More filesystem need this fix (xfs: use MMAPLOCK around filemap_map_pages())

[Amir Goldstein <amir73il@gmail.com>]

On Tue, Jun 23, 2020 at 8:21 AM Dave Chinner <david@fromorbit.com> wrote:
>
> From: Dave Chinner <dchinner@redhat.com>
>
> The page faultround path ->map_pages is implemented in XFS via
> filemap_map_pages(). This function checks that pages found in page
> cache lookups have not raced with truncate based invalidation by
> checking page->mapping is correct and page->index is within EOF.
>
> However, we've known for a long time that this is not sufficient to
> protect against races with invalidations done by operations that do
> not change EOF. e.g. hole punching and other fallocate() based
> direct extent manipulations. The way we protect against these
> races is we wrap the page fault operations in a XFS_MMAPLOCK_SHARED
> lock so they serialise against fallocate and truncate before calling
> into the filemap function that processes the fault.
>
> Do the same for XFS's ->map_pages implementation to close this
> potential data corruption issue.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_file.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 7b05f8fd7b3d..4b185a907432 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1266,10 +1266,23 @@ xfs_filemap_pfn_mkwrite(
>         return __xfs_filemap_fault(vmf, PE_SIZE_PTE, true);
>  }
>
> +static void
> +xfs_filemap_map_pages(
> +       struct vm_fault         *vmf,
> +       pgoff_t                 start_pgoff,
> +       pgoff_t                 end_pgoff)
> +{
> +       struct inode            *inode = file_inode(vmf->vma->vm_file);
> +
> +       xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> +       filemap_map_pages(vmf, start_pgoff, end_pgoff);
> +       xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> +}
> +
>  static const struct vm_operations_struct xfs_file_vm_ops = {
>         .fault          = xfs_filemap_fault,
>         .huge_fault     = xfs_filemap_huge_fault,
> -       .map_pages      = filemap_map_pages,
> +       .map_pages      = xfs_filemap_map_pages,
>         .page_mkwrite   = xfs_filemap_page_mkwrite,
>         .pfn_mkwrite    = xfs_filemap_pfn_mkwrite,
>  };
> --
> 2.26.2.761.g0e0b3e54be
>

It appears that ext4, f2fs, gfs2, orangefs, zonefs also need this fix

zonefs does not support hole punching, so it may not need to use
mmap_sem at all.

It is interesting to look at how this bug came to be duplicated in so
many filesystems, because there are lessons to be learned.

Commit f1820361f83d ("mm: implement ->map_pages for page cache")
added to ->map_pages() operation and its commit message said:

"...It should be safe to use filemap_map_pages() for ->map_pages() if
    filesystem use filemap_fault() for ->fault()."

At the time, all of the aforementioned filesystems used filemap_fault()
for ->fault().

But since then, ext4, xfs, f2fs and just recently gfs2 have added a filesystem
->fault() operation.

orangefs has added vm_operations since and zonefs was added since,
probably copying the mmap_sem handling from ext4. Both have a filesystem
->fault() operation.

It was surprising for me to see that some of the filesystem developers
signed on the added ->fault() operations are not strangers to mm. The
recent gfs2 change was even reviewed by an established mm developer
[1].

So what can we learn from this case study? How could we fix the interface to
avoid repeating the same mistake in the future?

Thanks,
Amir.

[1] https://lore.kernel.org/linux-fsdevel/20200703113801.GD25523@casper.infradead.org/