From: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx> Subject: mm/memory.c: recheck page table entry with page table lock held We clear the pte temporarily during read/modify/write update of the pte. If we take a page fault while the pte is cleared, the application can get SIGBUS. One such case is with remap_pfn_range without a backing vm_ops->fault callback. do_fault will return SIGBUS in that case. cpu 0 cpu1 mprotect() ptep_modify_prot_start()/pte cleared. . . page fault. . . prep_modify_prot_commit() Fix this by taking page table lock and rechecking for pte_none. [aneesh.kumar@xxxxxxxxxxxxx: fix crash observed with syzkaller run] Link: http://lkml.kernel.org/r/87va6bwlfg.fsf@xxxxxxxxxxxxx Link: http://lkml.kernel.org/r/20180926031858.9692-1-aneesh.kumar@xxxxxxxxxxxxx Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx> Cc: Ido Schimmel <idosch@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/memory.c | 34 ++++++++++++++++++++++++++++++---- 1 file changed, 30 insertions(+), 4 deletions(-) --- a/mm/memory.c~mm-recheck-page-table-entry-with-page-table-lock-held +++ a/mm/memory.c @@ -3496,10 +3496,36 @@ static vm_fault_t do_fault(struct vm_fau struct vm_area_struct *vma = vmf->vma; vm_fault_t ret; - /* The VMA was not fully populated on mmap() or missing VM_DONTEXPAND */ - if (!vma->vm_ops->fault) - ret = VM_FAULT_SIGBUS; - else if (!(vmf->flags & FAULT_FLAG_WRITE)) + /* + * The VMA was not fully populated on mmap() or missing VM_DONTEXPAND + */ + if (!vma->vm_ops->fault) { + /* + * If we find a migration pmd entry or a none pmd entry, which + * should never happen, return SIGBUS + */ + if (unlikely(!pmd_present(*vmf->pmd))) + ret = VM_FAULT_SIGBUS; + else { + vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, + vmf->pmd, + vmf->address, + &vmf->ptl); + /* + * Make sure this is not a temporary clearing of pte + * by holding ptl and checking again. A R/M/W update + * of pte involves: take ptl, clearing the pte so that + * we don't have concurrent modification by hardware + * followed by an update. + */ + if (unlikely(pte_none(*vmf->pte))) + ret = VM_FAULT_SIGBUS; + else + ret = VM_FAULT_NOPAGE; + + pte_unmap_unlock(vmf->pte, vmf->ptl); + } + } else if (!(vmf->flags & FAULT_FLAG_WRITE)) ret = do_read_fault(vmf); else if (!(vma->vm_flags & VM_SHARED)) ret = do_cow_fault(vmf); _
