4.19-stable review patch. If anyone has any objections, please let me know.
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From: James Houghton jthoughton@google.com
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 jthoughton@google.com Fixes: 031e6e6b4e12 ("arm64: hugetlb: Avoid unnecessary clearing in huge_ptep_set_access_flags") Cc: stable@vger.kernel.org Acked-by: Will Deacon will@kernel.org Reviewed-by: Ryan Roberts ryan.roberts@arm.com Link: https://lore.kernel.org/r/20231204172646.2541916-3-jthoughton@google.com Signed-off-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/arm64/include/asm/pgtable.h | 6 ++++++ 1 file changed, 6 insertions(+)
--- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -595,6 +595,12 @@ static inline pte_t pte_modify(pte_t pte if (pte_hw_dirty(pte)) pte = pte_mkdirty(pte); 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; }