From bbb9db73f0b8983c026c1be5e5d3f71582032c4f Mon Sep 17 00:00:00 2001 From: Sergei Antonov Date: Thu, 8 Sep 2022 23:48:09 +0300 Subject: [PATCH] mm: bring back update_mmu_cache() to finish_fault() Running this test program on ARMv4 a few times (sometimes just once) reproduces the bug. int main() { unsigned i; char paragon[SIZE]; void* ptr; memset(paragon, 0xAA, SIZE); ptr = mmap(NULL, SIZE, PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, -1, 0); if (ptr == MAP_FAILED) return 1; printf("ptr = %p\n", ptr); for (i=0;i<10000;i++){ memset(ptr, 0xAA, SIZE); if (memcmp(ptr, paragon, SIZE)) { printf("Unexpected bytes on iteration %u!!!\n", i); break; } } munmap(ptr, SIZE); } In the "ptr" buffer there appear runs of zero bytes which are aligned by 16 and their lengths are multiple of 16. Linux v5.11 does not have the bug, "git bisect" finds the first bad commit: 33fe6a60a178 ("mm: Cleanup faultaround and finish_fault() codepaths") Before the commit update_mmu_cache() was called during a call to filemap_map_pages() as well as finish_fault(). After the commit finish_fault() lacks it. Bring back update_mmu_cache() to finish_fault() to fix the bug. Also call update_mmu_tlb() only when returning VM_FAULT_NOPAGE to more closely reproduce the code of alloc_set_pte() function that existed before the commit. On many platforms update_mmu_cache() is nop: x86, see arch/x86/include/asm/pgtable ARMv6+, see arch/arm/include/asm/tlbflush.h So, it seems, few users ran into this bug. Link: https://lkml.kernel.org/r/20220908204809.2012451-1-saproj@gmail.com Fixes: 33fe6a60a178 ("mm: Cleanup faultaround and finish_fault() codepaths") Signed-off-by: Sergei Antonov Acked-by: Kirill A. Shutemov Cc: Will Deacon Cc: Signed-off-by: Andrew Morton --- mm/memory.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) diff --git a/mm/memory.c b/mm/memory.c index 4ba73f5aa8bb7..a78814413ac03 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4386,14 +4386,20 @@ vm_fault_t finish_fault(struct vm_fault *vmf) vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address, &vmf->ptl); - ret = 0; + /* Re-check under ptl */ - if (likely(!vmf_pte_changed(vmf))) + if (likely(!vmf_pte_changed(vmf))) { do_set_pte(vmf, page, vmf->address); - else + + /* no need to invalidate: a not-present page won't be cached */ + update_mmu_cache(vma, vmf->address, vmf->pte); + + ret = 0; + } else { + update_mmu_tlb(vma, vmf->address, vmf->pte); ret = VM_FAULT_NOPAGE; + } - update_mmu_tlb(vma, vmf->address, vmf->pte); pte_unmap_unlock(vmf->pte, vmf->ptl); return ret; } -- 2.39.5