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

[Philippe Mathieu-Daudé]

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"?

> +     * 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();