[PATCH urgent 0/2] x86/unwind: an unwinder fix and debugging improvement

[PATCH urgent 0/2] x86/unwind: an unwinder fix and debugging improvement

[Josh Poimboeuf]

Patch 1 is a fix for an unwinder issue found by Boris.  Patch 2 is a
debugging improvement which helped diagnose the problem.

Patch 1 needs to go into tip/urgent for 4.10.  I'd argue that patch 2
should also go into urgent because it only affects the error path and it
makes the new unwinder warnings much more useful.

Josh Poimboeuf (2):
  x86/unwind: adjust last frame check for aligned function stacks
  x86/unwind: dump stack data on warnings

 arch/x86/include/asm/unwind.h  |  2 +-
 arch/x86/kernel/unwind_frame.c | 47 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 47 insertions(+), 2 deletions(-)

-- 
2.7.4

[PATCH urgent 1/2] x86/unwind: adjust last frame check for aligned function stacks

[Josh Poimboeuf]

Somehow, CONFIG_PARAVIRT=n convinces gcc to change the
x86_64_start_kernel() prologue from:

  0000000000000129 <x86_64_start_kernel>:
   129:	55                   	push   %rbp
   12a:	48 89 e5             	mov    %rsp,%rbp

to:

  0000000000000124 <x86_64_start_kernel>:
   124:	4c 8d 54 24 08       	lea    0x8(%rsp),%r10
   129:	48 83 e4 f0          	and    $0xfffffffffffffff0,%rsp
   12d:	41 ff 72 f8          	pushq  -0x8(%r10)
   131:	55                   	push   %rbp
   132:	48 89 e5             	mov    %rsp,%rbp

This is an unusual pattern which aligns rsp (though in this case it's
already aligned) and saves the start_cpu() return address again on the
stack before storing the frame pointer.

The unwinder assumes the last stack frame header is at a certain offset,
but the above code breaks that assumption, resulting in the following
warning:

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

Fix it by checking for the last task stack frame at the aligned offset
in addition to the normal unaligned offset.

Fixes: acb4608ad186 ("x86/unwind: Create stack frames for saved syscall registers")
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/kernel/unwind_frame.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index ea7b7f9..33aeaae 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -46,7 +46,14 @@ static bool is_last_task_frame(struct unwind_state *state)
 	unsigned long bp = (unsigned long)state->bp;
 	unsigned long regs = (unsigned long)task_pt_regs(state->task);
 
-	return bp == regs - FRAME_HEADER_SIZE;
+	/*
+	 * We have to check for the last task frame at two different locations
+	 * because gcc can occasionally decide to realign the stack pointer and
+	 * change the offset of the stack frame by a word in the prologue of a
+	 * function called by head/entry code.
+	 */
+	return bp == regs - FRAME_HEADER_SIZE ||
+	       bp == regs - FRAME_HEADER_SIZE - sizeof(long);
 }
 
 /*
-- 
2.7.4

[PATCH urgent 2/2] x86/unwind: dump stack data on warnings

[Josh Poimboeuf]

The unwinder warnings are good at finding unexpected unwinder issues,
but they often don't give enough data to be able to fully diagnose them.
Print a one-time stack dump when a warning is detected.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/include/asm/unwind.h  |  2 +-
 arch/x86/kernel/unwind_frame.c | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)

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 33aeaae..20d4b4e 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -6,6 +6,37 @@
 
 #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++) {
+		word = READ_ONCE_NOCHECK(*sp);
+
+		prev_zero = zero;
+		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 +56,7 @@ 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;
 	}
 
@@ -74,6 +106,7 @@ 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.
@@ -87,6 +120,9 @@ 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;
 }
 
@@ -185,11 +221,13 @@ 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;
-- 
2.7.4

[tip:x86/urgent] x86/unwind: Adjust last frame check for aligned function stacks

[tip-bot for Josh Poimboeuf]

Commit-ID:  8023e0e2a48d45e8d5363081fad9f7ed4402f953
Gitweb:     http://git.kernel.org/tip/8023e0e2a48d45e8d5363081fad9f7ed4402f953
Author:     Josh Poimboeuf <jpoimboe@redhat.com>
AuthorDate: Fri, 16 Dec 2016 10:05:05 -0600
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Mon, 19 Dec 2016 11:47:05 +0100

x86/unwind: Adjust last frame check for aligned function stacks

Somehow, CONFIG_PARAVIRT=n convinces gcc to change the
x86_64_start_kernel() prologue from:

  0000000000000129 <x86_64_start_kernel>:
   129:	55                   	push   %rbp
   12a:	48 89 e5             	mov    %rsp,%rbp

to:

  0000000000000124 <x86_64_start_kernel>:
   124:	4c 8d 54 24 08       	lea    0x8(%rsp),%r10
   129:	48 83 e4 f0          	and    $0xfffffffffffffff0,%rsp
   12d:	41 ff 72 f8          	pushq  -0x8(%r10)
   131:	55                   	push   %rbp
   132:	48 89 e5             	mov    %rsp,%rbp

This is an unusual pattern which aligns rsp (though in this case it's
already aligned) and saves the start_cpu() return address again on the
stack before storing the frame pointer.

The unwinder assumes the last stack frame header is at a certain offset,
but the above code breaks that assumption, resulting in the following
warning:

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

Fix it by checking for the last task stack frame at the aligned offset
in addition to the normal unaligned offset.

Fixes: acb4608ad186 ("x86/unwind: Create stack frames for saved syscall registers")
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/9d7b4eb8cf55a7d6002cb738f25c23e7429c99a0.1481904011.git.jpoimboe@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 arch/x86/kernel/unwind_frame.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index ea7b7f9..33aeaae 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -46,7 +46,14 @@ static bool is_last_task_frame(struct unwind_state *state)
 	unsigned long bp = (unsigned long)state->bp;
 	unsigned long regs = (unsigned long)task_pt_regs(state->task);
 
-	return bp == regs - FRAME_HEADER_SIZE;
+	/*
+	 * We have to check for the last task frame at two different locations
+	 * because gcc can occasionally decide to realign the stack pointer and
+	 * change the offset of the stack frame by a word in the prologue of a
+	 * function called by head/entry code.
+	 */
+	return bp == regs - FRAME_HEADER_SIZE ||
+	       bp == regs - FRAME_HEADER_SIZE - sizeof(long);
 }
 
 /*

[tip:x86/urgent] x86/unwind: Dump stack data on warnings

[tip-bot for Josh Poimboeuf]

Commit-ID:  8b5e99f02264130782a10ba5c0c759797fb064ee
Gitweb:     http://git.kernel.org/tip/8b5e99f02264130782a10ba5c0c759797fb064ee
Author:     Josh Poimboeuf <jpoimboe@redhat.com>
AuthorDate: Fri, 16 Dec 2016 10:05:06 -0600
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Mon, 19 Dec 2016 11:47:05 +0100

x86/unwind: Dump stack data on warnings

The unwinder warnings are good at finding unexpected unwinder issues,
but they often don't give enough data to be able to fully diagnose them.
Print a one-time stack dump when a warning is detected.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/15607370e3ddb1732b6a73d5c65937864df16ac8.1481904011.git.jpoimboe@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 arch/x86/include/asm/unwind.h  |  2 +-
 arch/x86/kernel/unwind_frame.c | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)

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 33aeaae..20d4b4e 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -6,6 +6,37 @@
 
 #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++) {
+		word = READ_ONCE_NOCHECK(*sp);
+
+		prev_zero = zero;
+		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 +56,7 @@ 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;
 	}
 
@@ -74,6 +106,7 @@ 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.
@@ -87,6 +120,9 @@ 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;
 }
 
@@ -185,11 +221,13 @@ bad_address:
 			"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;