Re: WARNING: kernel stack frame pointer at ffffffff82e03f40 in swapper:0 has bad value (null)

[Josh Poimboeuf]

On Sat, Dec 10, 2016 at 06:28:02PM +0100, Borislav Petkov wrote:
> On Sat, Dec 10, 2016 at 11:04:44AM -0600, Josh Poimboeuf wrote:
> > That's a new one.  Was there anything else printed?
> 
> It is the first line that appears in dmesg when I boot:
> 
> [    0.000000] WARNING: kernel stack frame pointer at ffffffffb5e03f40 in swapper:0 has bad value           (null)
> [    0.000000] Linux version 4.9.0-rc8+ (boris@gondor) (gcc version 6.2.0 20161109 (Debian 6.2.0-13) ) #1 SMP PREEMPT Sat Dec 10 13:25:29 CET 2016
> [    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.9.0-rc8+ root=/dev/sda7 ro earlyprintk=ttyS0,115200 console=ttyS0,115200 console=tty0 root=/dev/sda7 log_buf_len=10M resume=/dev/sda5 no_console_suspend ignore_loglevel
> [    0.000000] KERNEL supported cpus:
> [    0.000000]   Intel GenuineIntel
> [    0.000000]   AMD AuthenticAMD
> [    0.000000]   Centaur CentaurHauls
> [    0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
> ...
> 
> > Were you doing anything special when it happened?
> 
> Not really - just booting ;-)
> 
> > Do you see it reliably?
> 
> 2 of 2 boots.
> 
> > I should probably figure out a way to dump more data for that warning.
> 
> Sure, I can test patches.

Can you try with this?


diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
index c5a7f3a..6fa75b1 100644
--- a/arch/x86/include/asm/unwind.h
+++ b/arch/x86/include/asm/unwind.h
@@ -12,7 +12,7 @@ struct unwind_state {
 	struct task_struct *task;
 	int graph_idx;
 #ifdef CONFIG_FRAME_POINTER
-	unsigned long *bp;
+	unsigned long *bp, *orig_sp;
 	struct pt_regs *regs;
 #else
 	unsigned long *sp;
diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index ea7b7f9..1c93984 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -6,6 +6,36 @@
 
 #define FRAME_HEADER_SIZE (sizeof(long) * 2)
 
+static void unwind_dump(struct unwind_state *state, unsigned long *sp)
+{
+	static bool dumped_before = false;
+	bool prev_zero, zero = false;
+	unsigned long word;
+
+	if (dumped_before)
+		return;
+
+	dumped_before = true;
+
+	printk_deferred("unwind stack type:%d next_sp:%p mask:%lx graph_idx:%d\n",
+			state->stack_info.type, state->stack_info.next_sp,
+			state->stack_mask, state->graph_idx);
+
+	for (sp = state->orig_sp; sp < state->stack_info.end; sp++) {
+		prev_zero = zero;
+		word = READ_ONCE_NOCHECK(*sp);
+		zero = word == 0;
+
+		if (zero) {
+			if (!prev_zero)
+				printk_deferred("%p: %016x ...\n", sp, 0);
+			continue;
+		}
+
+		printk_deferred("%p: %016lx (%pB)\n", sp, word, (void *)word);
+	}
+}
+
 unsigned long unwind_get_return_address(struct unwind_state *state)
 {
 	unsigned long addr;
@@ -25,6 +55,7 @@ unsigned long unwind_get_return_address(struct unwind_state *state)
 			"WARNING: unrecognized kernel stack return address %p at %p in %s:%d\n",
 			(void *)addr, addr_p, state->task->comm,
 			state->task->pid);
+		unwind_dump(state, addr_p);
 		return 0;
 	}
 
@@ -67,6 +98,7 @@ static bool update_stack_state(struct unwind_state *state, void *addr,
 			       size_t len)
 {
 	struct stack_info *info = &state->stack_info;
+	enum stack_type orig_type = info->type;
 
 	/*
 	 * If addr isn't on the current stack, switch to the next one.
@@ -80,6 +112,9 @@ static bool update_stack_state(struct unwind_state *state, void *addr,
 				   &state->stack_mask))
 			return false;
 
+	if (!state->orig_sp || info->type != orig_type)
+		state->orig_sp = addr;
+
 	return true;
 }
 
@@ -178,11 +213,13 @@ bool unwind_next_frame(struct unwind_state *state)
 			"WARNING: kernel stack regs at %p in %s:%d has bad 'bp' value %p\n",
 			state->regs, state->task->comm,
 			state->task->pid, next_frame);
+		unwind_dump(state, (unsigned long *)state->regs);
 	} else {
 		printk_deferred_once(KERN_WARNING
 			"WARNING: kernel stack frame pointer at %p in %s:%d has bad value %p\n",
 			state->bp, state->task->comm,
 			state->task->pid, next_frame);
+		unwind_dump(state, state->bp);
 	}
 the_end:
 	state->stack_info.type = STACK_TYPE_UNKNOWN;