Subject: Re: [PATCH v2 7/7] ARM: mm: Fix PXN process with LPAE feature

From: "Russell King (Oracle)" <[email-masked]>
Date: Wed, 2 Jun 2021 11:52:56 +0100

Hi,

On Wed, Jun 02, 2021 at 03:02:46PM +0800, Kefeng Wang wrote:
> When user code execution with privilege mode, it will lead to
> infinite loop in the page fault handler if ARM_LPAE enabled,
> 
> The issue could be reproduced with
>   "echo EXEC_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT"
> 
> Lets' fix it by adding the check in do_page_fault() and panic
> when ARM_LPAE enabled.
> 
> Fixes: 1d4d37159d01 ("ARM: 8235/1: Support for the PXN CPU feature on ARMv7")
> Signed-off-by: Kefeng Wang <[email-masked]>
> ---
>  arch/arm/mm/fault.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index 7cfa9a59d3ec..279bbeb33b48 100644
> --- a/arch/arm/mm/fault.c
> +++ b/arch/arm/mm/fault.c
> @@ -257,8 +257,14 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
>  		vm_flags = VM_WRITE;
>  	}
>  
> -	if (fsr & FSR_LNX_PF)
> +	if (fsr & FSR_LNX_PF) {
>  		vm_flags = VM_EXEC;
> +#ifdef CONFIG_ARM_LPAE
> +		if (addr && addr < TASK_SIZE && !user_mode(regs))
> +			die_kernel_fault("execution of user memory",
> +					 addr, fsr, regs);
> +#endif
> +	}

Do we need to do this test here?

Also, is this really LPAE specific? We have similar protection on 32-bit
ARM using domains to disable access to userspace except when the user
accessors are being used, so I would expect kernel-mode execution to
also cause a fault there.

-- 
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