Manually set NX bit of specific PTE

Question

On Ubuntu with kernel 4.16.7 I am writing a custom system call and, I want to set the NX bit of a specific Page Table Entry. So far I have this piece of code, where I am doing a page table walk to get the PTE I want and then try to set its NX bit:

pgd = pgd_offset(mm, addr);
if (pgd_none(*pgd) || pgd_bad(*pgd)){
    printk("Invalid pgd");
    return -1;
}

p4d = p4d_offset(pgd, addr);
if (p4d_none(*p4d) || p4d_bad(*p4d)){
    printk("Invalid p4d");
    return -1;
}

pud = pud_offset(p4d, addr);
if (pud_none(*pud) || pud_bad(*pud)){
    printk("Invalid pud");
    return -1;
}

pmd = pmd_offset(pud, addr);
if (pmd_none(*pmd) || pmd_bad(*pmd)){
    printk("Invalid pmd");
    return -1;
}

ptep = pte_offset_map(pmd, addr);
if (!ptep){
    printk("Invalid ptep");
    return -1;
}
pte = *ptep;

if (pte_present(pte)){
    printk("pte_set_flags");
    printk("NX bit before:  %d", pte_exec(pte));
    // pte_set_flags(pte, _PAGE_NX);
    // printk("NX bit after :  %d", pte_exec(pte));
    printk("pte_clear_flags");
    // pte_clear_flags(pte, _PAGE_NX); // Same as pte_mkexec()
    pte_mkexec(pte);
    printk("NX bit after :  %d", pte_exec(pte));
    page = pte_page(pte);
    if (page){
        printk("Page frame struct is @ %p", page);
    }

    pte_unmap(ptep);
}

but it doesn't work. All the printk commands show the same result. Any insight?

Mar Tsan · Accepted Answer · 2018-08-17T09:21:22.047

So I found it by myself. The code is as follows:

struct vm_area_struct *vma;
unsigned long oldflags, newflags, pfn;

vma = find_extend_vma(mm, addr);
oldflags = vma->vm_flags;
newflags = oldflags &= ~VM_EXEC;

//...
//...
//...

if (pte_present(pte)){
    printk("NX bit before: %d", pte_exec(pte));

    pte = pte_modify(pte, vm_get_page_prot(newflags));
    printk("NX bit after:  %d", pte_exec(pte));

    pfn = pte_pfn(pte);

    flush_cache_page(vma, addr, pfn);
    set_pte(ptep, pte);
    flush_tlb_page(vma, addr);
    update_mmu_cache(vma, addr, ptep);

    pte_unmap(ptep);
}

and thus the NX bit of a specific PTE is changed.

Manually set NX bit of specific PTE

1 Answers1