Re: [PATCH] filemap: replace pte_offset_map() with pte_offset_map_nolock()

["zhangpeng (AS)" <zhangpeng362@huawei.com>]

On 2024/7/5 2:40, Andrew Morton wrote:

> On Tue, 25 Jun 2024 14:06:43 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
>
>> On Wed, 13 Mar 2024 09:29:13 +0800 Peng Zhang <zhangpeng362@huawei.com> wrote:
>>
>>> From: ZhangPeng <zhangpeng362@huawei.com>
>>>
>>> The vmf->ptl in filemap_fault_recheck_pte_none() is still set from
>>> handle_pte_fault(). But at the same time, we did a pte_unmap(vmf->pte).
>>> After a pte_unmap(vmf->pte) unmap and rcu_read_unlock(), the page table
>>> may be racily changed and vmf->ptl maybe fails to protect the actual
>>> page table.
>>> Fix this by replacing pte_offset_map() with pte_offset_map_nolock().
>>>
>>> ...
>>>
>>> --- a/mm/filemap.c
>>> +++ b/mm/filemap.c
>>> @@ -3207,7 +3207,8 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
>>>   	if (!(vmf->flags & FAULT_FLAG_ORIG_PTE_VALID))
>>>   		return 0;
>>>   
>>> -	ptep = pte_offset_map(vmf->pmd, vmf->address);
>>> +	ptep = pte_offset_map_nolock(vma->vm_mm, vmf->pmd, vmf->address,
>>> +				     &vmf->ptl);
>>>   	if (unlikely(!ptep))
>>>   		return VM_FAULT_NOPAGE;
>>>   
>> whoops, I'm still sitting on this because I didn't know whether we
>> should backport it.
>>
>> And...  guess what I say next.  Can we please describe what are the
>> userspace visible effects of the bug?
>>
> Nobody?
>
> Oh well, I'll add cc:stable amd move this into mm-hotfixes.
>
Sorry for the late reply.

As David said, the PTL pointer might be stale so if we continue to use it infilemap_fault_recheck_pte_none(), it might trigger UAF.
Also, if the PTL fails, the issue fixed by commit 58f327f2ce80 ("filemap: avoid unnecessary major faults in filemap_fault()") might reappear.

-- 
Best Regards,
Peng