[PATCH] arm64: fix the overlap between the kernel image and vmalloc address

[Laura Abbott]

On 04/24/2017 08:51 AM, Mark Rutland wrote:
> On Mon, Apr 24, 2017 at 09:28:48PM +0800, zhong jiang wrote:
>> On 2017/4/24 18:44, Mark Rutland wrote:
>>> So the issue is that we have the callchain below for a kernel image
>>> address:
>>>
>>> read_kcore()
>>> ->is_vmalloc_or_module_addr() // returns true
>>> ->vread()
>>> -->aligned_vread()
>>> --->vmalloc_to_page()
>>>
>>> In is_vmalloc{,or_module}_addr() we just check the addr against
>>> VMALLOC_START and VMALLOC_END, so they will return true for a kernel
>>> image address.
>>>
>>> Then, we call vmalloc_to_page(). While this only handles mappings made
>>> at page granularity, the kernel image mapping may have used sections. So
>>> this tries a bogus walk to the pte level.
> 
>>> Should we special-case kernel image handling, e.g. with new
>>> is_kernel_image_addr() / kernel_image_to_page() helpers?
> 
>>   yes ,  it seems to the best way to implents it  without performance back.
> 
>> The following patch is the implment. Any thought?
>>
>> Signed-off-by: zhong jiang <zhongjiang at huawei.com>
>>
>>   diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index b84615b..851ac35 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -475,6 +475,15 @@ static inline bool is_vmalloc_addr(const void *x)
>>         return false;
>>  #endif
>>  }
>> +
>> +static inline bool is_kernel_image_addr(const void *x)
>> +{
>> +       unsigned long addr = (unsigned long)x;
>> +
>> +       return addr >= (unsigned long)_stext && addr < (unsigned long)_end;
>> +
>> +}
>> +
>>  #ifdef CONFIG_MMU
>>  extern int is_vmalloc_or_module_addr(const void *x);
>>  #else
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 3ca82d4..9a9ef65 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -229,6 +229,42 @@ int is_vmalloc_or_module_addr(const void *x)
>>         return is_vmalloc_addr(x);
>>  }
>>
>> +static struct page *kernel_image_to_page(const void *kernel_addr, pgd_t *pgd)
>> +{
>> +       unsigned long addr = (unsigned long)kernel_addr;
>> +       struct page *page = NULL;
>> +       pud_t *pud;
>> +       pmd_t *pmd;
>> +       pte_t *pte;
>> +
>> +       if (pgd_none(*pgd))
>> +               goto out;
>> +
>> +       pud = pud_offset(pgd, addr);
>> +       if (pud_none(*pud))
>> +               goto out;
>> +
>> +       if (pud_sect(*pud))
>> +               return pud_page(*pud);
> 
> The *_sect() helpers are arch-specific, so we cannot use them in generic
> code. This would need to be architecture-specific.
> 
> Secondly, this will return head page of the section regardless of which
> page in the section the address corresponds to
> 
>> +
>> +       pmd = pmd_offset(*pmd, addr);
>> +       if (pmd_none(*pmd))
>> +               goto out;
>> +
>> +       if (pmd_sect(*pmd))
>> +               return pmd_page(*pmd);
> 
> Likewise on both counts.
> 
>> +
>> +       pte = pte_offset_kernel(pmd, addr);
>> +       if (pte_none(*pte))
>> +               goto out;
>> +
>> +       page = pte_page(*pte);
>> +
>> +out:
>> +       return page;
>> +
>> +}
> 
> Given we know what the address should map to, I don't think we need to
> walk the page tables here. I think this can be:
> 
> static struct page *kernel_image_to_page(const void *addr)
> {
> 	return virt_to_page(lm_alias(vmalloc_addr));
> }
> 
>> +
>>  /*
>>    * Walk a vmap address to the struct page it maps.
>>    */
>> @@ -244,6 +280,9 @@ struct page *vmalloc_to_page(const void *vmalloc_addr)
>>          */
>>         VIRTUAL_BUG_ON(!is_vmalloc_or_module_addr(vmalloc_addr));
>>
>> +       if (is_kernel_image_addr(vmalloc_addr))
>> +               return kernel_image_to_page(vmalloc_addr, pgd);
> 
> It's not clear to me that this is the right place for this to live.
> 
> It might be best to code the kernel image logic directly in kcore (and
> kmem), assuming everyone's OK with that approach.
> 

That will fix kcore and kmem but this will show up in other places too.
We've gone through and made sure that virt_addr_valid returns
true if and only if virt_to_page returns a valid address. I don't know
if we can make as strong a claim about is_vmalloc_addr and
vmalloc_to_page in all cases but is_vmalloc_addr should not return true
for the kernel image. That would at least let kcore fall back to
kern_addr_valid which should correctly handle the kernel image.
The suggestion to move the kernel image out of VMALLOC_START/VMALLOC_END
seems like the best approach although I haven't tried a prototype
at all.

Thanks,
Laura