linux-kernel - Re: [PATCH] arm64/mm: fold check for KFENCE into can_set_direct

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Message-ID: <a9c04a95-a12a-2b10-b923-d90d9e5f4ac2@arm.com>
Date:   Thu, 22 Sep 2022 08:21:38 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: fold check for KFENCE into can_set_direct_map()



On 9/21/22 20:49, Mike Rapoport wrote:
> Hi Anshuman,
> 
> On Wed, Sep 21, 2022 at 05:09:19PM +0530, Anshuman Khandual wrote:
>>
>>
>> On 9/21/22 13:18, Mike Rapoport wrote:
>>> From: Mike Rapoport <rppt@...ux.ibm.com>
>>>
>>> KFENCE requires linear map to be mapped at page granularity, so that it
>>> is possible to protect/unprotect single pages, just like with
>>> rodata_full and DEBUG_PAGEALLOC.
>>>
>>> Instead of repating
>>>
>>> 	can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE)
>>>
>>> make can_set_direct_map() handle the KFENCE case.
>>>
>>> This also prevents potential false positives in kernel_page_present()
>>> that may return true for non-present page if CONFIG_KFENCE is enabled.
>>>
>>> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
>>> ---
>>>  arch/arm64/mm/mmu.c      | 8 ++------
>>>  arch/arm64/mm/pageattr.c | 8 +++++++-
>>>  2 files changed, 9 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index e7ad44585f40..c5065abec55a 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -535,7 +535,7 @@ static void __init map_mem(pgd_t *pgdp)
>>>  	 */
>>>  	BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
>>>  
>>> -	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
>>> +	if (can_set_direct_map())
>>>  		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>>>  
>>>  	/*
>>> @@ -1547,11 +1547,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
>>>  
>>>  	VM_BUG_ON(!mhp_range_allowed(start, size, true));
>>>  
>>> -	/*
>>> -	 * KFENCE requires linear map to be mapped at page granularity, so that
>>> -	 * it is possible to protect/unprotect single pages in the KFENCE pool.
>>> -	 */
>>> -	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
>>> +	if (can_set_direct_map())
>>>  		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>>>  
>>>  	__create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
>>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>>> index 64e985eaa52d..d107c3d434e2 100644
>>> --- a/arch/arm64/mm/pageattr.c
>>> +++ b/arch/arm64/mm/pageattr.c
>>> @@ -21,7 +21,13 @@ bool rodata_full __ro_after_init = IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED
>>>  
>>>  bool can_set_direct_map(void)
>>>  {
>>> -	return rodata_full || debug_pagealloc_enabled();
>>> +	/*
>>> +	 * rodata_full, DEBUG_PAGEALLOC and KFENCE require linear map to be
>>> +	 * mapped at page granularity, so that it is possible to
>>> +	 * protect/unprotect single pages.
>>> +	 */
>>> +	return rodata_full || debug_pagealloc_enabled() ||
>>> +		IS_ENABLED(CONFIG_KFENCE);
>>>  }
>>
>> Changing can_set_direct_map() also changes behaviour for other functions such as 
>>
>> set_direct_map_default_noflush()
>> set_direct_map_invalid_noflush()
>> __kernel_map_pages()
>>
>> Is that okay ?
>  
> Yes. Since KFENCE disables block mappings, these will actually change the
> page tables.
> Actually, before this change the test for can_set_direct_map() in these
> functions was false negative when CONFIG_KFENCE=y

Okay but then should not this have a "Fixes:" tag as well ?

>   
>>>  static int change_page_range(pte_t *ptep, unsigned long addr, void *data)
>