On Sun, Jun 10, 2018 at 06:24:59AM +0100, Al Viro wrote:
> On Tue, May 29, 2018 at 04:43:19PM +0200, Miklos Szeredi wrote:
> > Implement stacked mmap.
> >
> > Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> > ---
> > fs/overlayfs/file.c | 28 ++++++++++++++++++++++++++++
> > 1 file changed, 28 insertions(+)
> >
> > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> > index 7b47dce4b072..4057bbf2e141 100644
> > --- a/fs/overlayfs/file.c
> > +++ b/fs/overlayfs/file.c
> > @@ -255,6 +255,33 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
> > return ret;
> > }
> >
> > +static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
> > +{
> > + struct fd real;
> > + const struct cred *old_cred;
> > + int ret;
> > +
> > + ret = ovl_real_fdget(file, &real);
> > + if (ret)
> > + return ret;
> > +
> > + /* transfer ref: */
> > + fput(vma->vm_file);
> > + vma->vm_file = get_file(real.file);
> > + fdput(real);
> > +
> > + if (!vma->vm_file->f_op->mmap)
> > + return -ENODEV;
>
> That's broken. ->mmap() failure will fput(file), not fput(vma->vm_file).
> What's more, _here_ your "corner case" is a huge DoS - open file r/o,
> then have somebody else trigger copyup, then do tons of MAP_PRIVATE
> mmaps on the r/o descriptor. *EACH* *OF* *THEM* will open a separate
> struct file and stash into into new vmas.
>
> NAK with extreme prejudice, sensu PTerry...
Okay, okay, got it now. Incremental below. It's a step back (mmap after
copy-up will get old data), but not a regression from current state. Obviously
need to fix properly and I think that's doable together with dealing with shared
map coherency.
diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index f801e1175a0b..b5a6bcc1bcfa 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -282,26 +282,30 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
{
- struct fd real;
+ struct file *realfile = file->private_data;
const struct cred *old_cred;
int ret;
- ret = ovl_real_fdget(file, &real);
- if (ret)
- return ret;
+ if (!realfile->f_op->mmap)
+ return -ENODEV;
- /* transfer ref: */
- fput(vma->vm_file);
- vma->vm_file = get_file(real.file);
- fdput(real);
+ if (WARN_ON(file != vma->vm_file))
+ return -EIO;
- if (!vma->vm_file->f_op->mmap)
- return -ENODEV;
+ vma->vm_file = get_file(realfile);
old_cred = ovl_override_creds(file_inode(file)->i_sb);
ret = call_mmap(vma->vm_file, vma);
revert_creds(old_cred);
+ if (ret) {
+ /* Drop reference count from new vm_file value */
+ fput(realfile);
+ } else {
+ /* Drop reference count from previous vm_file value */
+ fput(file);
+ }
+
ovl_file_accessed(file);
return ret;
--
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