[PATCH] powerpc/livepatch: relax reliable stack tracer exception check

[Joe Lawrence]

The ppc64le reliable stack tracer iterates over a given task's stack,
verifying a set of conditions to determine whether the trace is
'reliable'.  These checks include the presence of any exception frames.

We should be careful when inspecting the bottom-most stack frame (the
first to be unwound), particularly for scheduled-out tasks.  As Nicolai
Stange explains, "If I'm reading the code in _switch() correctly, the
first frame is completely uninitialized except for the pointer back to
the caller's stack frame."  If a previous do_IRQ() invocation, for
example, has left a residual exception-marker on the first frame, the
stack tracer would incorrectly report this task's trace as unreliable.

save_stack_trace_tsk_reliable() already skips the first frame when
verifying the saved Link Register.  It should do the same when looking
for the STACK_FRAME_REGS_MARKER.  The task it is unwinding will be
either 'current', in which case the tracer will have never been called
from an exception path, or the task will be inactive and its first
frame's contents will be uninitialized (aside from backchain pointer).

Fixes: df78d3f61480 ("powerpc/livepatch: Implement reliable stack tracing for the consistency model")
Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
---
 arch/powerpc/kernel/stacktrace.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
index e2c50b55138f..0793e75c45a6 100644
--- a/arch/powerpc/kernel/stacktrace.c
+++ b/arch/powerpc/kernel/stacktrace.c
@@ -84,6 +84,12 @@ save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace)
 EXPORT_SYMBOL_GPL(save_stack_trace_regs);
 
 #ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
+/*
+ * This function returns an error if it detects any unreliable features of the
+ * stack.  Otherwise it guarantees that the stack trace is reliable.
+ *
+ * If the task is not 'current', the caller *must* ensure the task is inactive.
+ */
 int
 save_stack_trace_tsk_reliable(struct task_struct *tsk,
 				struct stack_trace *trace)
@@ -143,7 +149,7 @@ save_stack_trace_tsk_reliable(struct task_struct *tsk,
 			return 1;
 
 		/* Mark stacktraces with exception frames as unreliable. */
-		if (sp <= stack_end - STACK_INT_FRAME_SIZE &&
+		if (!firstframe && sp <= stack_end - STACK_INT_FRAME_SIZE &&
 		    stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {
 			return 1;
 		}
-- 
2.20.1