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From:   Joe Lawrence <joe.lawrence@redhat.com>
To:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        live-patching@vger.kernel.org
Cc:     Balbir Singh <bsingharora@gmail.com>,
        Jiri Kosina <jkosina@suse.cz>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nicolai Stange <nstange@suse.de>, Torsten Duwe <duwe@lst.de>
Subject: [PATCH 2/4] powerpc/livepatch: relax reliable stack tracer checks for first-frame
Date:   Tue, 22 Jan 2019 10:57:22 -0500
Message-Id: <20190122155724.27557-3-joe.lawrence@redhat.com>
In-Reply-To: <20190122155724.27557-1-joe.lawrence@redhat.com>
References: <20190122155724.27557-1-joe.lawrence@redhat.com>
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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")
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
---
 arch/powerpc/kernel/stacktrace.c | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
index e2c50b55138f..06688f4d557b 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 +158,26 @@ 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) {
+			firstframe = 0;
+			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
@@ -183,6 +198,7 @@ save_stack_trace_tsk_reliable(struct task_struct *tsk,
 		else
 			trace->skip--;
 
+next:
 		if (newsp == stack_end)
 			break;
 
-- 
2.20.1