linux-kernel - Re: [PATCH] arm64: pageattr: Explicitly bail out when changing permissions for vmalloc

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Message-ID: <deedf5e2-4a25-4c1f-a5d8-a661a2eb16d2@arm.com>
Date: Sat, 29 Mar 2025 09:46:56 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Mike Rapoport <rppt@...nel.org>, Dev Jain <dev.jain@....com>
Cc: catalin.marinas@....com, will@...nel.org, gshan@...hat.com,
 steven.price@....com, suzuki.poulose@....com, tianyaxiong@...inos.cn,
 ardb@...nel.org, david@...hat.com, urezki@...il.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: pageattr: Explicitly bail out when changing
 permissions for vmalloc_huge mappings

On 28/03/2025 18:50, Mike Rapoport wrote:
> On Fri, Mar 28, 2025 at 11:51:03AM +0530, Dev Jain wrote:
>> arm64 uses apply_to_page_range to change permissions for kernel VA mappings,
> 
>                                                      for vmalloc mappings ^
> 
> arm64 does not allow changing permissions to any VA address right now.
> 
>> which does not support changing permissions for leaf mappings. This function
>> will change permissions until it encounters a leaf mapping, and will bail
>> out. To avoid this partial change, explicitly disallow changing permissions
>> for VM_ALLOW_HUGE_VMAP mappings.
>>
>> Signed-off-by: Dev Jain <dev.jain@....com>

I wonder if we want a Fixes: tag here? It's certainly a *latent* bug.

>> ---
>>  arch/arm64/mm/pageattr.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>> index 39fd1f7ff02a..8337c88eec69 100644
>> --- a/arch/arm64/mm/pageattr.c
>> +++ b/arch/arm64/mm/pageattr.c
>> @@ -96,7 +96,7 @@ static int change_memory_common(unsigned long addr, int numpages,
>>  	 * we are operating on does not result in such splitting.
>>  	 *
>>  	 * Let's restrict ourselves to mappings created by vmalloc (or vmap).
>> -	 * Those are guaranteed to consist entirely of page mappings, and
>> +	 * Disallow VM_ALLOW_HUGE_VMAP vmalloc mappings so that
> 
> I'd keep mention of page mappings in the comment, e.g
> 
> 	* Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
> 	* mappings are updated and splitting is never needed.
> 
> With this and changelog updates Ryan asked for
> 
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> 
> 
>>  	 * splitting is never needed.
>>  	 *
>>  	 * So check whether the [addr, addr + size) interval is entirely
>> @@ -105,7 +105,7 @@ static int change_memory_common(unsigned long addr, int numpages,
>>  	area = find_vm_area((void *)addr);
>>  	if (!area ||
>>  	    end > (unsigned long)kasan_reset_tag(area->addr) + area->size ||
>> -	    !(area->flags & VM_ALLOC))
>> +	    ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC))
>>  		return -EINVAL;
>>  
>>  	if (!numpages)
>> -- 
>> 2.30.2
>>
>