Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model

From	Miroslav Benes <mbenes@suse.cz>
Newsgroups	linux.kernel
Subject	Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model
Date	2016-05-10 13:50 +0200
Message-ID	<rxekP-1Bh-29@gated-at.bofh.it> (permalink)
References	<rt12N-1W5-3@gated-at.bofh.it> <rt12O-1W5-9@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Thu, 28 Apr 2016, Josh Poimboeuf wrote:

> Change livepatch to use a basic per-task consistency model.  This is the
> foundation which will eventually enable us to patch those ~10% of
> security patches which change function or data semantics.  This is the
> biggest remaining piece needed to make livepatch more generally useful.
> 
> This code stems from the design proposal made by Vojtech [1] in November
> 2014.  It's 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 (CONFIG_RELIABLE_STACKTRACE and
>    CONFIG_STACK_VALIDATION).
> 
> 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) Applying patches for architectures which don't yet have
>       CONFIG_RELIABLE_STACKTRACE.  In this case you'll have to signal
>       most of the tasks on the system.  However this isn't a complete
>       solution, because there's currently no way to patch kthreads
>       without CONFIG_RELIABLE_STACKTRACE.
> 
>    Note: since idle "swapper" tasks don't ever exit the kernel, they
>    instead have a kpatch_patch_task() call in the idle loop which allows

s/kpatch_patch_task()/klp_patch_task()/

[...]

> --- a/Documentation/livepatch/livepatch.txt
> +++ b/Documentation/livepatch/livepatch.txt
> @@ -72,7 +72,8 @@ 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.
> +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
> @@ -86,20 +87,103 @@ 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.
> +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 (CONFIG_RELIABLE_STACKTRACE and
> +   CONFIG_STACK_VALIDATION).
> +
> +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) Applying patches for architectures which don't yet have
> +      CONFIG_RELIABLE_STACKTRACE.  In this case you'll have to signal
> +      most of the tasks on the system.  However this isn't a complete
> +      solution, because there's currently no way to patch kthreads
> +      without CONFIG_RELIABLE_STACKTRACE.
> +
> +   Note: since idle "swapper" tasks don't ever exit the kernel, they
> +   instead have a kpatch_patch_task() call in the idle loop which allows

s/kpatch_patch_task()/klp_patch_task()/

Otherwise all the code that touches livepatch looks good to me. Apart from 
the things mentioned in another emails.

Miroslav

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[RFC PATCH v2 00/18] livepatch: hybrid consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
  [RFC PATCH v2 14/18] livepatch: remove unnecessary object loaded check Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
  [RFC PATCH v2 10/18] livepatch/powerpc: add TIF_PATCH_PENDING thread flag Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
    Re: [RFC PATCH v2 10/18] livepatch/powerpc: add TIF_PATCH_PENDING  thread flag Petr Mladek <pmladek@suse.com> - 2016-05-03 11:10 +0200
      Re: [RFC PATCH v2 10/18] livepatch/powerpc: add TIF_PATCH_PENDING  thread flag Miroslav Benes <mbenes@suse.cz> - 2016-05-03 14:10 +0200
  [RFC PATCH v2 16/18] livepatch: store function sizes Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
  [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
    Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-29 20:10 +0200
      Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-29 22:20 +0200
        Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 22:30 +0200
          Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-29 22:40 +0200
            Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 23:30 +0200
              Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-29 23:40 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Jiri Kosina <jikos@kernel.org> - 2016-04-30 00:20 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-30 01:00 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-30 02:20 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-30 00:50 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-30 02:10 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-02 16:00 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-05-02 18:00 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-02 19:40 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-05-02 20:20 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Ingo Molnar <mingo@kernel.org> - 2016-05-02 20:40 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-02 21:50 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Jiri Kosina <jikos@kernel.org> - 2016-05-02 22:00 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Jiri Kosina <jikos@kernel.org> - 2016-05-02 22:10 +0200
                Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-05-03 02:50 +0200
                RE: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking David Laight <David.Laight@ACULAB.COM> - 2016-05-04 17:20 +0200
      Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ  tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 22:20 +0200
  [RFC PATCH v2 09/18] livepatch/x86: add TIF_PATCH_PENDING thread flag Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
    Re: [RFC PATCH v2 09/18] livepatch/x86: add TIF_PATCH_PENDING thread flag Andy Lutomirski <luto@amacapital.net> - 2016-04-29 20:10 +0200
      Re: [RFC PATCH v2 09/18] livepatch/x86: add TIF_PATCH_PENDING thread  flag Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 22:20 +0200
  [RFC PATCH v2 02/18] x86/asm/head: use a common function for starting CPUs Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
  [RFC PATCH v2 13/18] livepatch: separate enabled and patched states Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
    Re: [RFC PATCH v2 13/18] livepatch: separate enabled and patched  states Petr Mladek <pmladek@suse.com> - 2016-05-03 11:40 +0200
      Re: [RFC PATCH v2 13/18] livepatch: separate enabled and patched  states Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-03 15:50 +0200
  [RFC PATCH v2 06/18] x86: dump_trace() error handling Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
    Re: [RFC PATCH v2 06/18] x86: dump_trace() error handling Minfei Huang <mnghuan@gmail.com> - 2016-04-29 15:50 +0200
      Re: [RFC PATCH v2 06/18] x86: dump_trace() error handling Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 16:10 +0200
  [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 23:00 +0200
    Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the  stack for idle tasks Brian Gerst <brgerst@gmail.com> - 2016-04-29 20:50 +0200
      Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the  stack for idle tasks Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 22:30 +0200
    Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the  stack for idle tasks Andy Lutomirski <luto@kernel.org> - 2016-04-29 21:40 +0200
      Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the  stack for idle tasks Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 23:00 +0200
        Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the  stack for idle tasks Andy Lutomirski <luto@amacapital.net> - 2016-04-29 23:40 +0200
          Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the  stack for idle tasks Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-30 01:30 +0200
            Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the  stack for idle tasks Andy Lutomirski <luto@amacapital.net> - 2016-04-30 02:20 +0200
  [RFC PATCH v2 04/18] x86: move _stext marker before head code Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 23:00 +0200
  [RFC PATCH v2 01/18] x86/asm/head: clean up initial stack variable Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 23:00 +0200
  Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency  model Petr Mladek <pmladek@suse.com> - 2016-05-04 10:50 +0200
    Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency  model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 18:00 +0200
      Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency  model Miroslav Benes <mbenes@suse.cz> - 2016-05-05 11:50 +0200
      Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency  model Petr Mladek <pmladek@suse.com> - 2016-05-05 15:10 +0200
  barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task  consistency model Petr Mladek <pmladek@suse.com> - 2016-05-04 14:40 +0200
    Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Peter Zijlstra <peterz@infradead.org> - 2016-05-04 16:00 +0200
      Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 19:00 +0200
    Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-04 16:20 +0200
      Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 19:30 +0200
        Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-05 13:30 +0200
        Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Miroslav Benes <mbenes@suse.cz> - 2016-05-09 17:50 +0200
    Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 19:10 +0200
      Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-05 12:30 +0200
  klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-04 16:50 +0200
    Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to  a per-task consistency model Jiri Kosina <jikos@kernel.org> - 2016-05-04 17:00 +0200
    Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 20:00 +0200
      Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-05 14:00 +0200
        Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-06 14:40 +0200
          Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a  per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-09 14:30 +0200
  Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency  model Petr Mladek <pmladek@suse.com> - 2016-05-06 13:40 +0200
    Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency  model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-06 14:50 +0200
  Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency  model Miroslav Benes <mbenes@suse.cz> - 2016-05-09 11:50 +0200
  Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency  model Miroslav Benes <mbenes@suse.cz> - 2016-05-10 13:50 +0200

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