x86/kprobes: Fix ordering while text-patching

Kprobes does something like:

register:
	arch_arm_kprobe()
	  text_poke(INT3)
          /* guarantees nothing, INT3 will become visible at some point, maybe */

        kprobe_optimizer()
	  /* guarantees the bytes after INT3 are unused */
	  synchronize_rcu_tasks();
	  text_poke_bp(JMP32);
	  /* implies IPI-sync, kprobe really is enabled */

unregister:
	__disarm_kprobe()
	  unoptimize_kprobe()
	    text_poke_bp(INT3 + tail);
	    /* implies IPI-sync, so tail is guaranteed visible */
          arch_disarm_kprobe()
            text_poke(old);
	    /* guarantees nothing, old will maybe become visible */

	synchronize_rcu()

        free-stuff

Now the problem is that on register, the synchronize_rcu_tasks() does
not imply sufficient to guarantee all CPUs have already observed INT3
(although in practice this is exceedingly unlikely not to have
happened) (similar to how MEMBARRIER_CMD_PRIVATE_EXPEDITED does not
imply MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE).

Worse, even if it did, we'd have to do 2 synchronize calls to provide
the guarantee we're looking for, the first to ensure INT3 is visible,
the second to guarantee nobody is then still using the instruction
bytes after INT3.

Similar on unregister; the synchronize_rcu() between
__unregister_kprobe_top() and __unregister_kprobe_bottom() does not
guarantee all CPUs are free of the INT3 (and observe the old text).

Therefore, sprinkle some IPI-sync love around. This guarantees that
all CPUs agree on the text and RCU once again provides the required
guaranteed.

Tested-by: Alexei Starovoitov <[email protected]>
Tested-by: Steven Rostedt (VMware) <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Mathieu Desnoyers <[email protected]>
Acked-by: Masami Hiramatsu <[email protected]>
Acked-by: Alexei Starovoitov <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions