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

[Kefeng Wang <wangkefeng.wang@huawei.com>]

Hi Russell,  any comments, thanks.

On 2021/6/3 17:38, Kefeng Wang wrote:
>
> On 2021/6/2 23:58, Russell King (Oracle) wrote:
>> On Wed, Jun 02, 2021 at 11:13:14PM +0800, Kefeng Wang wrote:
>>>    IFSR format when using the Short-descriptor translation table format
>>>
>>>      Domain fault      01001            First level   01011     
>>> Second level
>>>
>>>      Permission fault 01101            First level   01111 Second level
>>>
>>>    IFSR format when using the Long-descriptor translation table format
>>>
>>>     0011LL Permission fault. LL bits indicate levelb.
>>>
>>> After check the ARM spec, I think for the permission fault, we 
>>> should panic
>>> with or without LPAE, will change to
>> As I explained in one of the previous patches, the page tables that get
>> used for mapping kernel space are the _tasks_ own page tables. Any new
>> kernel mappings are lazily copied to the task page tables - such as
>> when a module is loaded.
>>
>> The first time we touch a page, we could end up with a page translation
>> fault. This will call do_page_fault(), and so with your proposal,
>> loading a module will potentially cause a kernel panic in this case,
>> probably leading to systems that panic early during userspace boot.
>
> Could we add some FSR_FS check， only panic when the permission fault， 
> eg，
>
> +static inline bool is_permission_fault(unsigned int fsr)
> +{
> +       int fs = fsr_fs(fsr);
> +#ifdef CONFIG_ARM_LPAE
> +       if ((fs & FS_PERM_NOLL_MASK) == FS_PERM_NOLL)
> +               return true;
> +#else
> +       if (fs == FS_L1_PERM || fs == FS_L2_PERM )
> +               return true;
> +#endif
> +       return false;
> +}
> +
>  static int __kprobes
>  do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs 
> *regs)
>  {
> @@ -255,8 +268,7 @@ do_page_fault(unsigned long addr, unsigned int 
> fsr, struct pt_regs *regs)
>
>         if (fsr & FSR_LNX_PF) {
>                 vm_flags = VM_EXEC;
> -
> -               if (!user_mode(regs))
> +               if (is_permission_fault && !user_mode(regs))
>                         die_kernel_fault("execution of memory",
>                                          mm, addr, fsr, regs);
>         }
>
> diff --git a/arch/arm/mm/fault.h b/arch/arm/mm/fault.h
> index 9ecc2097a87a..187954b4acca 100644
> --- a/arch/arm/mm/fault.h
> +++ b/arch/arm/mm/fault.h
> @@ -14,6 +14,8 @@
>
>  #ifdef CONFIG_ARM_LPAE
>  #define FSR_FS_AEA             17
> +#define FS_PERM_NOLL           0xC
> +#define FS_PERM_NOLL_MASK      0x3C
>
>  static inline int fsr_fs(unsigned int fsr)
>  {
> @@ -21,6 +23,8 @@ static inline int fsr_fs(unsigned int fsr)
>  }
>  #else
>  #define FSR_FS_AEA             22
> +#define FS_L1_PERM             0xD
> +#define FS_L2_PERM             0xF
>
> and suggestion or proper solution to solve the issue？
>
>>