Re: [Qemu-devel] [Virtio-fs] [PATCH 2/5] virtiofsd: prevent lo_lookup() NULL pointer dereference

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Mon, Jul 29, 2019 at 08:35:36PM +0800, piaojun wrote:
> Hi Stefan,
> 
> On 2019/7/26 17:11, Stefan Hajnoczi wrote:
> > Most lo_do_lookup() have already checked that the parent inode exists.
> > lo_lookup() hasn't and can therefore hit a NULL pointer dereference when
> > lo_inode(req, parent) returns NULL.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  contrib/virtiofsd/passthrough_ll.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/contrib/virtiofsd/passthrough_ll.c b/contrib/virtiofsd/passthrough_ll.c
> > index 9ae1381618..277a17fc03 100644
> > --- a/contrib/virtiofsd/passthrough_ll.c
> > +++ b/contrib/virtiofsd/passthrough_ll.c
> > @@ -766,6 +766,10 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
> >  	struct lo_data *lo = lo_data(req);
> >  	struct lo_inode *inode, *dir = lo_inode(req, parent);
> >  
> > +	if (!dir) {
> > +		return EBADF;
> > +	}
> > +
> 
> I worry about that dir will be released or set NULL just after NULL
> checking. Or could we use some lock to prevent the simultaneity?

Yes, I agree.  I haven't audited lo_inode yet, but it needs a refcount
and/or lock to ensure accesses are safe.  I'll do that and other things
in a separate patch series.

Stefan

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