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 */