Re: [PATCH 101/104] virtiofsd: prevent FUSE_INIT/FUSE_DESTROY races

[Philippe Mathieu-Daudé]

On 1/17/20 4:28 PM, Dr. David Alan Gilbert wrote:
> * Philippe Mathieu-Daudé (address@hidden) wrote:
> > On 12/12/19 5:39 PM, Dr. David Alan Gilbert (git) wrote:
> > > From: Stefan Hajnoczi <address@hidden>
> > >
> > > When running with multiple threads it can be tricky to handle
> > > FUSE_INIT/FUSE_DESTROY in parallel with other request types or in
> > > parallel with themselves.  Serialize FUSE_INIT and FUSE_DESTROY so that
> > > malicious clients cannot trigger race conditions.
> > >
> > > Signed-off-by: Stefan Hajnoczi <address@hidden>
> > > ---
> > >    tools/virtiofsd/fuse_i.h        |  1 +
> > >    tools/virtiofsd/fuse_lowlevel.c | 18 ++++++++++++++++++
> > >    2 files changed, 19 insertions(+)
> > >
> > > diff --git a/tools/virtiofsd/fuse_i.h b/tools/virtiofsd/fuse_i.h
> > > index d0679508cd..8a4a05b319 100644
> > > --- a/tools/virtiofsd/fuse_i.h
> > > +++ b/tools/virtiofsd/fuse_i.h
> > > @@ -61,6 +61,7 @@ struct fuse_session {
> > >        struct fuse_req list;
> > >        struct fuse_req interrupts;
> > >        pthread_mutex_t lock;
> > > +    pthread_rwlock_t init_rwlock;
> > >        int got_destroy;
> > >        int broken_splice_nonblock;
> > >        uint64_t notify_ctr;
> > > diff --git a/tools/virtiofsd/fuse_lowlevel.c b/tools/virtiofsd/fuse_lowlevel.c
> > > index 10f478b00c..9f01c05e3e 100644
> > > --- a/tools/virtiofsd/fuse_lowlevel.c
> > > +++ b/tools/virtiofsd/fuse_lowlevel.c
> > > @@ -2431,6 +2431,19 @@ void fuse_session_process_buf_int(struct fuse_session 
> > > *se,
> > >        req->ctx.pid = in->pid;
> > >        req->ch = ch ? fuse_chan_get(ch) : NULL;
> > > +    /*
> > > +     * INIT and DESTROY requests are serialized, all other request types
> > > +     * run in parallel.  This prevents races between FUSE_INIT and ordinary
> > > +     * requests, FUSE_INIT and FUSE_INIT, FUSE_INIT and FUSE_DESTROY, and
> >
> > typo "FUSE_INIT and FUSE_INIT" -> "FUSE_INIT and CUSE_INIT"?
>
> No, don't think so; I think it's suggesting a race between two
> FUSE_INIT's.

And CUSE_INIT is a subtype of FUSE_INIT, OK.

> Dave
>
> > > +     * FUSE_DESTROY and FUSE_DESTROY.
> > > +     */
> > > +    if (in->opcode == FUSE_INIT || in->opcode == CUSE_INIT ||
> > > +        in->opcode == FUSE_DESTROY) {
> > > +        pthread_rwlock_wrlock(&se->init_rwlock);
> > > +    } else {
> > > +        pthread_rwlock_rdlock(&se->init_rwlock);
> > > +    }
> > > +
> > >        err = EIO;
> > >        if (!se->got_init) {
> > >            enum fuse_opcode expected;
> > > @@ -2488,10 +2501,13 @@ void fuse_session_process_buf_int(struct fuse_session 
> > > *se,
> > >        } else {
> > >            fuse_ll_ops[in->opcode].func(req, in->nodeid, &iter);
> > >        }
> > > +
> > > +    pthread_rwlock_unlock(&se->init_rwlock);
> > >        return;
> > >    reply_err:
> > >        fuse_reply_err(req, err);
> > > +    pthread_rwlock_unlock(&se->init_rwlock);
> > >    }
> > >    #define LL_OPTION(n, o, v)                     \
> > > @@ -2538,6 +2554,7 @@ void fuse_session_destroy(struct fuse_session *se)
> > >                se->op.destroy(se->userdata);
> > >            }
> > >        }
> > > +    pthread_rwlock_destroy(&se->init_rwlock);
> > >        pthread_mutex_destroy(&se->lock);
> > >        free(se->cuse_data);
> > >        if (se->fd != -1) {
> > > @@ -2631,6 +2648,7 @@ struct fuse_session *fuse_session_new(struct fuse_args 
> > > *args,
> > >        list_init_req(&se->list);
> > >        list_init_req(&se->interrupts);
> > >        fuse_mutex_init(&se->lock);
> > > +    pthread_rwlock_init(&se->init_rwlock, NULL);
> > >        memcpy(&se->op, op, op_size);
> > >        se->owner = getuid();
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK