Re: [PATCH] mm/hugetlb: avoid get wrong ptep caused by race

From: Mike Kravetz
Date: Thu Feb 20 2020 - 19:23:52 EST

On 2/19/20 6:30 PM, Longpeng (Mike) wrote:
> å 2020/2/20 3:33, Mike Kravetz åé:
>> + Kirill
>> On 2/18/20 5:58 PM, Sean Christopherson wrote:
>>> On Wed, Feb 19, 2020 at 09:39:59AM +0800, Longpeng (Mike) wrote:
>>> The race and the fix make sense. I assumed dereferencing garbage from the
>>> huge page was the issue, but I wasn't 100% that was the case, which is why
>>> I asked about alternative fixes.
>>>> We change the code from
>>>> if (pud_huge(*pud) || !pud_present(*pud))
>>>> to
>>>> if (pud_huge(*pud)
>>>> return (pte_t *)pud;
>>>> busy loop for 500ms
>>>> if (!pud_present(*pud))
>>>> return (pte_t *)pud;
>>>> and the panic will be hit quickly.
>>>> ARM64 has already use READ/WRITE_ONCE to access the pagetable, look at this
>>>> commit 20a004e7 (arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables).
>>>> The root cause is: 'if (pud_huge(*pud) || !pud_present(*pud))' read entry from
>>>> pud twice and the *pud maybe change in a race, so if we only read the pud once.
>>>> I use READ_ONCE here is just for safe, to prevents the complier mischief if
>>>> possible.
>>> FWIW, I'd be in favor of going the READ/WRITE_ONCE() route for x86, e.g.
>>> convert everything as a follow-up patch (or patches). I'm fairly confident
>>> that KVM's usage of lookup_address_in_mm() is safe, but I wouldn't exactly
>>> bet my life on it. I'd much rather the failing scenario be that KVM uses
>>> a sub-optimal page size as opposed to exploding on a bad pointer.
>> Longpeng(Mike) asked in another e-mail specifically about making similar
>> changes to lookup_address_in_mm(). Replying here as there is more context.
>> I 'think' lookup_address_in_mm is safe from this issue. Why? IIUC, the
>> problem with the huge_pte_offset routine is that the pud changes from
>> pud_none() to pud_huge() in the middle of
>> 'if (pud_huge(*pud) || !pud_present(*pud))'. In the case of
>> lookup_address_in_mm, we know pud was not pud_none() as it was previously
>> checked. I am not aware of any other state transitions which could cause
>> us trouble. However, I am no expert in this area.

Bad copy/paste by me. Longpeng(Mike) was asking about lookup_address_in_pgd.

> So... I need just fix huge_pte_offset in mm/hugetlb.c, right?

Let's start with just a fix for huge_pte_offset() as you can easily reproduce
that issue by adding a delay.

> Is it possible the pud changes from pud_huge() to pud_none() while another CPU
> is walking the pagetable ?

I believe it is possible. If we hole punch a hugetlbfs file, we will clear
the corresponding pud's. Hence, we can go from pud_huge() to pud_none().
Unless I am missing something, that does imply we could have issues in places
such as lookup_address_in_pgd:

pud = pud_offset(p4d, address);
if (pud_none(*pud))
return NULL;

*level = PG_LEVEL_1G;
if (pud_large(*pud) || !pud_present(*pud))
return (pte_t *)pud;

I hope I am wrong, but it seems like pud_none(*pud) could become true after
the initial check, and before the (pud_large) check. If so, there could be
a problem (addressing exception) when the code continues and looks up the pmd.

pmd = pmd_offset(pud, address);
if (pmd_none(*pmd))
return NULL;

It has been mentioned before that there are many page table walks like this.
What am I missing that prevents races like this? Or, have we just been lucky?
Mike Kravetz