Re: [PATCH] filemap: replace pte_offset_map() with pte_offset_map_nolock()

From: Andrew Morton
Date: Tue Jun 25 2024 - 17:06:50 EST


On Wed, 13 Mar 2024 09:29:13 +0800 Peng Zhang <zhangpeng362@xxxxxxxxxx> wrote:

> From: ZhangPeng <zhangpeng362@xxxxxxxxxx>
>
> The vmf->ptl in filemap_fault_recheck_pte_none() is still set from
> handle_pte_fault(). But at the same time, we did a pte_unmap(vmf->pte).
> After a pte_unmap(vmf->pte) unmap and rcu_read_unlock(), the page table
> may be racily changed and vmf->ptl maybe fails to protect the actual
> page table.
> Fix this by replacing pte_offset_map() with pte_offset_map_nolock().
>
> ...
>
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3207,7 +3207,8 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
> if (!(vmf->flags & FAULT_FLAG_ORIG_PTE_VALID))
> return 0;
>
> - ptep = pte_offset_map(vmf->pmd, vmf->address);
> + ptep = pte_offset_map_nolock(vma->vm_mm, vmf->pmd, vmf->address,
> + &vmf->ptl);
> if (unlikely(!ptep))
> return VM_FAULT_NOPAGE;
>

whoops, I'm still sitting on this because I didn't know whether we
should backport it.

And... guess what I say next. Can we please describe what are the
userspace visible effects of the bug?