[PATCH v2] x86/show_trace_log_lvl: ensure stack pointer is aligned, again

[PATCH v2] 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 | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 0bf6779187dd..f82d896eb99d 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);
 
 	/*
@@ -214,7 +213,10 @@ 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 = PTR_ALIGN(stack ?: get_stack_pointer(task, regs), sizeof(long));
+	     stack;
+	     stack = PTR_ALIGN(stack_info.next_sp, sizeof(long)))
+	{
 		const char *stack_name;
 
 		if (get_stack_info(stack, task, &stack_info, &visit_mask)) {
-- 
2.25.1.362.g51ebf55

Re: [PATCH v2] x86/show_trace_log_lvl: ensure stack pointer is aligned, again

[Oleg Nesterov]

Josh,

This version does exactly what you requested in reply to v1.

	https://lore.kernel.org/lkml/20230428043158.r5omehiaqawcac2y@treble/

What else should we do? Make V3 following David's suggestion

	https://lore.kernel.org/lkml/aa8537d81dc747a097e9e30491b5081b@AcuMS.aculab.com/

? Or what?

On 04/29, 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 | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
> index 0bf6779187dd..f82d896eb99d 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);
>  
>  	/*
> @@ -214,7 +213,10 @@ 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 = PTR_ALIGN(stack ?: get_stack_pointer(task, regs), sizeof(long));
> +	     stack;
> +	     stack = PTR_ALIGN(stack_info.next_sp, sizeof(long)))
> +	{
>  		const char *stack_name;
>  
>  		if (get_stack_info(stack, task, &stack_info, &visit_mask)) {
> -- 
> 2.25.1.362.g51ebf55
>