Re: [Qemu-devel] [PATCH v2 5/5] 9p: forbid empty extension string

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Aug 26, 2016 at 02:00:37PM -0500, Eric Blake wrote:
> On 08/26/2016 10:07 AM, Greg Kurz wrote:
> > A buggy guest using the 9p2000.u protocol can issue a create request and
> > pass an empty string as the extension argument. This causes QEMU to crash
> > in the case of a hard link or a special file, and leads to undefined
> > behavior, depending on the backend, in the case of a symbolic link.
> > 
> > This patch causes the request to fail with EINVAL in these scenarios.
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> >  hw/9pfs/9p.c |   21 +++++++++++++++++++--
> >  1 file changed, 19 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> > index 7b1dfe4e47cb..dc65c3125006 100644
> > --- a/hw/9pfs/9p.c
> > +++ b/hw/9pfs/9p.c
> > @@ -2150,6 +2150,11 @@ static void v9fs_create(void *opaque)
> >          }
> >          fidp->fid_type = P9_FID_DIR;
> >      } else if (perm & P9_STAT_MODE_SYMLINK) {
> > +        if (extension.data == NULL) {
> > +            err = -EINVAL;
> > +            goto out;
> > +        }
> 
> POSIX specifically requires implementations to support creating a
> symlink whose target is the empty string.  Linux doesn't [yet] permit
> it, but BSD does.  On systems where creating such a symlink is legal,
> POSIX requires that such a symlink either be treated as "." if
> dereferenced, or be treated as ENOENT on attempt to dereference.  But
> since such links can be created, readlink() should be able to read them
> without error.
> 
> I would argue that we should NOT forbid empty symlinks on creation (but
> pass back any error from the underlying host OS); but instead check that
> dereferencing such a symlink behaves sanely if it was created.
> Meanwhile, a client should not be relying on the behavior (since Linux
> disobeys POSIX, portable clients should already be avoiding empty symlinks).
> 
> http://austingroupbugs.net/view.php?id=649

Given 9p is only supported on Linux hosts, I think this patch's approach
is OK for now, and it's certainly much simpler than worrying about
the fallout of allowing empty names.

A TODO that documents your suggestions, and including
the considerations in the comment would be
a good idea.



> > @@ -2161,8 +2166,15 @@ static void v9fs_create(void *opaque)
> >          }
> >          v9fs_path_copy(&fidp->path, &path);
> >      } else if (perm & P9_STAT_MODE_LINK) {
> > -        int32_t ofid = atoi(extension.data);
> > -        V9fsFidState *ofidp = get_fid(pdu, ofid);
> > +        V9fsFidState *ofidp;
> > +
> > +        if (extension.data == NULL) {
> > +            err = -EINVAL;
> > +            goto out;
> > +        }
> 
> Rejecting an empty destination on hard link or device creation, however,
> is indeed appropriate.
> 
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>