linux-kernel - Re: [PATCH v8 2/4] tee: generic TEE subsystem

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Date:	Fri, 12 Feb 2016 04:51:59 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jens Wiklander <jens.wiklander@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	devicetree@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	valentin.manea@...wei.com, jean-michel.delorme@...com,
	emmanuel.michel@...com, javier@...igon.com,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Mark Rutland <mark.rutland@....com>,
	Michal Simek <michal.simek@...inx.com>,
	Rob Herring <robh+dt@...nel.org>,
	Will Deacon <will.deacon@....com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v8 2/4] tee: generic TEE subsystem

On Thu, Feb 11, 2016 at 06:14:35PM +0100, Jens Wiklander wrote:

> +static int tee_ioctl_shm_alloc(struct tee_context *ctx,
> +		struct tee_ioctl_shm_alloc_data __user *udata)
> +{
> +	long ret;
> +	struct tee_ioctl_shm_alloc_data data;
> +	struct tee_shm *shm;
> +
> +	if (copy_from_user(&data, udata, sizeof(data)))
> +		return -EFAULT;
> +
> +	/* Currently no input flags are supported */
> +	if (data.flags)
> +		return -EINVAL;
> +
> +	data.fd = -1;
> +
> +	shm = tee_shm_alloc(ctx->teedev, data.size,
> +			    TEE_SHM_MAPPED | TEE_SHM_DMA_BUF);
> +	if (IS_ERR(shm))
> +		return PTR_ERR(shm);
> +
> +	data.flags = shm->flags;
> +	data.size = shm->size;
> +	data.fd = tee_shm_get_fd(shm);
> +	if (data.fd < 0) {
> +		ret = data.fd;
> +		goto err;
> +	}
> +
> +	if (copy_to_user(udata, &data, sizeof(data))) {
> +		ret = -EFAULT;
> +		goto err;
> +	}
> +	/*
> +	 * When user space closes the file descriptor the shared memory
> +	 * should be freed
> +	 */
> +	tee_shm_put(shm);
> +	return 0;
> +err:
> +	if (data.fd >= 0)
> +		tee_shm_put_fd(data.fd);

This is completely broken.  Don't ever use that pattern.  Once something
is in descriptor table, that's _it_.  You are already past the point of
no return and there is no way to clean up.

In ABIs like that (and struct containing descriptor *is* a bad ABI design)
solution is
	* allocate a descriptor
	* do everything that might fail, including copy_to_user()/put_user(),
etc.
	* if failed, release unused descriptor and do fput(), if you already
have a struct file reference that needs to be released.
	* FINALLY, when nothing no failures are possible, fd_install() the
sucker in place.

And yes, dma_buf_fd() encourages that kind of braindamage.  It's tolerable
only in one case - when we are about to return descriptor number directly
as return value of syscall and really can't fail anymore.  Not the case
here.