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    livepatch: change to a per-task consistency model · d83a7cb3
    Josh Poimboeuf 提交于
    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.czSigned-off-by: NJosh Poimboeuf <jpoimboe@redhat.com>
    Acked-by: NMiroslav Benes <mbenes@suse.cz>
    Acked-by: Ingo Molnar <mingo@kernel.org>        # for the scheduler changes
    Signed-off-by: NJiri Kosina <jkosina@suse.cz>
    d83a7cb3
sched.h 42.7 KB