Re: [PATCH v1] mm: Fix race between __split_huge_pmd_locked() and GUP-fast

[Ryan Roberts <ryan.roberts@arm.com>]

On 26/04/2024 05:19, Anshuman Khandual wrote:
> On 4/25/24 22:37, Ryan Roberts wrote:
>> __split_huge_pmd_locked() can be called for a present THP, devmap or
>> (non-present) migration entry. It calls pmdp_invalidate()
>> unconditionally on the pmdp and only determines if it is present or not
>> based on the returned old pmd. This is a problem for the migration entry
>> case because pmd_mkinvalid(), called by pmdp_invalidate() must only be
>> called for a present pmd.
> 
> pmdp_invalidate() must be called only for present PMD - is this expected
> by core MM ? Does this cause any problem otherwise ?

I'm saying that only calling pmdp_invalidate() on a pte_present()==true pte is
the only semantic that makes sense. And, yes, it causes a problem if called on a
pte_present()==false pte - that's exactly what I'm describing in this commit log.

To labour the point, this is the logical type hierachy of PTEs (and block-mapped
PMDs) as I see it:

---8<----

pte
 |- present
 |   |- valid
 |   |- invalid
 |
 |- not_present
     |- none
     |- swap_pte

present: All fields must be interpretted the way the HW sees them. e.g.
         pte_pfn(), pte_write(), pte_dirty(), pte_young(), pte_mkwrite(),
         pte_mkold() can all be legitimately used to query and modify the pte.

  valid: The HW may access the pte, interpret the fields and create a TLB entry,
         etc.

  invalid: The HW will never access the pte or create a TLB entry for it.

not_present: The fields are SW-defined. HW never accesses the PTE.

  none: Unused; represents a hole

  swap_pte: Contains a swap entry and swap pte bits. The contained swap entry
            may 1 of a few different types e.g. actual swap entry, migration
            entry, hw poison, etc.

---8<----

We test present vs not_present with pte_present()

We test none vs swap_pte with pte_none()

valid vs invalid is slightly more vague. The core-mm can move a PMD from valid
-> invalid by calling pmd_mkinvalid(). But it can't query the state. And it
can't do this generically for a PTE.


Based on that lot, it makes no sense to me that we should permit calling
pmd_mkinvalid() on a non-present pte. Indeed, we don't permit calling
pte_mkwrite() etc on a non-present pte. And those functions are not defensive;
they don't check that the pte is present before making the change. They just
trust that the core-mm will not call them for non-present ptes.

The alternative approach would be to make pmdp_invalidate() defensive so that it
checks the pmd is present before making any changes. But it doesn't semantically
make sense to invalidate a non-present pmd in the first place so why call it
under these circumstances? There is also a practical problem in that some arches
implement their own pmdp_invalidate() so you would want to make all those
defensive too, which would grow the size of the change.


> 
>>
>> On arm64 at least, pmd_mkinvalid() will mark the pmd such that any
>> future call to pmd_present() will return true. And therefore any
> 
> IIRC the following semantics needs to be followed as expected by core MM.
> 
> -------------------------------------------------------------------------
> |	PMD states	|	pmd_present	|	pmd_trans_huge	|
> -------------------------------------------------------------------------
> |	Mapped		|	Yes		|	Yes		|
> -------------------------------------------------------------------------
> |	Splitting	|	Yes		|	Yes		|
> -------------------------------------------------------------------------
> |	Migration/Swap	|	No		|	No		|
> -------------------------------------------------------------------------

Indeed, the problem, as I see it, is if pmd_mkinvalid() is called on a
"Migration/Swap" pmd, then a future call to pmd_present() will return Yes, which
is clearly wrong. pmd_trans_huge() will also return Yes due to:

static inline int pmd_trans_huge(pmd_t pmd)
{
	return pmd_val(pmd) && pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT);
}

At least this happens for arm64. Although Zi suggests other arches look like
they will do this too in the other email.

The reason is that arm64's pmd_mkinvalid() unconditionally sets
PMD_PRESENT_INVALID (bit 59) and clears PMD_SECT_VALID (bit 0) in the pte. So
next time pmd_present() is called it will see PMD_PRESENT_INVALID is set and
return true.

> 
> 
>> lockless pgtable walker could see the migration entry pmd in this state
>> and start interpretting the fields as if it were present, leading to
>> BadThings (TM). GUP-fast appears to be one such lockless pgtable walker.
> 
> Could you please explain how bad things might happen ?

See 2 places where pmdp_get_lockless() is called in gup.c, without the PTL.
These could both return the swap pte for which pmd_mkinvalid() has been called.
In both cases, this would lead to the pmd_present() check eroneously returning
true, eventually causing incorrect interpretation of the pte fields. e.g.:

gup_pmd_range()
  pmd_t pmd = pmdp_get_lockless(pmdp);
  gup_huge_pmd(pmd, ...)
    page = nth_page(pmd_page(orig), (addr & ~PMD_MASK) >> PAGE_SHIFT);

page is guff.

Let me know what you think!

Thanks,
Ryan


>  
>> I suspect the same is possible on other architectures.
>>
>> Fix this by only calling pmdp_invalidate() for a present pmd. And for
>> good measure let's add a warning to the generic implementation of
>> pmdp_invalidate(). I've manually reviewed all other
>> pmdp_invalidate[_ad]() call sites and believe all others to be
>> conformant.
>>
>> This is a theoretical bug found during code review. I don't have any
>> test case to trigger it in practice.
>>
>> Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path")
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>
>> Applies on top of v6.9-rc5. Passes all the mm selftests on arm64.
>>
>> Thanks,
>> Ryan
>>
>>
>>  mm/huge_memory.c     | 5 +++--
>>  mm/pgtable-generic.c | 2 ++
>>  2 files changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 89f58c7603b2..80939ad00718 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2513,12 +2513,12 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>  	 * for this pmd), then we flush the SMP TLB and finally we write the
>>  	 * non-huge version of the pmd entry with pmd_populate.
>>  	 */
>> -	old_pmd = pmdp_invalidate(vma, haddr, pmd);
>>
>> -	pmd_migration = is_pmd_migration_entry(old_pmd);
>> +	pmd_migration = is_pmd_migration_entry(*pmd);
>>  	if (unlikely(pmd_migration)) {
>>  		swp_entry_t entry;
>>
>> +		old_pmd = *pmd;
>>  		entry = pmd_to_swp_entry(old_pmd);
>>  		page = pfn_swap_entry_to_page(entry);
>>  		write = is_writable_migration_entry(entry);
>> @@ -2529,6 +2529,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>  		soft_dirty = pmd_swp_soft_dirty(old_pmd);
>>  		uffd_wp = pmd_swp_uffd_wp(old_pmd);
>>  	} else {
>> +		old_pmd = pmdp_invalidate(vma, haddr, pmd);
>>  		page = pmd_page(old_pmd);
>>  		folio = page_folio(page);
>>  		if (pmd_dirty(old_pmd)) {
>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>> index 4fcd959dcc4d..74e34ea90656 100644
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -198,6 +198,7 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp)
>>  pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>>  		     pmd_t *pmdp)
>>  {
>> +	VM_WARN_ON(!pmd_present(*pmdp));
>>  	pmd_t old = pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp));
>>  	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>>  	return old;
>> @@ -208,6 +209,7 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>>  pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address,
>>  			 pmd_t *pmdp)
>>  {
>> +	VM_WARN_ON(!pmd_present(*pmdp));
>>  	return pmdp_invalidate(vma, address, pmdp);
>>  }
>>  #endif
>> --
>> 2.25.1
>>
>>