[PATCH PTI] Fixup "x86/entry/64: Use a per-CPU trampoline stack for IDT entries"

[PATCH PTI] Fixup "x86/entry/64: Use a per-CPU trampoline stack for IDT entries"

[Andy Lutomirski]

Add to commit message:

Using a trampoline stack would obnoxious for Xen PV because Xen PV
enters entry_SYSCALL_64_after_hwframe on the stack indicated by sp0.
This could be fixed, but I think it's nice to ensure the entry code
can still work without a trampoline stack.  So this patch doesn't
use the entry trampoline stack on Xen.

The regs != eregs check in sync_regs is optional, but I think it's
better than copying the whole set of regs over itself.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---

Boris, this is lightly tested and should fix the problem you're seeing.

 arch/x86/include/asm/switch_to.h | 3 +++
 arch/x86/kernel/traps.c          | 9 +++++----
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index fab453ad2460..cbc71e73bd32 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -93,6 +93,9 @@ static inline void update_sp0(struct task_struct *task)
 	/* On x86_64, sp0 always points to the entry trampoline stack, which is constant: */
 #ifdef CONFIG_X86_32
 	load_sp0(task->thread.sp0);
+#else
+	if (static_cpu_has(X86_FEATURE_XENPV))
+		load_sp0(task_top_of_stack(task));
 #endif
 }
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 40cc3dc5967a..ee9ca0ad4388 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -619,14 +619,15 @@ NOKPROBE_SYMBOL(do_int3);
 
 #ifdef CONFIG_X86_64
 /*
- * Help handler running on IST stack to switch off the IST stack if the
- * interrupted code was in user mode. The actual stack switch is done in
- * entry_64.S
+ * Help handler running on a per-cpu (IST or entry trampoline) stack
+ * to switch to the normal thread stack if the interrupted code was in
+ * user mode. The actual stack switch is done in entry_64.S
  */
 asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs)
 {
 	struct pt_regs *regs = (struct pt_regs *)this_cpu_read(cpu_current_top_of_stack) - 1;
-	*regs = *eregs;
+	if (regs != eregs)
+		*regs = *eregs;
 	return regs;
 }
 NOKPROBE_SYMBOL(sync_regs);
-- 
2.13.6

Re: [PATCH PTI] Fixup "x86/entry/64: Use a per-CPU trampoline stack for IDT entries"

[Juergen Gross]

On 06/12/17 07:00, Andy Lutomirski wrote:
> Add to commit message:
> 
> Using a trampoline stack would obnoxious for Xen PV because Xen PV
> enters entry_SYSCALL_64_after_hwframe on the stack indicated by sp0.
> This could be fixed, but I think it's nice to ensure the entry code
> can still work without a trampoline stack.  So this patch doesn't
> use the entry trampoline stack on Xen.
> 
> The regs != eregs check in sync_regs is optional, but I think it's
> better than copying the whole set of regs over itself.
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>

I verified the crash occurring without the patch when started as a
pv guest.

Your patch makes pv guest boot again. HVM guest is working, too.

You can add my:

Tested-by: Juergen Gross <jgross@suse.com>


Juergen

> ---
> 
> Boris, this is lightly tested and should fix the problem you're seeing.
> 
>  arch/x86/include/asm/switch_to.h | 3 +++
>  arch/x86/kernel/traps.c          | 9 +++++----
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index fab453ad2460..cbc71e73bd32 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -93,6 +93,9 @@ static inline void update_sp0(struct task_struct *task)
>  	/* On x86_64, sp0 always points to the entry trampoline stack, which is constant: */
>  #ifdef CONFIG_X86_32
>  	load_sp0(task->thread.sp0);
> +#else
> +	if (static_cpu_has(X86_FEATURE_XENPV))
> +		load_sp0(task_top_of_stack(task));
>  #endif
>  }
>  
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 40cc3dc5967a..ee9ca0ad4388 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -619,14 +619,15 @@ NOKPROBE_SYMBOL(do_int3);
>  
>  #ifdef CONFIG_X86_64
>  /*
> - * Help handler running on IST stack to switch off the IST stack if the
> - * interrupted code was in user mode. The actual stack switch is done in
> - * entry_64.S
> + * Help handler running on a per-cpu (IST or entry trampoline) stack
> + * to switch to the normal thread stack if the interrupted code was in
> + * user mode. The actual stack switch is done in entry_64.S
>   */
>  asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs)
>  {
>  	struct pt_regs *regs = (struct pt_regs *)this_cpu_read(cpu_current_top_of_stack) - 1;
> -	*regs = *eregs;
> +	if (regs != eregs)
> +		*regs = *eregs;
>  	return regs;
>  }
>  NOKPROBE_SYMBOL(sync_regs);
>