On Wed, Jan 20, 2021 at 12:20 PM Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>
> On Tue, Jan 05, 2021 at 08:47:41AM +0200, Amir Goldstein wrote:
> > On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng <icenowy@xxxxxxx> wrote:
> > >
> > > The function ovl_dir_real_file() currently uses the semaphore of the
> > > inode to synchronize write to the upperfile cache field.
> >
> > Although the inode lock is a rw_sem it is referred to as the "inode lock"
> > and you also left semaphore in the commit subject.
> > No need to re-post. This can be fixed on commit.
> >
> > >
> > > However, this function will get called by ovl_ioctl_set_flags(), which
> > > utilizes the inode semaphore too. In this case ovl_dir_real_file() will
> > > try to claim a lock that is owned by a function in its call stack, which
> > > won't get released before ovl_dir_real_file() returns.
> > >
> > > Define a dedicated semaphore for the upperfile cache, so that the
> > > deadlock won't happen.
> > >
> > > Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
> > > Cc: stable@xxxxxxxxxxxxxxx # v5.10
> > > Signed-off-by: Icenowy Zheng <icenowy@xxxxxxx>
> > > ---
> > > Changes in v2:
> > > - Fixed missing replacement in error handling path.
> > > Changes in v3:
> > > - Use mutex instead of semaphore.
> > >
> > > fs/overlayfs/readdir.c | 10 +++++-----
> > > 1 file changed, 5 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> > > index 01620ebae1bd..3980f9982f34 100644
> > > --- a/fs/overlayfs/readdir.c
> > > +++ b/fs/overlayfs/readdir.c
> > > @@ -56,6 +56,7 @@ struct ovl_dir_file {
> > > struct list_head *cursor;
> > > struct file *realfile;
> > > struct file *upperfile;
> > > + struct mutex upperfile_mutex;
> >
> > That's a very specific name.
> > This mutex protects members of struct ovl_dir_file, which could evolve
> > into struct ovl_file one day (because no reason to cache only dir upper file),
> > so I would go with a more generic name, but let's leave it to Miklos to decide.
> >
> > He could have a different idea altogether for fixing this bug.
>
> How about this (untested) patch?
>
Much better :)
> It's a cleanup as well as a fix, but maybe we should separate the cleanup from
> the fix...
>
> Thanks,
> Miklos
> ---
>
> fs/overlayfs/readdir.c | 23 +++++++----------------
> 1 file changed, 7 insertions(+), 16 deletions(-)
>
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -865,7 +865,7 @@ struct file *ovl_dir_real_file(const str
>
> struct ovl_dir_file *od = file->private_data;
> struct dentry *dentry = file->f_path.dentry;
> - struct file *realfile = od->realfile;
> + struct file *old, *realfile = od->realfile;
>
> if (!OVL_TYPE_UPPER(ovl_path_type(dentry)))
> return want_upper ? NULL : realfile;
> @@ -874,29 +874,20 @@ struct file *ovl_dir_real_file(const str
> * Need to check if we started out being a lower dir, but got copied up
> */
> if (!od->is_upper) {
> - struct inode *inode = file_inode(file);
> -
> realfile = READ_ONCE(od->upperfile);
> if (!realfile) {
> struct path upperpath;
>
> ovl_path_upper(dentry, &upperpath);
> realfile = ovl_dir_open_realfile(file, &upperpath);
> + if (IS_ERR(realfile))
> + return realfile;
>
> - inode_lock(inode);
> - if (!od->upperfile) {
> - if (IS_ERR(realfile)) {
> - inode_unlock(inode);
> - return realfile;
> - }
> - smp_store_release(&od->upperfile, realfile);
> - } else {
> - /* somebody has beaten us to it */
> - if (!IS_ERR(realfile))
> - fput(realfile);
> - realfile = od->upperfile;
> + old = cmpxchg_release(&od->upperfile, NULL, realfile);
> + if (old) {
> + fput(realfile);
> + realfile = old;
> }
> - inode_unlock(inode);
> }
> }
>
