contpte_ptep_set_access_flags() compared the gathered ptep_get() value against the requested entry to detect no-ops. ptep_get() ORs AF/dirty from all sub-PTEs in the CONT block, so a dirty sibling can make the target appear already-dirty. When the gathered value matches entry, the function returns 0 even though the target sub-PTE still has PTE_RDONLY set in hardware. For a CPU with FEAT_HAFDBS this gathered view is fine, since hardware may set AF/dirty on any sub-PTE and CPU TLB behavior is effectively gathered across the CONT range. But page-table walkers that evaluate each descriptor individually (e.g. a CPU without DBM support, or an SMMU without HTTU, or with HA/HD disabled in CD.TCR) can keep faulting on the unchanged target sub-PTE, causing an infinite fault loop. Gathering can therefore cause false no-ops when only a sibling has been updated: - write faults: target still has PTE_RDONLY (needs PTE_RDONLY cleared) - read faults: target still lacks PTE_AF Fix by checking each sub-PTE against the requested AF/dirty/write state (the same bits consumed by __ptep_set_access_flags()), using raw per-PTE values rather than the gathered ptep_get() view, before returning no-op. Keep using the raw target PTE for the write-bit unfold decision. Per Arm ARM (DDI 0487) D8.7.1 ("The Contiguous bit"), any sub-PTE in a CONT range may become the effective cached translation and software must maintain consistent attributes across the range. Fixes: 4602e5757bcc ("arm64/mm: wire up PTE_CONT for user mappings") Cc: Ryan Roberts Cc: Catalin Marinas Cc: Will Deacon Cc: Jason Gunthorpe Cc: John Hubbard Cc: Zi Yan Cc: Breno Leitao Cc: stable@vger.kernel.org Reviewed-by: Alistair Popple Reviewed-by: James Houghton Reviewed-by: Ryan Roberts Reviewed-by: Catalin Marinas Tested-by: Breno Leitao Signed-off-by: Piotr Jaroszynski --- Changes in v2: - Clarify commit message/comments: issue affects per-descriptor walkers (CPU without DBM support, or SMMU without HTTU / with HA/HD disabled). - Clarify sub-PTE comparison semantics: use raw per-PTE values and match bits consumed by __ptep_set_access_flags() (AF, DIRTY, write permission). - Add Reviewed-by/Tested-by trailers from the v1 thread. --- arch/arm64/mm/contpte.c | 53 +++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 49 insertions(+), 4 deletions(-) diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c index b929a455103f..1519d090d5ea 100644 --- a/arch/arm64/mm/contpte.c +++ b/arch/arm64/mm/contpte.c @@ -599,6 +599,27 @@ void contpte_clear_young_dirty_ptes(struct vm_area_struct *vma, } EXPORT_SYMBOL_GPL(contpte_clear_young_dirty_ptes); +static bool contpte_all_subptes_match_access_flags(pte_t *ptep, pte_t entry) +{ + pte_t *cont_ptep = contpte_align_down(ptep); + /* + * PFNs differ per sub-PTE. Match only bits consumed by + * __ptep_set_access_flags(): AF, DIRTY and write permission. + */ + const pteval_t cmp_mask = PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY; + pteval_t entry_cmp = pte_val(entry) & cmp_mask; + int i; + + for (i = 0; i < CONT_PTES; i++) { + pteval_t pte_cmp = pte_val(__ptep_get(cont_ptep + i)) & cmp_mask; + + if (pte_cmp != entry_cmp) + return false; + } + + return true; +} + int contpte_ptep_set_access_flags(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep, pte_t entry, int dirty) @@ -608,13 +629,37 @@ int contpte_ptep_set_access_flags(struct vm_area_struct *vma, int i; /* - * Gather the access/dirty bits for the contiguous range. If nothing has - * changed, its a noop. + * Check whether all sub-PTEs in the CONT block already match the + * requested access flags/write permission, using raw per-PTE values + * rather than the gathered ptep_get() view. + * + * __ptep_set_access_flags() can update AF, dirty and write + * permission, but only to make the mapping more permissive. + * + * ptep_get() gathers AF/dirty state across the whole CONT block, + * which is correct for a CPU with FEAT_HAFDBS. But page-table + * walkers that evaluate each descriptor individually (e.g. a CPU + * without DBM support, or an SMMU without HTTU, or with HA/HD + * disabled in CD.TCR) can keep faulting on the target sub-PTE if + * only a sibling has been updated. Gathering can therefore cause + * false no-ops when only a sibling has been updated: + * - write faults: target still has PTE_RDONLY (needs PTE_RDONLY cleared) + * - read faults: target still lacks PTE_AF + * + * Per Arm ARM (DDI 0487) D8.7.1, any sub-PTE in a CONT range may + * become the effective cached translation, so all entries must have + * consistent attributes. Check the full CONT block before returning + * no-op, and when any sub-PTE mismatches, proceed to update the whole + * range. */ - orig_pte = pte_mknoncont(ptep_get(ptep)); - if (pte_val(orig_pte) == pte_val(entry)) + if (contpte_all_subptes_match_access_flags(ptep, entry)) return 0; + /* + * Use raw target pte (not gathered) for write-bit unfold decision. + */ + orig_pte = pte_mknoncont(__ptep_get(ptep)); + /* * We can fix up access/dirty bits without having to unfold the contig * range. But if the write bit is changing, we must unfold. --- base-commit: c107785c7e8dbabd1c18301a1c362544b5786282 change-id: 20260305-contpte-fault-loop-76ed911b01c0 Best regards, -- Piotr Jaroszynski