Re: [PATCH v3] misc: sram: Add DMA-BUF Heap exporting of SRAM areas

[Robin Murphy <robin.murphy@arm.com>]

On 2023-07-13 20:13, Andrew Davis wrote:
> This new export type exposes to userspace the SRAM area as a DMA-BUF Heap,
> this allows for allocations of DMA-BUFs that can be consumed by various
> DMA-BUF supporting devices.
> 
> Signed-off-by: Andrew Davis <afd@ti.com>
> ---
> 
> Changes from v2:
>   - Make sram_dma_heap_allocate static (kernel test robot)
>   - Rebase on v6.5-rc1
> 
>   drivers/misc/Kconfig         |   7 +
>   drivers/misc/Makefile        |   1 +
>   drivers/misc/sram-dma-heap.c | 245 +++++++++++++++++++++++++++++++++++
>   drivers/misc/sram.c          |   6 +
>   drivers/misc/sram.h          |  16 +++
>   5 files changed, 275 insertions(+)
>   create mode 100644 drivers/misc/sram-dma-heap.c
> 
> diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
> index 75e427f124b28..ee34dfb61605f 100644
> --- a/drivers/misc/Kconfig
> +++ b/drivers/misc/Kconfig
> @@ -448,6 +448,13 @@ config SRAM
>   config SRAM_EXEC
>   	bool
>   
> +config SRAM_DMA_HEAP
> +	bool "Export on-chip SRAM pools using DMA-Heaps"
> +	depends on DMABUF_HEAPS && SRAM
> +	help
> +	  This driver allows the export of on-chip SRAM marked as both pool
> +	  and exportable to userspace using the DMA-Heaps interface.
> +
>   config DW_XDATA_PCIE
>   	depends on PCI
>   	tristate "Synopsys DesignWare xData PCIe driver"
> diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
> index f2a4d1ff65d46..5e7516bfaa8de 100644
> --- a/drivers/misc/Makefile
> +++ b/drivers/misc/Makefile
> @@ -47,6 +47,7 @@ obj-$(CONFIG_VMWARE_VMCI)	+= vmw_vmci/
>   obj-$(CONFIG_LATTICE_ECP3_CONFIG)	+= lattice-ecp3-config.o
>   obj-$(CONFIG_SRAM)		+= sram.o
>   obj-$(CONFIG_SRAM_EXEC)		+= sram-exec.o
> +obj-$(CONFIG_SRAM_DMA_HEAP)	+= sram-dma-heap.o
>   obj-$(CONFIG_GENWQE)		+= genwqe/
>   obj-$(CONFIG_ECHO)		+= echo/
>   obj-$(CONFIG_CXL_BASE)		+= cxl/
> diff --git a/drivers/misc/sram-dma-heap.c b/drivers/misc/sram-dma-heap.c
> new file mode 100644
> index 0000000000000..c054c04dff33e
> --- /dev/null
> +++ b/drivers/misc/sram-dma-heap.c
> @@ -0,0 +1,245 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * SRAM DMA-Heap userspace exporter
> + *
> + * Copyright (C) 2019-2022 Texas Instruments Incorporated - https://www.ti.com/
> + *	Andrew Davis <afd@ti.com>
> + */
> +
> +#include <linux/dma-mapping.h>
> +#include <linux/err.h>
> +#include <linux/genalloc.h>
> +#include <linux/io.h>
> +#include <linux/mm.h>
> +#include <linux/scatterlist.h>
> +#include <linux/slab.h>
> +#include <linux/dma-buf.h>
> +#include <linux/dma-heap.h>
> +
> +#include "sram.h"
> +
> +struct sram_dma_heap {
> +	struct dma_heap *heap;
> +	struct gen_pool *pool;
> +};
> +
> +struct sram_dma_heap_buffer {
> +	struct gen_pool *pool;
> +	struct list_head attachments;
> +	struct mutex attachments_lock;
> +	unsigned long len;
> +	void *vaddr;
> +	phys_addr_t paddr;
> +};
> +
> +struct dma_heap_attachment {
> +	struct device *dev;
> +	struct sg_table *table;
> +	struct list_head list;
> +};
> +
> +static int dma_heap_attach(struct dma_buf *dmabuf,
> +			   struct dma_buf_attachment *attachment)
> +{
> +	struct sram_dma_heap_buffer *buffer = dmabuf->priv;
> +	struct dma_heap_attachment *a;
> +	struct sg_table *table;
> +
> +	a = kzalloc(sizeof(*a), GFP_KERNEL);
> +	if (!a)
> +		return -ENOMEM;
> +
> +	table = kmalloc(sizeof(*table), GFP_KERNEL);
> +	if (!table) {
> +		kfree(a);
> +		return -ENOMEM;
> +	}
> +	if (sg_alloc_table(table, 1, GFP_KERNEL)) {
> +		kfree(table);
> +		kfree(a);
> +		return -ENOMEM;
> +	}
> +	sg_set_page(table->sgl, pfn_to_page(PFN_DOWN(buffer->paddr)), buffer->len, 0);

What happens if someone (reasonably) assumes that this struct page 
pointer isn't completely made up, and dereferences it?

(That's if pfn_to_page() itself doesn't blow up, which it potentially 
might, at least under CONFIG_SPARSEMEM)

I think this needs to be treated as P2PDMA if it's going to have any 
hope of working robustly.

> +
> +	a->table = table;
> +	a->dev = attachment->dev;
> +	INIT_LIST_HEAD(&a->list);
> +
> +	attachment->priv = a;
> +
> +	mutex_lock(&buffer->attachments_lock);
> +	list_add(&a->list, &buffer->attachments);
> +	mutex_unlock(&buffer->attachments_lock);
> +
> +	return 0;
> +}
> +
> +static void dma_heap_detatch(struct dma_buf *dmabuf,
> +			     struct dma_buf_attachment *attachment)
> +{
> +	struct sram_dma_heap_buffer *buffer = dmabuf->priv;
> +	struct dma_heap_attachment *a = attachment->priv;
> +
> +	mutex_lock(&buffer->attachments_lock);
> +	list_del(&a->list);
> +	mutex_unlock(&buffer->attachments_lock);
> +
> +	sg_free_table(a->table);
> +	kfree(a->table);
> +	kfree(a);
> +}
> +
> +static struct sg_table *dma_heap_map_dma_buf(struct dma_buf_attachment *attachment,
> +					     enum dma_data_direction direction)
> +{
> +	struct dma_heap_attachment *a = attachment->priv;
> +	struct sg_table *table = a->table;
> +
> +	/*
> +	 * As this heap is backed by uncached SRAM memory we do not need to
> +	 * perform any sync operations on the buffer before allowing device
> +	 * domain access. For this reason we use SKIP_CPU_SYNC and also do
> +	 * not use or provide begin/end_cpu_access() dma-buf functions.
> +	 */

I bet that works really well with SWIOTLB ;)

Also note that SKIP_CPU_SYNC is only a hint, so the DMA API 
implementation is still at liberty to do the fatally dangerous things 
you're hoping it won't (e.g. dereferencing sg_virt()...)

Thanks,
Robin.

> +	if (!dma_map_sg_attrs(attachment->dev, table->sgl, table->nents,
> +			      direction, DMA_ATTR_SKIP_CPU_SYNC))
> +		return ERR_PTR(-ENOMEM);
> +
> +	return table;
> +}
> +
> +static void dma_heap_unmap_dma_buf(struct dma_buf_attachment *attachment,
> +				   struct sg_table *table,
> +				   enum dma_data_direction direction)
> +{
> +	dma_unmap_sg_attrs(attachment->dev, table->sgl, table->nents,
> +			   direction, DMA_ATTR_SKIP_CPU_SYNC);
> +}
> +
> +static void dma_heap_dma_buf_release(struct dma_buf *dmabuf)
> +{
> +	struct sram_dma_heap_buffer *buffer = dmabuf->priv;
> +
> +	gen_pool_free(buffer->pool, (unsigned long)buffer->vaddr, buffer->len);
> +	kfree(buffer);
> +}
> +
> +static int dma_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
> +{
> +	struct sram_dma_heap_buffer *buffer = dmabuf->priv;
> +	int ret;
> +
> +	/* SRAM mappings are not cached */
> +	vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> +
> +	ret = vm_iomap_memory(vma, buffer->paddr, buffer->len);
> +	if (ret)
> +		pr_err("Could not map buffer to userspace\n");
> +
> +	return ret;
> +}
> +
> +static int dma_heap_vmap(struct dma_buf *dmabuf, struct iosys_map *map)
> +{
> +	struct sram_dma_heap_buffer *buffer = dmabuf->priv;
> +
> +	iosys_map_set_vaddr(map, buffer->vaddr);
> +
> +	return 0;
> +}
> +
> +static const struct dma_buf_ops sram_dma_heap_buf_ops = {
> +	.attach = dma_heap_attach,
> +	.detach = dma_heap_detatch,
> +	.map_dma_buf = dma_heap_map_dma_buf,
> +	.unmap_dma_buf = dma_heap_unmap_dma_buf,
> +	.release = dma_heap_dma_buf_release,
> +	.mmap = dma_heap_mmap,
> +	.vmap = dma_heap_vmap,
> +};
> +
> +static struct dma_buf *sram_dma_heap_allocate(struct dma_heap *heap,
> +					      unsigned long len,
> +					      unsigned long fd_flags,
> +					      unsigned long heap_flags)
> +{
> +	struct sram_dma_heap *sram_dma_heap = dma_heap_get_drvdata(heap);
> +	struct sram_dma_heap_buffer *buffer;
> +
> +	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
> +	struct dma_buf *dmabuf;
> +	int ret = 0;
> +
> +	buffer = kzalloc(sizeof(*buffer), GFP_KERNEL);
> +	if (!buffer)
> +		return ERR_PTR(-ENOMEM);
> +	buffer->pool = sram_dma_heap->pool;
> +	INIT_LIST_HEAD(&buffer->attachments);
> +	mutex_init(&buffer->attachments_lock);
> +	buffer->len = len;
> +
> +	buffer->vaddr = (void *)gen_pool_alloc(buffer->pool, buffer->len);
> +	if (!buffer->vaddr) {
> +		ret = -ENOMEM;
> +		goto free_buffer;
> +	}
> +
> +	buffer->paddr = gen_pool_virt_to_phys(buffer->pool, (unsigned long)buffer->vaddr);
> +	if (buffer->paddr == -1) {
> +		ret = -ENOMEM;
> +		goto free_pool;
> +	}
> +
> +	/* create the dmabuf */
> +	exp_info.exp_name = dma_heap_get_name(heap);
> +	exp_info.ops = &sram_dma_heap_buf_ops;
> +	exp_info.size = buffer->len;
> +	exp_info.flags = fd_flags;
> +	exp_info.priv = buffer;
> +	dmabuf = dma_buf_export(&exp_info);
> +	if (IS_ERR(dmabuf)) {
> +		ret = PTR_ERR(dmabuf);
> +		goto free_pool;
> +	}
> +
> +	return dmabuf;
> +
> +free_pool:
> +	gen_pool_free(buffer->pool, (unsigned long)buffer->vaddr, buffer->len);
> +free_buffer:
> +	kfree(buffer);
> +
> +	return ERR_PTR(ret);
> +}
> +
> +static struct dma_heap_ops sram_dma_heap_ops = {
> +	.allocate = sram_dma_heap_allocate,
> +};
> +
> +int sram_add_dma_heap(struct sram_dev *sram,
> +		      struct sram_reserve *block,
> +		      phys_addr_t start,
> +		      struct sram_partition *part)
> +{
> +	struct sram_dma_heap *sram_dma_heap;
> +	struct dma_heap_export_info exp_info;
> +
> +	dev_info(sram->dev, "Exporting SRAM Heap '%s'\n", block->label);
> +
> +	sram_dma_heap = kzalloc(sizeof(*sram_dma_heap), GFP_KERNEL);
> +	if (!sram_dma_heap)
> +		return -ENOMEM;
> +	sram_dma_heap->pool = part->pool;
> +
> +	exp_info.name = kasprintf(GFP_KERNEL, "sram_%s", block->label);
> +	exp_info.ops = &sram_dma_heap_ops;
> +	exp_info.priv = sram_dma_heap;
> +	sram_dma_heap->heap = dma_heap_add(&exp_info);
> +	if (IS_ERR(sram_dma_heap->heap)) {
> +		int ret = PTR_ERR(sram_dma_heap->heap);
> +		kfree(sram_dma_heap);
> +		return ret;
> +	}
> +
> +	return 0;
> +}
> diff --git a/drivers/misc/sram.c b/drivers/misc/sram.c
> index 5757adf418b1d..6dd173a2fba8e 100644
> --- a/drivers/misc/sram.c
> +++ b/drivers/misc/sram.c
> @@ -120,6 +120,12 @@ static int sram_add_partition(struct sram_dev *sram, struct sram_reserve *block,
>   		ret = sram_add_pool(sram, block, start, part);
>   		if (ret)
>   			return ret;
> +
> +		if (block->export) {
> +			ret = sram_add_dma_heap(sram, block, start, part);
> +			if (ret)
> +				return ret;
> +		}
>   	}
>   	if (block->export) {
>   		ret = sram_add_export(sram, block, start, part);
> diff --git a/drivers/misc/sram.h b/drivers/misc/sram.h
> index 397205b8bf6ff..062bdd25fa068 100644
> --- a/drivers/misc/sram.h
> +++ b/drivers/misc/sram.h
> @@ -60,4 +60,20 @@ static inline int sram_add_protect_exec(struct sram_partition *part)
>   	return -ENODEV;
>   }
>   #endif /* CONFIG_SRAM_EXEC */
> +
> +#ifdef CONFIG_SRAM_DMA_HEAP
> +int sram_add_dma_heap(struct sram_dev *sram,
> +		      struct sram_reserve *block,
> +		      phys_addr_t start,
> +		      struct sram_partition *part);
> +#else
> +static inline int sram_add_dma_heap(struct sram_dev *sram,
> +				    struct sram_reserve *block,
> +				    phys_addr_t start,
> +				    struct sram_partition *part)
> +{
> +	return 0;
> +}
> +#endif /* CONFIG_SRAM_DMA_HEAP */
> +
>   #endif /* __SRAM_H */