Re: KASAN: use-after-free Read in filemap_fault

[Dmitry Vyukov]

On Sat, Dec 29, 2018 at 12:51 AM Kirill A. Shutemov
<kirill@xxxxxxxxxxxxx> wrote:
>
> On Fri, Dec 28, 2018 at 01:09:38PM -0800, Andrew Morton wrote:
> > On Fri, 28 Dec 2018 12:51:04 -0800 syzbot <syzbot+b437b5a429d680cf2217@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > > Hello,
> > >
> > > syzbot found the following crash on:
> >
> > uh-oh.  Josef, could you please take a look?
> >
> > :     page = find_get_page(mapping, offset);
> > :     if (likely(page) && !(vmf->flags & FAULT_FLAG_TRIED)) {
> > :             /*
> > :              * We found the page, so try async readahead before
> > :              * waiting for the lock.
> > :              */
> > :             fpin = do_async_mmap_readahead(vmf, page);
> > :     } else if (!page) {
> > :             /* No page in the page cache at all */
> > :             fpin = do_sync_mmap_readahead(vmf);
> > :             count_vm_event(PGMAJFAULT);
> > :             count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
> >
> > vmf->vma has been freed at this point.
> >
> > :             ret = VM_FAULT_MAJOR;
> > : retry_find:
> > :             page = pagecache_get_page(mapping, offset,
> > :                                       FGP_CREAT|FGP_FOR_MMAP,
> > :                                       vmf->gfp_mask);
> > :             if (!page) {
> > :                     if (fpin)
> > :                             goto out_retry;
> > :                     return vmf_error(-ENOMEM);
> > :             }
> > :     }
> >
>
> Here's a fixup for "filemap: drop the mmap_sem for all blocking operations".

If you are going to squash this, please add:

Tested-by: syzbot+b437b5a429d680cf2217@xxxxxxxxxxxxxxxxxxxxxxxxx


> do_sync_mmap_readahead() drops mmap_sem now, so by the time of
> dereferencing vmf->vma for count_memcg_event_mm() the VMA can be gone.
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 00a9315f45d4..65c85c47bdb1 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2554,10 +2554,10 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>                 fpin = do_async_mmap_readahead(vmf, page);
>         } else if (!page) {
>                 /* No page in the page cache at all */
> -               fpin = do_sync_mmap_readahead(vmf);
>                 count_vm_event(PGMAJFAULT);
>                 count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
>                 ret = VM_FAULT_MAJOR;
> +               fpin = do_sync_mmap_readahead(vmf);
>  retry_find:
>                 page = pagecache_get_page(mapping, offset,
>                                           FGP_CREAT|FGP_FOR_MMAP,
> --
>  Kirill A. Shutemov
>
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