Re: [PATCH] mm: softdirty: write protect PTEs created for read faults after VM_SOFTDIRTY cleared

[Kirill A. Shutemov]

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@xxxxxxxxxx>
> ---
>  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@xxxxxxxxxxxxxxx>

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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