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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