Manually set NX bit of specific PTE

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                          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?
                        

Asked by Mar Tsan (21 rep)

Aug 8, 2018, 09:05 AM
Last activity: Dec 20, 2018, 06:55 AM

Manually set NX bit of specific PTE

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