Re: [PATCH 093/104] virtiofsd: introduce inode refcount to prevent use-after-free

[Dr. David Alan Gilbert]

* Stefan Hajnoczi (address@hidden) wrote:
> On Thu, Jan 16, 2020 at 09:25:42PM +0900, Misono Tomohiro wrote:
> > > From: Stefan Hajnoczi <address@hidden>
> > > 
> > > If thread A is using an inode it must not be deleted by thread B when
> > > processing a FUSE_FORGET request.
> > > 
> > > The FUSE protocol itself already has a counter called nlookup that is
> > > used in FUSE_FORGET messages.  We cannot trust this counter since the
> > > untrusted client can manipulate it via FUSE_FORGET messages.
> > > 
> > > Introduce a new refcount to keep inodes alive for the required lifespan.
> > > lo_inode_put() must be called to release a reference.  FUSE's nlookup
> > > counter holds exactly one reference so that the inode stays alive as
> > > long as the client still wants to remember it.
> > > 
> > > Note that the lo_inode->is_symlink field is moved to avoid creating a
> > > hole in the struct due to struct field alignment.
> > > 
> > > Signed-off-by: Stefan Hajnoczi <address@hidden>
> > > ---
> > >  tools/virtiofsd/passthrough_ll.c | 168 ++++++++++++++++++++++++++-----
> > >  1 file changed, 145 insertions(+), 23 deletions(-)
> > > 
> > > diff --git a/tools/virtiofsd/passthrough_ll.c 
> > > b/tools/virtiofsd/passthrough_ll.c
> > > index b19c9ee328..8f4ab8351c 100644
> > > --- a/tools/virtiofsd/passthrough_ll.c
> > > +++ b/tools/virtiofsd/passthrough_ll.c
> > > @@ -99,7 +99,13 @@ struct lo_key {
> > >  
> > >  struct lo_inode {
> > >      int fd;
> > > -    bool is_symlink;
> > > +
> > > +    /*
> > > +     * Atomic reference count for this object.  The nlookup field holds a
> > > +     * reference and release it when nlookup reaches 0.
> > > +     */
> > > +    gint refcount;
> > > +
> > >      struct lo_key key;
> > >  
> > >      /*
> > > @@ -118,6 +124,8 @@ struct lo_inode {
> > >      fuse_ino_t fuse_ino;
> > >      pthread_mutex_t plock_mutex;
> > >      GHashTable *posix_locks; /* protected by lo_inode->plock_mutex */
> > > +
> > > +    bool is_symlink;
> > >  };
> > >  
> > >  struct lo_cred {
> > > @@ -473,6 +481,23 @@ static ssize_t lo_add_inode_mapping(fuse_req_t req, 
> > > struct lo_inode *inode)
> > >      return elem - lo_data(req)->ino_map.elems;
> > >  }
> > >  
> > > +static void lo_inode_put(struct lo_data *lo, struct lo_inode **inodep)
> > > +{
> > > +    struct lo_inode *inode = *inodep;
> > > +
> > > +    if (!inode) {
> > > +        return;
> > > +    }
> > > +
> > > +    *inodep = NULL;
> > > +
> > > +    if (g_atomic_int_dec_and_test(&inode->refcount)) {
> > > +        close(inode->fd);
> > > +        free(inode);
> > > +    }
> > > +}
> > > +
> > > +/* Caller must release refcount using lo_inode_put() */
> > >  static struct lo_inode *lo_inode(fuse_req_t req, fuse_ino_t ino)
> > >  {
> > >      struct lo_data *lo = lo_data(req);
> > > @@ -480,6 +505,9 @@ static struct lo_inode *lo_inode(fuse_req_t req, 
> > > fuse_ino_t ino)
> > >  
> > >      pthread_mutex_lock(&lo->mutex);
> > >      elem = lo_map_get(&lo->ino_map, ino);
> > > +    if (elem) {
> > > +        g_atomic_int_inc(&elem->inode->refcount);
> > > +    }
> > >      pthread_mutex_unlock(&lo->mutex);
> > >  
> > >      if (!elem) {
> > > @@ -489,10 +517,23 @@ static struct lo_inode *lo_inode(fuse_req_t req, 
> > > fuse_ino_t ino)
> > >      return elem->inode;
> > >  }
> > >  
> > > +/*
> > > + * TODO Remove this helper and force callers to hold an inode refcount 
> > > until
> > > + * they are done with the fd.  This will be done in a later patch to make
> > > + * review easier.
> > > + */
> > >  static int lo_fd(fuse_req_t req, fuse_ino_t ino)
> > >  {
> > >      struct lo_inode *inode = lo_inode(req, ino);
> > > -    return inode ? inode->fd : -1;
> > > +    int fd;
> > > +
> > > +    if (!inode) {
> > > +        return -1;
> > > +    }
> > > +
> > > +    fd = inode->fd;
> > > +    lo_inode_put(lo_data(req), &inode);
> > > +    return fd;
> > >  }
> > >  
> > >  static void lo_init(void *userdata, struct fuse_conn_info *conn)
> > > @@ -547,6 +588,10 @@ static void lo_getattr(fuse_req_t req, fuse_ino_t 
> > > ino,
> > >      fuse_reply_attr(req, &buf, lo->timeout);
> > >  }
> > >  
> > > +/*
> > > + * Increments parent->nlookup and caller must release refcount using
> > > + * lo_inode_put(&parent).
> > > + */
> > >  static int lo_parent_and_name(struct lo_data *lo, struct lo_inode *inode,
> > >                                char path[PATH_MAX], struct lo_inode 
> > > **parent)
> > >  {
> > > @@ -584,6 +629,7 @@ retry:
> > >          p = &lo->root;
> > >          pthread_mutex_lock(&lo->mutex);
> > >          p->nlookup++;
> > > +        g_atomic_int_inc(&p->refcount);
> > >          pthread_mutex_unlock(&lo->mutex);
> > >      } else {
> > >          *last = '\0';
> > 
> > We need lo_ionde_put() in error path, right?:
> > https://gitlab.com/virtio-fs/qemu/blob/virtio-fs-as-posted-2019-12-12/tools/virtiofsd/passthrough_ll.c#L680
> 
> Yes, thanks for spotting this bug!  The lo_parent_and_name() code should
> look like this:
> 
>   fail_unref:
>       unref_inode_lolocked(lo, p, 1);
>       lo_inode_put(lo, &p);
>   ...

I've merged that one in.

> > nit: if yes, unref_inode_lolocked() is always paired with lo_inode_put().
> > So how about combine them in one function? As p->nloockup and p->refcount
> > are both incremented in one place (lo_find/lo_parent_and_name) in these 
> > case,
> > it seems natural for me to decrement them in one function as well.
> 
> Nice idea.  I would also drop the nlookup argument - this function will
> only be used with nlookup=1.

I'll leave that to you if you want to send a patch on top.

Dave

> Stefan


--
Dr. David Alan Gilbert / address@hidden / Manchester, UK