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    SubjectRe: [PATCH] mm: softdirty: write protect PTEs created for read faults after VM_SOFTDIRTY cleared
    On Wed, Aug 20, 2014 at 05:46:22PM -0400, Peter Feiner wrote:
    > In readable+writable+shared VMAs, PTEs created for read faults have
    > their write bit set. If the read fault happens after VM_SOFTDIRTY is
    > cleared, then the PTE's softdirty bit will remain clear after
    > subsequent writes.
    >
    > Here's a simple code snippet to demonstrate the bug:
    >
    > char* m = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE,
    > MAP_ANONYMOUS | MAP_SHARED, -1, 0);
    > system("echo 4 > /proc/$PPID/clear_refs"); /* clear VM_SOFTDIRTY */
    > assert(*m == '\0'); /* new PTE allows write access */
    > assert(!soft_dirty(x));
    > *m = 'x'; /* should dirty the page */
    > assert(soft_dirty(x)); /* fails */
    >
    > With this patch, new PTEs created for read faults are write protected
    > if the VMA has VM_SOFTDIRTY clear.
    >
    > Signed-off-by: Peter Feiner <pfeiner@google.com>
    > ---
    > mm/memory.c | 2 ++
    > 1 file changed, 2 insertions(+)
    >
    > diff --git a/mm/memory.c b/mm/memory.c
    > index ab3537b..282a959 100644
    > --- a/mm/memory.c
    > +++ b/mm/memory.c
    > @@ -2755,6 +2755,8 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
    > entry = maybe_mkwrite(pte_mkdirty(entry), vma);
    > else if (pte_file(*pte) && pte_file_soft_dirty(*pte))
    > entry = pte_mksoft_dirty(entry);
    > + else if (!(vma->vm_flags & VM_SOFTDIRTY))
    > + entry = pte_wrprotect(entry);

    It basically means VM_SOFTDIRTY require writenotify on the vma.

    What about patch below? Untested. And it seems it'll introduce bug similar
    to bug fixed by c9d0bf241451, *but* IIUC we have it already in mprotect()
    code path.

    I'll look more careful tomorrow.

    Not-signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

    diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    index dfc791c42d64..67d509a15969 100644
    --- a/fs/proc/task_mmu.c
    +++ b/fs/proc/task_mmu.c
    @@ -851,8 +851,9 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
    if (type == CLEAR_REFS_MAPPED && !vma->vm_file)
    continue;
    if (type == CLEAR_REFS_SOFT_DIRTY) {
    - if (vma->vm_flags & VM_SOFTDIRTY)
    - vma->vm_flags &= ~VM_SOFTDIRTY;
    + vma->vm_flags &= ~VM_SOFTDIRTY;
    + vma->vm_page_prot = vm_get_page_prot(
    + vma->vm_flags & ~VM_SHARED);
    }
    walk_page_range(vma->vm_start, vma->vm_end,
    &clear_refs_walk);
    --
    Kirill A. Shutemov

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