[PATCH] arm64: contpte: fix set_access_flags() no-op check for SMMU/ATS faults

[Piotr Jaroszynski <pjaroszynski@nvidia.com>]

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 CPU page-table walks this is benign: with FEAT_HAFDBS the hardware
may set AF/dirty on any sub-PTE and the CPU TLB treats the gathered
result as authoritative for the entire range. But an SMMU without HTTU
(or with HA/HD disabled in CD.TCR) evaluates each descriptor
individually and will keep raising F_PERMISSION 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 all sub-PTEs' access flags individually (not via the
gathered view) before returning no-op, and use the raw target PTE for
the write-bit unfold decision. The access-flag mask matches the one
used by __ptep_set_access_flags().

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")

Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: stable@vger.kernel.org
Signed-off-by: Piotr Jaroszynski <pjaroszynski@nvidia.com>
---
 arch/arm64/mm/contpte.c | 47 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 43 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index bcac4f55f9c1..9868bfe4607c 100644
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -390,6 +390,23 @@ 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);
+	const pteval_t access_mask = PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY;
+	pteval_t entry_access = pte_val(entry) & access_mask;
+	int i;
+
+	for (i = 0; i < CONT_PTES; i++) {
+		pteval_t pte_access = pte_val(__ptep_get(cont_ptep + i)) & access_mask;
+
+		if (pte_access != entry_access)
+			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)
@@ -399,13 +416,35 @@ 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 have the
+	 * requested access flags, using raw per-PTE values rather than the
+	 * gathered ptep_get() view.
+	 *
+	 * ptep_get() gathers AF/dirty state across the whole CONT block,
+	 * which is correct for CPU TLB semantics: with FEAT_HAFDBS the
+	 * hardware may set AF/dirty on any sub-PTE and the CPU TLB treats
+	 * the gathered result as authoritative for the entire range. But an
+	 * SMMU without HTTU (or with HA/HD disabled in CD.TCR) evaluates
+	 * each descriptor individually and will keep faulting on the target
+	 * sub-PTE if its flags haven't actually 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.
-- 
2.22.1.7.gac84d6e93c.dirty