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

[Anshuman Khandual]

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 ?

> 
> 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		|
-------------------------------------------------------------------------


> 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 ?
 
> 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@xxxxxxx>
> ---
> 
> 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
> 
>