[PATCH] powerpc/livepatch: relax reliable stack tracer checks for first-frame

[Joe Lawrence]

The bottom-most stack frame (the first to be unwound) may be largely
uninitialized, for the "Power Architecture 64-Bit ELF V2 ABI" only
requires its backchain pointer to be set.

The reliable stack tracer should be careful when verifying this frame:
skip checks on STACK_FRAME_LR_SAVE and STACK_FRAME_MARKER offsets that
may contain uninitialized residual data.

Fixes: df78d3f61480 ("powerpc/livepatch: Implement reliable stack tracing for the consistency model")
Suggested-by: Nicolai Stange <nstange@xxxxxxx>
Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
---
 arch/powerpc/kernel/stacktrace.c | 33 +++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
index e2c50b55138f..46096687a5a8 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)
@@ -142,12 +148,6 @@ save_stack_trace_tsk_reliable(struct task_struct *tsk,
 		if (sp & 0xF)
 			return 1;
 
-		/* Mark stacktraces with exception frames as unreliable. */
-		if (sp <= stack_end - STACK_INT_FRAME_SIZE &&
-		    stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {
-			return 1;
-		}
-
 		newsp = stack[0];
 		/* Stack grows downwards; unwinder may only go up. */
 		if (newsp <= sp)
@@ -158,11 +158,21 @@ save_stack_trace_tsk_reliable(struct task_struct *tsk,
 			return 1; /* invalid backlink, too far up. */
 		}
 
+		/* We can only trust the bottom frame's backlink, the rest
+		 * of the frame may be uninitialized, continue to the next. */
+		if (firstframe--)
+			goto next;
+
+		/* Mark stacktraces with exception frames as unreliable. */
+		if (sp <= stack_end - STACK_INT_FRAME_SIZE &&
+		    stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {
+			return 1;
+		}
+
 		/* Examine the saved LR: it must point into kernel code. */
 		ip = stack[STACK_FRAME_LR_SAVE];
-		if (!firstframe && !__kernel_text_address(ip))
+		if (!__kernel_text_address(ip))
 			return 1;
-		firstframe = 0;
 
 		/*
 		 * FIXME: IMHO these tests do not belong in
@@ -183,12 +193,13 @@ save_stack_trace_tsk_reliable(struct task_struct *tsk,
 		else
 			trace->skip--;
 
-		if (newsp == stack_end)
-			break;
-
 		if (trace->nr_entries >= trace->max_entries)
 			return -E2BIG;
 
+next:
+		if (newsp == stack_end)
+			break;
+
 		sp = newsp;
 	}
 	return 0;
-- 
2.20.1