Re: [Qemu-devel] [PATCH 2/4] virtiofsd: prevent FUSE_INIT/FUSE_DESTROY races

[Dr. David Alan Gilbert]

* Stefan Hajnoczi (address@hidden) wrote:
> 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>

(It took me a few minutes getting my head around the different _destroy
functions)

Reviewed-by: Dr. David Alan Gilbert <address@hidden>

> ---
>  contrib/virtiofsd/fuse_i.h        |  1 +
>  contrib/virtiofsd/fuse_lowlevel.c | 17 +++++++++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/contrib/virtiofsd/fuse_i.h b/contrib/virtiofsd/fuse_i.h
> index ff6e1b75ba..dcde9feb97 100644
> --- a/contrib/virtiofsd/fuse_i.h
> +++ b/contrib/virtiofsd/fuse_i.h
> @@ -63,6 +63,7 @@ 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/contrib/virtiofsd/fuse_lowlevel.c 
> b/contrib/virtiofsd/fuse_lowlevel.c
> index 6ebf237baa..483a1bc9be 100644
> --- a/contrib/virtiofsd/fuse_lowlevel.c
> +++ b/contrib/virtiofsd/fuse_lowlevel.c
> @@ -2473,6 +2473,18 @@ 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
> +      * 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;
> @@ -2524,10 +2536,13 @@ void fuse_session_process_buf_int(struct fuse_session 
> *se,
>               do_write_buf(req, in->nodeid, &iter, bufv);
>       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) \
> @@ -2569,6 +2584,7 @@ void fuse_session_destroy(struct fuse_session *se)
>               if (se->op.destroy)
>                       se->op.destroy(se->userdata, se);
>       }
> +     pthread_rwlock_destroy(&se->init_rwlock);
>       pthread_mutex_destroy(&se->lock);
>       free(se->cuse_data);
>       if (se->fd != -1)
> @@ -2656,6 +2672,7 @@ 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();
> -- 
> 2.21.0
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK