Re: [PATCH] ovl: fix oops in ovl_indexdir_cleanup() with nfs_export=on

[Murphy Zhou <jencce.kernel@gmail.com>]

Copying Miklos' another email.

On Mon, Jul 06, 2020 at 03:15:06PM +0300, Amir Goldstein wrote:
> On Sun, Jun 21, 2020 at 9:38 AM Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > Mounting with nfs_export=on, xfstests overlay/031 triggers a kernel panic
> > since v5.8-rc1 overlayfs updates.
> 
> Ping.
> 
> This is a kernel OOPS regression in v5.8-rc1.
> 
> Push tested branch ovl-fixes based on v5.8-rc3 with this change.
> It contains two other non-critical fixes, which are pretty obvious,
> so you may want to consider taking them as well.
> 
> Thanks,
> Amir.
> 
> >
> >  overlayfs: orphan index entry (index/00fb1..., ftype=4000, nlink=2)
> >  BUG: kernel NULL pointer dereference, address: 0000000000000030
> >  RIP: 0010:ovl_cleanup_and_whiteout+0x28/0x220 [overlay]
> >
> > Bisect point at commit c21c839b8448 ("ovl: whiteout inode sharing")
> >
> > Minimal reproducer:
> > --------------------------------------------------
> > rm -rf l u w m
> > mkdir -p l u w m
> > mkdir -p l/testdir
> > touch l/testdir/testfile
> > mount -t overlay -o lowerdir=l,upperdir=u,workdir=w,nfs_export=on overlay m
> > echo 1 > m/testdir/testfile
> > umount m
> > rm -rf u/testdir
> > mount -t overlay -o lowerdir=l,upperdir=u,workdir=w,nfs_export=on overlay m
> > umount m
> > --------------------------------------------------
> >
> > When mount with nfs_export=on, and fail to verify an orphan index, we're
> > cleaning this index from indexdir by calling ovl_cleanup_and_whiteout().
> > This dereferences ofs->workdir, that was earlier set to NULL.
> >
> > The design was that ovl->workdir will point at ovl->indexdir, but we are
> > assigning ofs->indexdir to ofs->workdir only after ovl_indexdir_cleanup().
> > There is no reason not to do it sooner, because once we get success from
> > ofs->indexdir = ovl_workdir_create(... there is no turning back.
> >
> > Reported-and-tested-by: Murphy Zhou <jencce.kernel@gmail.com>
> > Fixes: commit c21c839b8448 ("ovl: whiteout inode sharing")
> > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > ---
> >  fs/overlayfs/super.c | 16 +++++++---------
> >  1 file changed, 7 insertions(+), 9 deletions(-)
> >
> > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> > index 91476bc422f9..15939ab39c1c 100644
> > --- a/fs/overlayfs/super.c
> > +++ b/fs/overlayfs/super.c
> > @@ -1354,6 +1354,12 @@ static int ovl_get_indexdir(struct super_block *sb, struct ovl_fs *ofs,
> >
> >         ofs->indexdir = ovl_workdir_create(ofs, OVL_INDEXDIR_NAME, true);
> >         if (ofs->indexdir) {
> > +               /* index dir will act also as workdir */
> > +               iput(ofs->workdir_trap);
> > +               ofs->workdir_trap = NULL;
> > +               dput(ofs->workdir);
> > +               ofs->workdir = dget(ofs->indexdir);
> > +
> >                 err = ovl_setup_trap(sb, ofs->indexdir, &ofs->indexdir_trap,
> >                                      "indexdir");
> >                 if (err)
> > @@ -1843,20 +1849,12 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
> >                 sb->s_flags |= SB_RDONLY;
> >
> >         if (!(ovl_force_readonly(ofs)) && ofs->config.index) {
> > -               /* index dir will act also as workdir */
> > -               dput(ofs->workdir);
> > -               ofs->workdir = NULL;
> > -               iput(ofs->workdir_trap);
> > -               ofs->workdir_trap = NULL;
> > -
> >                 err = ovl_get_indexdir(sb, ofs, oe, &upperpath);
> >                 if (err)
> >                         goto out_free_oe;
> >
> >                 /* Force r/o mount with no index dir */
> > -               if (ofs->indexdir)
> > -                       ofs->workdir = dget(ofs->indexdir);
> > -               else
> > +               if (!ofs->indexdir)
> >                         sb->s_flags |= SB_RDONLY;
> >         }
> >
> > --
> > 2.17.1
> >

-- 
Murphy