Re: [PATCH 1/2] swiotlb: Remove alloc_size argument to swiotlb_tbl_map_single()

[Petr Tesařík]

On Sun,  7 Apr 2024 21:11:41 -0700
mhkelley58@xxxxxxxxx wrote:

> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> 
> Currently swiotlb_tbl_map_single() takes alloc_align_mask and
> alloc_size arguments to specify an swiotlb allocation that is
> larger than mapping_size. This larger allocation is used solely
> by iommu_dma_map_single() to handle untrusted devices that should
> not have DMA visibility to memory pages that are partially used
> for unrelated kernel data.
> 
> Having two arguments to specify the allocation is redundant. While
> alloc_align_mask naturally specifies the alignment of the starting
> address of the allocation, it can also implicitly specify the size
> by rounding up the mapping_size to that alignment.
> 
> Additionally, the current approach has an edge case bug.
> iommu_dma_map_page() already does the rounding up to compute the
> alloc_size argument. But swiotlb_tbl_map_single() then calculates
> the alignment offset based on the DMA min_align_mask, and adds
> that offset to alloc_size. If the offset is non-zero, the addition
> may result in a value that is larger than the max the swiotlb can
> allocate. If the rounding up is done _after_ the alignment offset is
> added to the mapping_size (and the original mapping_size conforms to
> the value returned by swiotlb_max_mapping_size), then the max that the
> swiotlb can allocate will not be exceeded.
> 
> In view of these issues, simplify the swiotlb_tbl_map_single() interface
> by removing the alloc_size argument. Most call sites pass the same
> value for mapping_size and alloc_size, and they pass alloc_align_mask
> as zero. Just remove the redundant argument from these callers, as they
> will see no functional change. For iommu_dma_map_page() also remove
> the alloc_size argument, and have swiotlb_tbl_map_single() compute
> the alloc_size by rounding up mapping_size after adding the offset
> based on min_align_mask. This has the side effect of fixing the
> edge case bug but with no other functional change.
> 
> Also add a sanity test on the alloc_align_mask. While IOMMU code
> currently ensures the granule is not larger than PAGE_SIZE, if
> that guarantee were to be removed in the future, the downstream
> effect on the swiotlb might go unnoticed until strange allocation
> failures occurred.
> 
> Tested on an ARM64 system with 16K page size and some kernel
> test-only hackery to allow modifying the DMA min_align_mask and
> the granule size that becomes the alloc_align_mask. Tested these
> combinations with a variety of original memory addresses and
> sizes, including those that reproduce the edge case bug:
> 
> * 4K granule and 0 min_align_mask
> * 4K granule and 0xFFF min_align_mask (4K - 1)
> * 16K granule and 0xFFF min_align_mask
> * 64K granule and 0xFFF min_align_mask
> * 64K granule and 0x3FFF min_align_mask (16K - 1)
> 
> With the changes, all combinations pass.
> 
> Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> ---
> I've haven't used any "Fixes:" tags. This patch really should be
> backported only if all the other recent swiotlb fixes get backported,
> and I'm unclear on whether that will happen.
> 
> I saw the brief discussion about removing the "dir" parameter from
> swiotlb_tbl_map_single(). That removal could easily be done as part
> of this patch, since it's already changing the swiotlb_tbl_map_single()
> parameters. But I think the conclusion of the discussion was to leave
> the "dir" parameter for symmetry with the swiotlb_sync_*() functions.
> Please correct me if that's wrong, and I'll respin this patch to do
> the removal.

Hi Michael,

sorry for taking so long to answer. Yes, there was no agreement on the
removal of the "dir" parameter, but I'm not sure it's because of
symmetry with swiotlb_sync_*(), because the topic was not really
discussed.

The discussion was about the KUnit test suite and whether direction is
a property of the bounce buffer or of each sync operation. Since DMA API
defines associates each DMA buffer with a direction, the direction
parameter passed to swiotlb_sync_*() should match what was passed to
swiotlb_tbl_map_single(), because that's how it is used by the generic
DMA code. In other words, if the parameter is kept, it should be kept
to match dma_map_*().

However, there is also symmetry with swiotlb_tbl_unmap_single(). This
function does use the parameter for the final sync. I believe there
should be a matching initial sync in swiotlb_tbl_map_single(). In
short, the buffer sync for DMA non-coherent devices should be moved from
swiotlb_map() to swiotlb_tbl_map_single(). If this sync is not needed,
then the caller can (and should) include DMA_ATTR_SKIP_CPU_SYNC in
the flags parameter.

To sum it up:

* Do *NOT* remove the "dir" parameter.
* Let me send a patch which moves the initial buffer sync.

Petr T

>  drivers/iommu/dma-iommu.c |  2 +-
>  drivers/xen/swiotlb-xen.c |  2 +-
>  include/linux/swiotlb.h   |  2 +-
>  kernel/dma/swiotlb.c      | 56
> +++++++++++++++++++++++++++++---------- 4 files changed, 45
> insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 07d087eecc17..c21ef1388499 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1165,7 +1165,7 @@ static dma_addr_t iommu_dma_map_page(struct
> device *dev, struct page *page, trace_swiotlb_bounced(dev, phys,
> size); 
>  		aligned_size = iova_align(iovad, size);
> -		phys = swiotlb_tbl_map_single(dev, phys, size,
> aligned_size,
> +		phys = swiotlb_tbl_map_single(dev, phys, size,
>  					      iova_mask(iovad), dir,
> attrs); 
>  		if (phys == DMA_MAPPING_ERROR)
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index 1c4ef5111651..6579ae3f6dac 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -216,7 +216,7 @@ static dma_addr_t xen_swiotlb_map_page(struct
> device *dev, struct page *page, */
>  	trace_swiotlb_bounced(dev, dev_addr, size);
>  
> -	map = swiotlb_tbl_map_single(dev, phys, size, size, 0, dir,
> attrs);
> +	map = swiotlb_tbl_map_single(dev, phys, size, 0, dir, attrs);
>  	if (map == (phys_addr_t)DMA_MAPPING_ERROR)
>  		return DMA_MAPPING_ERROR;
>  
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index ea23097e351f..14bc10c1bb23 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -43,7 +43,7 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask,
>  extern void __init swiotlb_update_mem_attributes(void);
>  
>  phys_addr_t swiotlb_tbl_map_single(struct device *hwdev, phys_addr_t
> phys,
> -		size_t mapping_size, size_t alloc_size,
> +		size_t mapping_size,
>  		unsigned int alloc_aligned_mask, enum
> dma_data_direction dir, unsigned long attrs);
>  
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index a5e0dfc44d24..046da973a7e2 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1340,15 +1340,40 @@ static unsigned long mem_used(struct
> io_tlb_mem *mem) 
>  #endif /* CONFIG_DEBUG_FS */
>  
> +/**
> + * swiotlb_tbl_map_single() - bounce buffer map a single contiguous
> physical area
> + * @dev:		Device which maps the buffer.
> + * @orig_addr:		Original (non-bounced) physical IO
> buffer address
> + * @mapping_size:	Requested size of the actual bounce buffer,
> excluding
> + *			any pre- or post-padding for alignment
> + * @alloc_align_mask:	Required start and end alignment of the
> allocated buffer
> + * @dir:		DMA direction
> + * @attrs:		Optional DMA attributes for the map
> operation
> + *
> + * Find and allocate a suitable sequence of IO TLB slots for the
> request.
> + * The allocated space starts at an alignment specified by
> alloc_align_mask,
> + * and the size of the allocated space is rounded up so that the
> total amount
> + * of allocated space is a multiple of (alloc_align_mask + 1). If
> + * alloc_align_mask is zero, the allocated space may be at any
> alignment and
> + * the size is not rounded up.
> + *
> + * The returned address is within the allocated space and matches
> the bits
> + * of orig_addr that are specified in the DMA min_align_mask for the
> device. As
> + * such, this returned address may be offset from the beginning of
> the allocated
> + * space. The bounce buffer space starting at the returned address
> for
> + * mapping_size bytes is initialized to the contents of the original
> IO buffer
> + * area. Any pre-padding (due to an offset) and any post-padding
> (due to
> + * rounding-up the size) is not initialized.
> + */
>  phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t
> orig_addr,
> -		size_t mapping_size, size_t alloc_size,
> -		unsigned int alloc_align_mask, enum
> dma_data_direction dir,
> -		unsigned long attrs)
> +		size_t mapping_size, unsigned int alloc_align_mask,
> +		enum dma_data_direction dir, unsigned long attrs)
>  {
>  	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
>  	unsigned int offset;
>  	struct io_tlb_pool *pool;
>  	unsigned int i;
> +	size_t size;
>  	int index;
>  	phys_addr_t tlb_addr;
>  	unsigned short pad_slots;
> @@ -1362,20 +1387,24 @@ phys_addr_t swiotlb_tbl_map_single(struct
> device *dev, phys_addr_t orig_addr, if
> (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) pr_warn_once("Memory
> encryption is active and system is using DMA bounce buffers\n"); 
> -	if (mapping_size > alloc_size) {
> -		dev_warn_once(dev, "Invalid sizes (mapping: %zd
> bytes, alloc: %zd bytes)",
> -			      mapping_size, alloc_size);
> -		return (phys_addr_t)DMA_MAPPING_ERROR;
> -	}
> +	/*
> +	 * The default swiotlb memory pool is allocated with
> PAGE_SIZE
> +	 * alignment. If a mapping is requested with larger
> alignment,
> +	 * the mapping may be unable to use the initial slot(s) in
> all
> +	 * sets of IO_TLB_SEGSIZE slots. In such case, a mapping
> request
> +	 * of or near the maximum mapping size would always fail.
> +	 */
> +	dev_WARN_ONCE(dev, alloc_align_mask > ~PAGE_MASK,
> +		"Alloc alignment may prevent fulfilling requests
> with max mapping_size\n"); 
>  	offset = swiotlb_align_offset(dev, alloc_align_mask,
> orig_addr);
> -	index = swiotlb_find_slots(dev, orig_addr,
> -				   alloc_size + offset,
> alloc_align_mask, &pool);
> +	size = ALIGN(mapping_size + offset, alloc_align_mask + 1);
> +	index = swiotlb_find_slots(dev, orig_addr, size,
> alloc_align_mask, &pool); if (index == -1) {
>  		if (!(attrs & DMA_ATTR_NO_WARN))
>  			dev_warn_ratelimited(dev,
>  	"swiotlb buffer is full (sz: %zd bytes), total %lu (slots),
> used %lu (slots)\n",
> -				 alloc_size, mem->nslabs,
> mem_used(mem));
> +				 size, mem->nslabs, mem_used(mem));
>  		return (phys_addr_t)DMA_MAPPING_ERROR;
>  	}
>  
> @@ -1388,7 +1417,7 @@ phys_addr_t swiotlb_tbl_map_single(struct
> device *dev, phys_addr_t orig_addr, offset &= (IO_TLB_SIZE - 1);
>  	index += pad_slots;
>  	pool->slots[index].pad_slots = pad_slots;
> -	for (i = 0; i < nr_slots(alloc_size + offset); i++)
> +	for (i = 0; i < (nr_slots(size) - pad_slots); i++)
>  		pool->slots[index + i].orig_addr =
> slot_addr(orig_addr, i); tlb_addr = slot_addr(pool->start, index) +
> offset; /*
> @@ -1543,8 +1572,7 @@ dma_addr_t swiotlb_map(struct device *dev,
> phys_addr_t paddr, size_t size, 
>  	trace_swiotlb_bounced(dev, phys_to_dma(dev, paddr), size);
>  
> -	swiotlb_addr = swiotlb_tbl_map_single(dev, paddr, size,
> size, 0, dir,
> -			attrs);
> +	swiotlb_addr = swiotlb_tbl_map_single(dev, paddr, size, 0,
> dir, attrs); if (swiotlb_addr == (phys_addr_t)DMA_MAPPING_ERROR)
>  		return DMA_MAPPING_ERROR;
>