From 2c8a21a124cab4391254680cda796036d061d9b9 Mon Sep 17 00:00:00 2001 From: James Houghton Date: Mon, 4 Dec 2023 17:26:46 +0000 Subject: [PATCH] arm64: mm: Always make sw-dirty PTEs hw-dirty in pte_modify commit 3c0696076aad60a2f04c019761921954579e1b0e upstream. It is currently possible for a userspace application to enter an infinite page fault loop when using HugeTLB pages implemented with contiguous PTEs when HAFDBS is not available. This happens because: 1. The kernel may sometimes write PTEs that are sw-dirty but hw-clean (PTE_DIRTY | PTE_RDONLY | PTE_WRITE). 2. If, during a write, the CPU uses a sw-dirty, hw-clean PTE in handling the memory access on a system without HAFDBS, we will get a page fault. 3. HugeTLB will check if it needs to update the dirty bits on the PTE. For contiguous PTEs, it will check to see if the pgprot bits need updating. In this case, HugeTLB wants to write a sequence of sw-dirty, hw-dirty PTEs, but it finds that all the PTEs it is about to overwrite are all pte_dirty() (pte_sw_dirty() => pte_dirty()), so it thinks no update is necessary. We can get the kernel to write a sw-dirty, hw-clean PTE with the following steps (showing the relevant VMA flags and pgprot bits): i. Create a valid, writable contiguous PTE. VMA vmflags: VM_SHARED | VM_READ | VM_WRITE VMA pgprot bits: PTE_RDONLY | PTE_WRITE PTE pgprot bits: PTE_DIRTY | PTE_WRITE ii. mprotect the VMA to PROT_NONE. VMA vmflags: VM_SHARED VMA pgprot bits: PTE_RDONLY PTE pgprot bits: PTE_DIRTY | PTE_RDONLY iii. mprotect the VMA back to PROT_READ | PROT_WRITE. VMA vmflags: VM_SHARED | VM_READ | VM_WRITE VMA pgprot bits: PTE_RDONLY | PTE_WRITE PTE pgprot bits: PTE_DIRTY | PTE_WRITE | PTE_RDONLY Make it impossible to create a writeable sw-dirty, hw-clean PTE with pte_modify(). Such a PTE should be impossible to create, and there may be places that assume that pte_dirty() implies pte_hw_dirty(). Signed-off-by: James Houghton Fixes: 031e6e6b4e12 ("arm64: hugetlb: Avoid unnecessary clearing in huge_ptep_set_access_flags") Cc: Acked-by: Will Deacon Reviewed-by: Ryan Roberts Link: https://lore.kernel.org/r/20231204172646.2541916-3-jthoughton@google.com Signed-off-by: Catalin Marinas Signed-off-by: Greg Kroah-Hartman --- arch/arm64/include/asm/pgtable.h | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index 7f7d9b1..07bdf5d 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -826,6 +826,12 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot) pte = set_pte_bit(pte, __pgprot(PTE_DIRTY)); pte_val(pte) = (pte_val(pte) & ~mask) | (pgprot_val(newprot) & mask); + /* + * If we end up clearing hw dirtiness for a sw-dirty PTE, set hardware + * dirtiness again. + */ + if (pte_sw_dirty(pte)) + pte = pte_mkdirty(pte); return pte; } -- 2.7.4