[PATCH v3] x86/show_trace_log_lvl: ensure stack pointer is aligned, again

[PATCH v3] x86/show_trace_log_lvl: ensure stack pointer is aligned, again

[Oleg Nesterov]

From: Vernon Lovejoy <vlovejoy@redhat.com>

The commit e335bb51cc15 ("x86/unwind: Ensure stack pointer is aligned")
tried to align the stack pointer in show_trace_log_lvl(), otherwise the
"stack < stack_info.end" check can't guarantee that the last read does
not go past the end of the stack.

However, we have the same problem with the initial value of the stack
pointer, it can also be unaligned. So without this patch this trivial
kernel module

	#include <linux/module.h>

	static int init(void)
	{
		asm volatile("sub    $0x4,%rsp");
		dump_stack();
		asm volatile("add    $0x4,%rsp");

		return -EAGAIN;
	}

	module_init(init);
	MODULE_LICENSE("GPL");

crashes the kernel.

Fixes: e335bb51cc15 ("x86/unwind: Ensure stack pointer is aligned")
Signed-off-by: Vernon Lovejoy <vlovejoy@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/x86/kernel/dumpstack.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 0bf6779187dd..f18ca44c904b 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -195,7 +195,6 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	printk("%sCall Trace:\n", log_lvl);
 
 	unwind_start(&state, task, regs, stack);
-	stack = stack ? : get_stack_pointer(task, regs);
 	regs = unwind_get_entry_regs(&state, &partial);
 
 	/*
@@ -214,9 +213,13 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	 * - hardirq stack
 	 * - entry stack
 	 */
-	for ( ; stack; stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
+	for (stack = stack ?: get_stack_pointer(task, regs);
+	     stack;
+	     stack = stack_info.next_sp) {
 		const char *stack_name;
 
+		stack = PTR_ALIGN(stack, sizeof(long));
+
 		if (get_stack_info(stack, task, &stack_info, &visit_mask)) {
 			/*
 			 * We weren't on a valid stack.  It's possible that
-- 
2.25.1.362.g51ebf55

Re: [PATCH v3] x86/show_trace_log_lvl: ensure stack pointer is aligned, again

[Oleg Nesterov]

Josh, this is boring.

We are changing this simple fix for the second time, precisely following
your recommendations. And for the second time, you mysteriously disappear
afterward.

Please tell us if you finally agree with this version or if you think we
should make changes again.

Thanks.

On 05/12, Oleg Nesterov wrote:
>
> From: Vernon Lovejoy <vlovejoy@redhat.com>
> 
> The commit e335bb51cc15 ("x86/unwind: Ensure stack pointer is aligned")
> tried to align the stack pointer in show_trace_log_lvl(), otherwise the
> "stack < stack_info.end" check can't guarantee that the last read does
> not go past the end of the stack.
> 
> However, we have the same problem with the initial value of the stack
> pointer, it can also be unaligned. So without this patch this trivial
> kernel module
> 
> 	#include <linux/module.h>
> 
> 	static int init(void)
> 	{
> 		asm volatile("sub    $0x4,%rsp");
> 		dump_stack();
> 		asm volatile("add    $0x4,%rsp");
> 
> 		return -EAGAIN;
> 	}
> 
> 	module_init(init);
> 	MODULE_LICENSE("GPL");
> 
> crashes the kernel.
> 
> Fixes: e335bb51cc15 ("x86/unwind: Ensure stack pointer is aligned")
> Signed-off-by: Vernon Lovejoy <vlovejoy@redhat.com>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  arch/x86/kernel/dumpstack.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
> index 0bf6779187dd..f18ca44c904b 100644
> --- a/arch/x86/kernel/dumpstack.c
> +++ b/arch/x86/kernel/dumpstack.c
> @@ -195,7 +195,6 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
>  	printk("%sCall Trace:\n", log_lvl);
>  
>  	unwind_start(&state, task, regs, stack);
> -	stack = stack ? : get_stack_pointer(task, regs);
>  	regs = unwind_get_entry_regs(&state, &partial);
>  
>  	/*
> @@ -214,9 +213,13 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
>  	 * - hardirq stack
>  	 * - entry stack
>  	 */
> -	for ( ; stack; stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
> +	for (stack = stack ?: get_stack_pointer(task, regs);
> +	     stack;
> +	     stack = stack_info.next_sp) {
>  		const char *stack_name;
>  
> +		stack = PTR_ALIGN(stack, sizeof(long));
> +
>  		if (get_stack_info(stack, task, &stack_info, &visit_mask)) {
>  			/*
>  			 * We weren't on a valid stack.  It's possible that
> -- 
> 2.25.1.362.g51ebf55
> 

Re: [PATCH v3] x86/show_trace_log_lvl: ensure stack pointer is aligned, again

[Josh Poimboeuf]

On Mon, May 15, 2023 at 08:26:08PM +0200, Oleg Nesterov wrote:
> Josh, this is boring.
> 
> We are changing this simple fix for the second time, precisely following
> your recommendations. And for the second time, you mysteriously disappear
> afterward.

Patience, please.  I'm not just sitting around waiting for your patches.

> Please tell us if you finally agree with this version or if you think we
> should make changes again.

Looks fine.

Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>

-- 
Josh

[tip: objtool/urgent] x86/show_trace_log_lvl: Ensure stack pointer is aligned, again

[tip-bot2 for Vernon Lovejoy]

The following commit has been merged into the objtool/urgent branch of tip:

Commit-ID:     2e4be0d011f21593c6b316806779ba1eba2cd7e0
Gitweb:        https://git.kernel.org/tip/2e4be0d011f21593c6b316806779ba1eba2cd7e0
Author:        Vernon Lovejoy <vlovejoy@redhat.com>
AuthorDate:    Fri, 12 May 2023 12:42:32 +02:00
Committer:     Josh Poimboeuf <jpoimboe@kernel.org>
CommitterDate: Tue, 16 May 2023 06:31:04 -07:00

x86/show_trace_log_lvl: Ensure stack pointer is aligned, again

The commit e335bb51cc15 ("x86/unwind: Ensure stack pointer is aligned")
tried to align the stack pointer in show_trace_log_lvl(), otherwise the
"stack < stack_info.end" check can't guarantee that the last read does
not go past the end of the stack.

However, we have the same problem with the initial value of the stack
pointer, it can also be unaligned. So without this patch this trivial
kernel module

	#include <linux/module.h>

	static int init(void)
	{
		asm volatile("sub    $0x4,%rsp");
		dump_stack();
		asm volatile("add    $0x4,%rsp");

		return -EAGAIN;
	}

	module_init(init);
	MODULE_LICENSE("GPL");

crashes the kernel.

Fixes: e335bb51cc15 ("x86/unwind: Ensure stack pointer is aligned")
Signed-off-by: Vernon Lovejoy <vlovejoy@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20230512104232.GA10227@redhat.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/x86/kernel/dumpstack.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 0bf6779..f18ca44 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -195,7 +195,6 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	printk("%sCall Trace:\n", log_lvl);
 
 	unwind_start(&state, task, regs, stack);
-	stack = stack ? : get_stack_pointer(task, regs);
 	regs = unwind_get_entry_regs(&state, &partial);
 
 	/*
@@ -214,9 +213,13 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	 * - hardirq stack
 	 * - entry stack
 	 */
-	for ( ; stack; stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
+	for (stack = stack ?: get_stack_pointer(task, regs);
+	     stack;
+	     stack = stack_info.next_sp) {
 		const char *stack_name;
 
+		stack = PTR_ALIGN(stack, sizeof(long));
+
 		if (get_stack_info(stack, task, &stack_info, &visit_mask)) {
 			/*
 			 * We weren't on a valid stack.  It's possible that