[PATCH AUTOSEL 5.4 17/22] nios2: force update_mmu_cache on spurious tlb-permission--related pagefaults

[Sasha Levin <sashal@kernel.org>]

From: Simon Schuster <schuster.simon@siemens-energy.com>

[ Upstream commit 2d8a3179ea035f9341b6a73e5ba4029fc67e983d ]

NIOS2 uses a software-managed TLB for virtual address translation. To
flush a cache line, the original mapping is replaced by one to physical
address 0x0 with no permissions (rwx mapped to 0) set. This can lead to
TLB-permission--related traps when such a nominally flushed entry is
encountered as a mapping for an otherwise valid virtual address within a
process (e.g. due to an MMU-PID-namespace rollover that previously
flushed the complete TLB including entries of existing, running
processes).

The default ptep_set_access_flags implementation from mm/pgtable-generic.c
only forces a TLB-update when the page-table entry has changed within the
page table:

	/*
	 * [...] We return whether the PTE actually changed, which in turn
	 * instructs the caller to do things like update__mmu_cache. [...]
	 */
	int ptep_set_access_flags(struct vm_area_struct *vma,
				  unsigned long address, pte_t *ptep,
				  pte_t entry, int dirty)
	{
		int changed = !pte_same(*ptep, entry);
		if (changed) {
			set_pte_at(vma->vm_mm, address, ptep, entry);
			flush_tlb_fix_spurious_fault(vma, address);
		}
		return changed;
	}

However, no cross-referencing with the TLB-state occurs, so the
flushing-induced pseudo entries that are responsible for the pagefault
in the first place are never pre-empted from TLB on this code path.

This commit fixes this behaviour by always requesting a TLB-update in
this part of the pagefault handling, fixing spurious page-faults on the
way. The handling is a straightforward port of the logic from the MIPS
architecture via an arch-specific ptep_set_access_flags function ported
from arch/mips/include/asm/pgtable.h.

Signed-off-by: Simon Schuster <schuster.simon@siemens-energy.com>
Signed-off-by: Andreas Oetken <andreas.oetken@siemens-energy.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

**YES** This commit should be backported to stable kernel trees for the
following reasons: **1. Fixes a Real User-Affecting Bug:** The commit
addresses infinite page fault loops that can occur when: - NIOS2's
software-managed TLB encounters spurious permission-related page faults
- TLB entries flushed to physical address 0x0 with no permissions
persist due to MMU-PID-namespace rollovers - The generic
`ptep_set_access_flags` implementation fails to flush these stale TLB
entries because it only flushes when the PTE actually changes in the
page table **2. Small, Contained Fix:** The code change is minimal and
surgical: - Adds an arch-specific `ptep_set_access_flags` function in
`arch/nios2/include/asm/pgtable.h:284-297` - Always returns `true` to
force `update_mmu_cache` execution, ensuring TLB invalidation even when
the PTE hasn't changed - Identical pattern to MIPS architecture
(arch/mips/include/asm/pgtable.h), proven and stable **3. Architecture-
Specific with No Side Effects:** - Only affects NIOS2 architecture
(`arch/nios2/include/asm/pgtable.h`) - No cross-architecture
dependencies or changes to core MM code - Uses existing `set_ptes()` and
`update_mmu_cache` infrastructure **4. Consistent with Similar
Backported Fixes:** - Similar commit #5 (xtensa: define update_mmu_tlb
function) was marked **YES** for backport with `Cc:
stable@vger.kernel.org # 5.12+` - Both fix TLB invalidation issues in
software-managed TLB architectures - Both address spurious page fault
scenarios **5. Meets Stable Tree Criteria:** - **Important bugfix**:
Prevents infinite page fault loops - **Minimal risk**: Architecture-
specific, follows proven MIPS pattern - **No architectural changes**:
Uses existing MM infrastructure - **Confined to subsystem**: Limited to
NIOS2 TLB management The fix directly addresses a critical reliability
issue where users experience system hangs due to infinite page faults,
making it an ideal candidate for stable backporting.

 arch/nios2/include/asm/pgtable.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/nios2/include/asm/pgtable.h b/arch/nios2/include/asm/pgtable.h
index 99985d8b71664..506bbc7730879 100644
--- a/arch/nios2/include/asm/pgtable.h
+++ b/arch/nios2/include/asm/pgtable.h
@@ -297,4 +297,20 @@ extern void __init mmu_init(void);
 extern void update_mmu_cache(struct vm_area_struct *vma,
 			     unsigned long address, pte_t *pte);
 
+static inline int pte_same(pte_t pte_a, pte_t pte_b);
+
+#define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
+static inline int ptep_set_access_flags(struct vm_area_struct *vma,
+					unsigned long address, pte_t *ptep,
+					pte_t entry, int dirty)
+{
+	if (!pte_same(*ptep, entry))
+		set_ptes(vma->vm_mm, address, ptep, entry, 1);
+	/*
+	 * update_mmu_cache will unconditionally execute, handling both
+	 * the case that the PTE changed and the spurious fault case.
+	 */
+	return true;
+}
+
 #endif /* _ASM_NIOS2_PGTABLE_H */
-- 
2.39.5