perf/x86/hw_breakpoints: Fix check for kernel-space breakpoints

The check looked wrong, although I think it was actually safe.  TASK_SIZE
is unnecessarily small for compat tasks, and it wasn't possible to make
a range breakpoint so large it started in user space and ended in kernel
space.

Nonetheless, let's fix up the check for the benefit of future
readers.  A breakpoint is in the kernel if either end is in the
kernel.

Signed-off-by: Andy Lutomirski <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/136be387950e78f18cea60e9d1bef74465d0ee8f.1438312874.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>

Author: amluto

arch/x86/kernel/hw_breakpoint.c

@@ -180,7 +180,11 @@ int arch_check_bp_in_kernelspace(struct perf_event *bp)
 	va = info->address;
 	len = bp->attr.bp_len;
 
-	return (va >= TASK_SIZE) && ((va + len - 1) >= TASK_SIZE);
+	/*
+	 * We don't need to worry about va + len - 1 overflowing:
+	 * we already require that va is aligned to a multiple of len.
+	 */
+	return (va >= TASK_SIZE_MAX) || ((va + len - 1) >= TASK_SIZE_MAX);
 }
 
 int arch_bp_generic_fields(int x86_len, int x86_type,