Re: [PATCH v3] mm: Fix race between __split_huge_pmd_locked() and GUP-fast

[Anshuman Khandual]

On 5/1/24 20:03, 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.
> 
> On arm64 at least, pmd_mkinvalid() will mark the pmd such that any
> future call to pmd_present() will return true. And therefore any
> 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.
> 
> x86 does not suffer the above problem, but instead pmd_mkinvalid() will
> corrupt the offset field of the swap entry within the swap pte. See link
> below for discussion of that problem.
> 
> Fix all of this by only calling pmdp_invalidate() for a present pmd. And
> for good measure let's add a warning to all implementations of
> pmdp_invalidate[_ad](). 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.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/all/0dd7827a-6334-439a-8fd0-43c98e6af22b@xxxxxxx/
> Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path")
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
> 
> Right v3; this goes back to the original approach in v1 to fix core-mm rather
> than push the fix into arm64, since we discovered that x86 can't handle
> pmd_mkinvalid() being called for non-present pmds either.

This is a better approach indeed.

> 
> I'm pulling in more arch maintainers because this version adds some warnings in
> arch code to help spot incorrect usage.
> 
> Although Catalin had already accepted v2 (fixing arm64) [2] into for-next/fixes,
> he's agreed to either remove or revert it.
> 
> 
> Changes since v1 [1]
> ====================
> 
>   - Improve pmdp_mkinvalid() docs to make it clear it can only be called for
>     present pmd (per JohnH, Zi Yan)
>   - Added warnings to arch overrides of pmdp_invalidate[_ad]() (per Zi Yan)
>   - Moved comment next to new location of pmpd_invalidate() (per Zi Yan)
> 
> 
> [1] https://lore.kernel.org/linux-mm/20240425170704.3379492-1-ryan.roberts@xxxxxxx/
> [2] https://lore.kernel.org/all/20240430133138.732088-1-ryan.roberts@xxxxxxx/
> 
> Thanks,
> Ryan
> 
> 
>  Documentation/mm/arch_pgtable_helpers.rst |  6 ++-
>  arch/powerpc/mm/book3s64/pgtable.c        |  1 +
>  arch/s390/include/asm/pgtable.h           |  4 +-
>  arch/sparc/mm/tlb.c                       |  1 +
>  arch/x86/mm/pgtable.c                     |  2 +
>  mm/huge_memory.c                          | 49 ++++++++++++-----------
>  mm/pgtable-generic.c                      |  2 +
>  7 files changed, 39 insertions(+), 26 deletions(-)
> 
> diff --git a/Documentation/mm/arch_pgtable_helpers.rst b/Documentation/mm/arch_pgtable_helpers.rst
> index 2466d3363af7..ad50ca6f495e 100644
> --- a/Documentation/mm/arch_pgtable_helpers.rst
> +++ b/Documentation/mm/arch_pgtable_helpers.rst
> @@ -140,7 +140,8 @@ PMD Page Table Helpers
>  +---------------------------+--------------------------------------------------+
>  | pmd_swp_clear_soft_dirty  | Clears a soft dirty swapped PMD                  |
>  +---------------------------+--------------------------------------------------+
> -| pmd_mkinvalid             | Invalidates a mapped PMD [1]                     |
> +| pmd_mkinvalid             | Invalidates a present PMD; do not call for       |
> +|                           | non-present PMD [1]                              |
>  +---------------------------+--------------------------------------------------+
>  | pmd_set_huge              | Creates a PMD huge mapping                       |
>  +---------------------------+--------------------------------------------------+
> @@ -196,7 +197,8 @@ PUD Page Table Helpers
>  +---------------------------+--------------------------------------------------+
>  | pud_mkdevmap              | Creates a ZONE_DEVICE mapped PUD                 |
>  +---------------------------+--------------------------------------------------+
> -| pud_mkinvalid             | Invalidates a mapped PUD [1]                     |
> +| pud_mkinvalid             | Invalidates a present PUD; do not call for       |
> +|                           | non-present PUD [1]                              |
>  +---------------------------+--------------------------------------------------+
>  | pud_set_huge              | Creates a PUD huge mapping                       |
>  +---------------------------+--------------------------------------------------+

LGTM but guess this will conflict with your other patch for mm/debug_vm_pgtable.c
if you choose to update pud_mkinvalid() description for pmd_leaf().

https://lore.kernel.org/all/20240501144439.1389048-1-ryan.roberts@xxxxxxx/

> diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
> index 83823db3488b..2975ea0841ba 100644
> --- a/arch/powerpc/mm/book3s64/pgtable.c
> +++ b/arch/powerpc/mm/book3s64/pgtable.c
> @@ -170,6 +170,7 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>  {
>  	unsigned long old_pmd;
> 
> +	VM_WARN_ON_ONCE(!pmd_present(*pmdp));
>  	old_pmd = pmd_hugepage_update(vma->vm_mm, address, pmdp, _PAGE_PRESENT, _PAGE_INVALID);
>  	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>  	return __pmd(old_pmd);
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 60950e7a25f5..480bea44559d 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -1768,8 +1768,10 @@ static inline pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma,
>  static inline pmd_t pmdp_invalidate(struct vm_area_struct *vma,
>  				   unsigned long addr, pmd_t *pmdp)
>  {
> -	pmd_t pmd = __pmd(pmd_val(*pmdp) | _SEGMENT_ENTRY_INVALID);
> +	pmd_t pmd;
> 
> +	VM_WARN_ON_ONCE(!pmd_present(*pmdp));
> +	pmd = __pmd(pmd_val(*pmdp) | _SEGMENT_ENTRY_INVALID);
>  	return pmdp_xchg_direct(vma->vm_mm, addr, pmdp, pmd);
>  }
> 
> diff --git a/arch/sparc/mm/tlb.c b/arch/sparc/mm/tlb.c
> index b44d79d778c7..ef69127d7e5e 100644
> --- a/arch/sparc/mm/tlb.c
> +++ b/arch/sparc/mm/tlb.c
> @@ -249,6 +249,7 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>  {
>  	pmd_t old, entry;
> 
> +	VM_WARN_ON_ONCE(!pmd_present(*pmdp));
>  	entry = __pmd(pmd_val(*pmdp) & ~_PAGE_VALID);
>  	old = pmdp_establish(vma, address, pmdp, entry);
>  	flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index d007591b8059..103cbccf1d7d 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -631,6 +631,8 @@ int pmdp_clear_flush_young(struct vm_area_struct *vma,
>  pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address,
>  			 pmd_t *pmdp)
>  {
> +	VM_WARN_ON_ONCE(!pmd_present(*pmdp));
> +
>  	/*
>  	 * No flush is necessary. Once an invalid PTE is established, the PTE's
>  	 * access and dirty bits cannot be updated.
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 89f58c7603b2..dd1fc105f70b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2493,32 +2493,11 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  		return __split_huge_zero_page_pmd(vma, haddr, pmd);
>  	}
> 
> -	/*
> -	 * Up to this point the pmd is present and huge and userland has the
> -	 * whole access to the hugepage during the split (which happens in
> -	 * place). If we overwrite the pmd with the not-huge version pointing
> -	 * to the pte here (which of course we could if all CPUs were bug
> -	 * free), userland could trigger a small page size TLB miss on the
> -	 * small sized TLB while the hugepage TLB entry is still established in
> -	 * the huge TLB. Some CPU doesn't like that.
> -	 * See http://support.amd.com/TechDocs/41322_10h_Rev_Gd.pdf, Erratum
> -	 * 383 on page 105. Intel should be safe but is also warns that it's
> -	 * only safe if the permission and cache attributes of the two entries
> -	 * loaded in the two TLB is identical (which should be the case here).
> -	 * But it is generally safer to never allow small and huge TLB entries
> -	 * for the same virtual address to be loaded simultaneously. So instead
> -	 * of doing "pmd_populate(); flush_pmd_tlb_range();" we first mark the
> -	 * current pmd notpresent (atomically because here the pmd_trans_huge
> -	 * must remain set at all times on the pmd until the split is complete
> -	 * 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 +2508,30 @@ 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 {
> +		/*
> +		 * Up to this point the pmd is present and huge and userland has
> +		 * the whole access to the hugepage during the split (which
> +		 * happens in place). If we overwrite the pmd with the not-huge
> +		 * version pointing to the pte here (which of course we could if
> +		 * all CPUs were bug free), userland could trigger a small page
> +		 * size TLB miss on the small sized TLB while the hugepage TLB
> +		 * entry is still established in the huge TLB. Some CPU doesn't
> +		 * like that. See
> +		 * http://support.amd.com/TechDocs/41322_10h_Rev_Gd.pdf, Erratum
> +		 * 383 on page 105. Intel should be safe but is also warns that
> +		 * it's only safe if the permission and cache attributes of the
> +		 * two entries loaded in the two TLB is identical (which should
> +		 * be the case here). But it is generally safer to never allow
> +		 * small and huge TLB entries for the same virtual address to be
> +		 * loaded simultaneously. So instead of doing "pmd_populate();
> +		 * flush_pmd_tlb_range();" we first mark the current pmd
> +		 * notpresent (atomically because here the pmd_trans_huge must
> +		 * remain set at all times on the pmd until the split is
> +		 * complete 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);
>  		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..a78a4adf711a 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_ONCE(!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_ONCE(!pmd_present(*pmdp));
>  	return pmdp_invalidate(vma, address, pmdp);
>  }
>  #endif

Rest LGTM but let's wait for this to run on multiple platforms.

Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>