lkml.org 
[lkml]   [2018]   [Jul]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    SubjectRe: [RFC PATCH v2 13/27] mm: Handle shadow stack page fault
    From
    Date
    On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
    > + if (is_shstk_mapping(vma->vm_flags))
    > + entry = pte_mkdirty_shstk(entry);
    > + else
    > + entry = pte_mkdirty(entry);
    > +
    > + entry = maybe_mkwrite(entry, vma);
    > if (ptep_set_access_flags(vma, vmf->address, vmf->pte, entry, 1))
    > update_mmu_cache(vma, vmf->address, vmf->pte);
    > pte_unmap_unlock(vmf->pte, vmf->ptl);
    > @@ -2526,7 +2532,11 @@ static int wp_page_copy(struct vm_fault *vmf)
    > }
    > flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
    > entry = mk_pte(new_page, vma->vm_page_prot);
    > - entry = maybe_mkwrite(pte_mkdirty(entry), vma);
    > + if (is_shstk_mapping(vma->vm_flags))
    > + entry = pte_mkdirty_shstk(entry);
    > + else
    > + entry = pte_mkdirty(entry);
    > + entry = maybe_mkwrite(entry, vma);

    Do we want to lift this hunk of code and put it elsewhere? Maybe:

    entry = pte_set_vma_features(entry, vma);

    and then:

    pte_t pte_set_vma_features(pte_t entry, struct vm_area_struct)
    {
    /*
    * Shadow stack PTEs are always dirty and always
    * writable. They have a different encoding for
    * this than normal PTEs, though.
    */
    if (is_shstk_mapping(vma->vm_flags))
    entry = pte_mkdirty_shstk(entry);
    else
    entry = pte_mkdirty(entry);

    entry = maybe_mkwrite(entry, vma);

    return entry;
    }

    > /*
    > * Clear the pte entry and flush it first, before updating the
    > * pte with the new entry. This will avoid a race condition
    > @@ -3201,6 +3211,14 @@ static int do_anonymous_page(struct vm_fault *vmf)
    > mem_cgroup_commit_charge(page, memcg, false, false);
    > lru_cache_add_active_or_unevictable(page, vma);
    > setpte:
    > + /*
    > + * If this is within a shadow stack mapping, mark
    > + * the PTE dirty. We don't use pte_mkdirty(),
    > + * because the PTE must have _PAGE_DIRTY_HW set.
    > + */
    > + if (is_shstk_mapping(vma->vm_flags))
    > + entry = pte_mkdirty_shstk(entry);
    > +
    > set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);

    I'm not sure this is the right spot to do this.

    The other code does pte_mkdirty_shstk() near where we do the
    pte_mkwrite(). Why not here? I think you might have picked this
    because it's a common path used by both allocated pages and zero pages.
    But, we can't have the zero pages in shadow stack areas since they can't
    be read-only. I think you need to move this up. Can you even
    consolidate it with the other two pte_mkdirt_shstk() call sites?

    > /* No need to invalidate - it was non-present before */
    > @@ -3983,6 +4001,14 @@ static int handle_pte_fault(struct vm_fault *vmf)
    > entry = vmf->orig_pte;
    > if (unlikely(!pte_same(*vmf->pte, entry)))
    > goto unlock;
    > +
    > + /*
    > + * Shadow stack PTEs are copy-on-access, so do_wp_page()
    > + * handling on them no matter if we have write fault or not.
    > + */

    I'd say this differently:

    Shadow stack PTEs can not be read-only and because of that can
    not have traditional copy-on-write semantics. This essentially
    performs a copy-on-write operation, but on *any* access, not
    just actual writes.

    \
     
     \ /
      Last update: 2018-07-11 01:07    [W:4.625 / U:0.076 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site