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livepatch: change to a per-task consistency model

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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 (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.

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

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Ingo Molnar <mingo@kernel.org>        # for the scheduler changes
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
This commit is contained in:
Josh Poimboeuf
2017-02-13 19:42:40 -06:00
committed by Jiri Kosina
parent f5e547f4ac
commit d83a7cb375
14 changed files with 944 additions and 46 deletions
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180
Documentation/livepatch/livepatch.txt
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@@ -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 +87,141 @@ 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
https://lkml.kernel.org/r/20141107140458.GA21774@suse.cz
stays consistent.
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.
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.
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.
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.
See also the section "Limitations" below.
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
@@ -134,7 +256,7 @@ 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:
@@ -156,6 +278,9 @@ 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.
@@ -172,10 +297,13 @@ into three levels:
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
@@ -239,9 +367,15 @@ 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.
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
@@ -261,6 +395,12 @@ 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.
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