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>
