Re: [PATCH] powerpc: Save AMR/IAMR when switching tasks

[Samuel Holland]

On 9/19/22 07:37, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes:
>> Le 16/09/2022 à 07:05, Samuel Holland a écrit :
>>> With CONFIG_PREEMPT=y (involuntary preemption enabled), it is possible
>>> to switch away from a task inside copy_{from,to}_user. This left the CPU
>>> with userspace access enabled until after the next IRQ or privilege
>>> level switch, when AMR/IAMR got reset to AMR_KU[AE]P_BLOCKED. Then, when
>>> switching back to the original task, the userspace access would fault:
>>
>> This is not supposed to happen. You never switch away from a task 
>> magically. Task switch will always happen in an interrupt, that means 
>> copy_{from,to}_user() get interrupted.
> 
> Unfortunately this isn't true when CONFIG_PREEMPT=y.
> 
> We can switch away without an interrupt via:
>   __copy_tofrom_user()
>     -> __copy_tofrom_user_power7()
>        -> exit_vmx_usercopy()
>           -> preempt_enable()
>              -> __preempt_schedule()
>                 -> preempt_schedule()
>                    -> preempt_schedule_common()
>                       -> __schedule()
> 
> I do some boot tests with CONFIG_PREEMPT=y, but I realise now those are
> all on Power8, which is a bit of an oversight on my part.
> 
> And clearly no one else tests it, until now :)
> 
> I think the root of our problem is that our KUAP lock/unlock is at too
> high a level, ie. we do it in C around the low-level copy to/from.
> 
> eg:
> 
> static inline unsigned long
> raw_copy_to_user(void __user *to, const void *from, unsigned long n)
> {
> 	unsigned long ret;
> 
> 	allow_write_to_user(to, n);
> 	ret = __copy_tofrom_user(to, (__force const void __user *)from, n);
> 	prevent_write_to_user(to, n);
> 	return ret;
> }
> 
> There's a reason we did that, which is that we have various different
> KUAP methods on different platforms, not a simple instruction like other
> arches.
> 
> But that means we have that exit_vmx_usercopy() being called deep in the
> guts of __copy_tofrom_user(), with KUAP disabled, and then we call into
> the preempt machinery and eventually schedule.
> 
> I don't see an easy way to fix that "properly", it would be a big change
> to all platforms to push the KUAP save/restore down into the low level
> asm code.
> 
> But I think the patch below does fix it, although it abuses things a
> little. Namely it only works because the 64s KUAP code can handle a
> double call to prevent, and doesn't need the addresses or size for the
> allow.
> 
> Still I think it might be our best option for an easy fix.
> 
> Samuel, can you try this on your system and check it works for you?

It looks like your patch works. Thanks for the correct fix!

I replaced my patch with the one below, and enabled
CONFIG_PPC_KUAP_DEBUG=y, and I was able to do several kernel builds
without any crashes or splats in dmesg.

I suppose the other calls to exit_vmx_usercopy() in copyuser_power7.S
would not cause a crash, because there is no userspace memory access
afterward, but couldn't they still leave KUAP erroneously unlocked?

Regards,
Samuel

> diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
> index 97a77b37daa3..c50080c6a136 100644
> --- a/arch/powerpc/include/asm/processor.h
> +++ b/arch/powerpc/include/asm/processor.h
> @@ -432,6 +432,7 @@ int speround_handler(struct pt_regs *regs);
>  /* VMX copying */
>  int enter_vmx_usercopy(void);
>  int exit_vmx_usercopy(void);
> +void exit_vmx_usercopy_continue(void);
>  int enter_vmx_ops(void);
>  void *exit_vmx_ops(void *dest);
>  
> diff --git a/arch/powerpc/lib/copyuser_power7.S b/arch/powerpc/lib/copyuser_power7.S
> index 28f0be523c06..77804860383c 100644
> --- a/arch/powerpc/lib/copyuser_power7.S
> +++ b/arch/powerpc/lib/copyuser_power7.S
> @@ -47,7 +47,7 @@
>  	ld	r15,STK_REG(R15)(r1)
>  	ld	r14,STK_REG(R14)(r1)
>  .Ldo_err3:
> -	bl	exit_vmx_usercopy
> +	bl	exit_vmx_usercopy_continue
>  	ld	r0,STACKFRAMESIZE+16(r1)
>  	mtlr	r0
>  	b	.Lexit
> diff --git a/arch/powerpc/lib/vmx-helper.c b/arch/powerpc/lib/vmx-helper.c
> index f76a50291fd7..78a18b8384ff 100644
> --- a/arch/powerpc/lib/vmx-helper.c
> +++ b/arch/powerpc/lib/vmx-helper.c
> @@ -8,6 +8,7 @@
>   */
>  #include <linux/uaccess.h>
>  #include <linux/hardirq.h>
> +#include <asm/kup.h>
>  #include <asm/switch_to.h>
>  
>  int enter_vmx_usercopy(void)
> @@ -34,12 +35,19 @@ int enter_vmx_usercopy(void)
>   */
>  int exit_vmx_usercopy(void)
>  {
> +	prevent_user_access(KUAP_READ_WRITE);
>  	disable_kernel_altivec();
>  	pagefault_enable();
>  	preempt_enable();
>  	return 0;
>  }
>  
> +void exit_vmx_usercopy_continue(void)
> +{
> +	exit_vmx_usercopy();
> +	allow_read_write_user(NULL, NULL, 0);
> +}
> +
>  int enter_vmx_ops(void)
>  {
>  	if (in_interrupt())
>