Re: [PATCH 3/3] uprobes: write_opcode()->__replace_page() can racewith try_to_unmap()

[Ingo Molnar]

* Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx> wrote:

> > ---
> > Subject: [PATCH] uprobes: write_opcode()->__replace_page() can race with try_to_unmap()
> > 
> > write_opcode() gets old_page via get_user_pages() and then calls
> > __replace_page() which assumes that this old_page is still mapped
> > after pte_offset_map_lock().
> > 
> > This is not true if this old_page was already try_to_unmap()'ed,
> > and in this case everything __replace_page() does with old_page
> > is wrong. Just for example, put_page() is not balanced.
> > 
> > I think it is possible to teach __replace_page() to handle this
> > unlikely case correctly, but this patch simply changes it to use
> > page_check_address() and return -EAGAIN if it fails. The caller
> > should notice this error code and retry.
> > 
> > Note: write_opcode() asks for the cleanups, I'll try to do this
> > in a separate patch.
> > 
> > Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> > ---
> >  kernel/events/uprobes.c |   41 +++++++++++++----------------------------
> >  1 files changed, 13 insertions(+), 28 deletions(-)
> > 
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index 9c53bc2..54c8780 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -135,33 +135,17 @@ static loff_t vma_address(struct vm_area_struct *vma, loff_t offset)
> >  static int __replace_page(struct vm_area_struct *vma, struct page *page, struct page *kpage)
> >  {
> >  	struct mm_struct *mm = vma->vm_mm;
> > -	pgd_t *pgd;
> > -	pud_t *pud;
> > -	pmd_t *pmd;
> > -	pte_t *ptep;
> > -	spinlock_t *ptl;
> >  	unsigned long addr;
> > -	int err = -EFAULT;
> > +	spinlock_t *ptl;
> > +	pte_t *ptep;
> > 
> >  	addr = page_address_in_vma(page, vma);
> >  	if (addr == -EFAULT)
> > -		goto out;
> > -
> > -	pgd = pgd_offset(mm, addr);
> > -	if (!pgd_present(*pgd))
> > -		goto out;
> > -
> > -	pud = pud_offset(pgd, addr);
> > -	if (!pud_present(*pud))
> > -		goto out;
> > +		return -EFAULT;
> > 
> > -	pmd = pmd_offset(pud, addr);
> > -	if (!pmd_present(*pmd))
> > -		goto out;
> > -
> > -	ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
> > +	ptep = page_check_address(page, mm, addr, &ptl, 0);
> >  	if (!ptep)
> > -		goto out;
> > +		return -EAGAIN;
> > 
> >  	get_page(kpage);
> >  	page_add_new_anon_rmap(kpage, vma, addr);
> > @@ -180,10 +164,8 @@ static int __replace_page(struct vm_area_struct *vma, struct page *page, struct
> >  		try_to_free_swap(page);
> >  	put_page(page);
> >  	pte_unmap_unlock(ptep, ptl);
> > -	err = 0;
> > 
> > -out:
> > -	return err;
> > +	return 0;
> >  }
> > 
> >  /**
> > @@ -228,9 +210,10 @@ static int write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
> >  	void *vaddr_old, *vaddr_new;
> >  	struct vm_area_struct *vma;
> >  	struct uprobe *uprobe;
> > +	unsigned long pgoff;
> >  	loff_t addr;
> >  	int ret;
> > -
> > +retry:
> 
> Just a check on coding style: Shouldnt we have a preceeding blank
> line before the goto label.

Yeah, that's most likely helpful to readability.

Thanks,

	Ingo
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