mjhjmtu

On Ubuntu with kernel 4.16.7 I am writing a custom system call and, as the title states, 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?

Thanks in advance!

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.