On Sat, Aug 23, 2014 at 06:11:59PM -0400, Peter Feiner wrote:
> For VMAs that don't want write notifications, 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, write notifications are enabled when VM_SOFTDIRTY is
> cleared. Furthermore, to avoid faults, write notifications are
> disabled when VM_SOFTDIRTY is reset.
>
> Signed-off-by: Peter Feiner <pfeiner@xxxxxxxxxx>
> ---
> v1 -> v2: Instead of checking VM_SOFTDIRTY in the fault handler, enable write
> notifications on vm_page_prot when we clear VM_SOFTDIRTY.
>
> fs/proc/task_mmu.c | 17 ++++++++++++++++-
> include/linux/mm.h | 15 +++++++++++++++
> mm/mmap.c | 10 +++++++++-
> 3 files changed, 40 insertions(+), 2 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index dfc791c..f1a5382 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -851,8 +851,23 @@ 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)
> + if (vma->vm_flags & VM_SOFTDIRTY) {
Why do we need the branch here. Does it save us anything?
Looks like we can update vm_flags and enable writenotify unconditionally.
Indentation level is high enough already.
> vma->vm_flags &= ~VM_SOFTDIRTY;
> + /*
> + * We don't have a write lock on
> + * mm->mmap_sem, so we race with the
> + * fault handler reading vm_page_prot.
> + * Therefore writable PTEs (that won't
> + * have soft-dirty set) can be created
> + * for read faults. However, since the
> + * PTE lock is held while vm_page_prot
> + * is read and while we write protect
> + * PTEs during our walk, any writable
> + * PTEs that slipped through will be
> + * write protected.
> + */
Hm.. Isn't this yet another bug?
Updating vma->vm_flags without down_write(&mm->mmap_sem) looks troublesome
to me. Am I wrong?
> + vma_enable_writenotify(vma);
> + }
> }
> walk_page_range(vma->vm_start, vma->vm_end,
> &clear_refs_walk);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8981cc8..5f26634 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1946,6 +1946,21 @@ static inline pgprot_t vm_get_page_prot(unsigned long vm_flags)
> }
> #endif
>
> +/* Enable write notifications without blowing away special flags. */
> +static inline void vma_enable_writenotify(struct vm_area_struct *vma)
> +{
> + vma->vm_page_prot = pgprot_modify(vma->vm_page_prot,
> + vm_get_page_prot(vma->vm_flags &
> + ~VM_SHARED));
I think this way is more readable:
pgprot_t newprot;
newprot = vm_get_page_prot(vma->vm_flags & ~VM_SHARED);
vma->vm_page_prot = pgprot_modify(vma->vm_page_prot, newprot);
> +}
> +
> +/* Disable write notifications without blowing away special flags. */
> +static inline void vma_disable_writenotify(struct vm_area_struct *vma)
> +{
> + vma->vm_page_prot = pgprot_modify(vma->vm_page_prot,
> + vm_get_page_prot(vma->vm_flags));
ditto.
> +}
> +
> #ifdef CONFIG_NUMA_BALANCING
> unsigned long change_prot_numa(struct vm_area_struct *vma,
> unsigned long start, unsigned long end);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c1f2ea4..abcac32 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1549,8 +1549,16 @@ munmap_back:
> * Can we just expand an old mapping?
> */
> vma = vma_merge(mm, prev, addr, addr + len, vm_flags, NULL, file, pgoff, NULL);
> - if (vma)
> + if (vma) {
> + if (!vma_wants_writenotify(vma)) {
> + /*
> + * We're going to reset VM_SOFTDIRTY, so we can disable
> + * write notifications.
> + */
> + vma_disable_writenotify(vma);
> + }
> goto out;
> + }
>
> /*
> * Determine the object being mapped and call the appropriate
> --
> 2.1.0.rc2.206.gedb03e5
>
--
Kirill A. Shutemov
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