PENDING	6	/* pending live patching update */

#define TIF_SYSCALL_AUDIT	7	/* syscall auditing active */

#define TIF_SINGLESTEP		8	/* singlestepping active */

#define TIF_NOHZ		9	/* in adaptive nohz mode */

#define _TIF_POLLING_NRFLAG	(1<<TIF_POLLING_NRFLAG)

#define _TIF_32BIT		(1<<TIF_32BIT)

#define _TIF_RESTORE_TM		(1<<TIF_RESTORE_TM)

#define _TIF_PATCH_PENDING	(1<<TIF_PATCH_PENDING)

#define _TIF_SYSCALL_AUDIT	(1<<TIF_SYSCALL_AUDIT)

#define _TIF_SINGLESTEP		(1<<TIF_SINGLESTEP)

#define _TIF_SECCOMP		(1<<TIF_SECCOMP)

#define _TIF_USER_WORK_MASK	(_TIF_SIGPENDING | _TIF_NEED_RESCHED | \

				 _TIF_NOTIFY_RESUME | _TIF_UPROBE | \

				 _TIF_RESTORE_TM)

				 _TIF_RESTORE_TM | _TIF_PATCH_PENDING)

#define _TIF_PERSYSCALL_MASK	(_TIF_RESTOREALL|_TIF_NOERROR)

/* Bits in local_flags */