Re: [PATCH 1/1] device-dax: Correct pgoff align in dax_set_mapping()

From: Joao Martins
Date: Mon Sep 30 2024 - 07:06:34 EST


On 27/09/2024 18:46, Andrew Morton wrote:
> (cc's added)
>
> On Fri, 27 Sep 2024 15:45:09 +0800 "Kun(llfl)" <llfl@xxxxxxxxxxxxxxxxx> wrote:
>
>> pgoff should be aligned using ALIGN_DOWN() instead of ALIGN(). Otherwise,
>> vmf->address not aligned to fault_size will be aligned to the next
>> alignment, that can result in memory failure getting the wrong address.
>>
>> Fixes: b9b5777f09be ("device-dax: use ALIGN() for determining pgoff")
>
> That's quite an old change. Can you suggest why it took this long to
> be discovered? What is your userspace doing to trigger this?
>
Likely we never reproduce in production because we always pin device-dax regions
in the region align they provide (Qemu does similarly with prealloc in
hugetlb/file backed memory). I think this bug requires that we touch *unpinned*
device-dax regions unaligned to the device-dax selected alignment (page size
i.e. 4K/2M/1G)

>> Signed-off-by: Kun(llfl) <llfl@xxxxxxxxxxxxxxxxx>
>> Tested-by: JianXiong Zhao <zhaojianxiong.zjx@xxxxxxxxxxxxxxx>

Thanks a lot for the catch and sorry for the pain that it may have caused you:

Reviewed-by: Joao Martins <joao.m.martins@xxxxxxxxxx>

>> ---
>> drivers/dax/device.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/dax/device.c b/drivers/dax/device.c
>> index 9c1a729cd77e..6d74e62bbee0 100644
>> --- a/drivers/dax/device.c
>> +++ b/drivers/dax/device.c
>> @@ -86,7 +86,7 @@ static void dax_set_mapping(struct vm_fault *vmf, pfn_t pfn,
>> nr_pages = 1;
>>
>> pgoff = linear_page_index(vmf->vma,
>> - ALIGN(vmf->address, fault_size));
>> + ALIGN_DOWN(vmf->address, fault_size));
>>
>> for (i = 0; i < nr_pages; i++) {
>> struct page *page = pfn_to_page(pfn_t_to_pfn(pfn) + i);
>> --
>> 2.43.0