On 02/05/2024 04:03, Anshuman Khandual wrote: > > > 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/ Indeed, a reason to avoid adding docs to that patch :) > >> 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> Thanks!
