linux-kernel - Re: [PATCH v4 5/5] iommu/dma: Force swiotlb_max_mapping

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Date: Mon, 26 Feb 2024 19:35:59 +0000
From: Robin Murphy <robin.murphy@....com>
To: Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Cc: kernel-team@...roid.com, iommu@...ts.linux.dev,
 Christoph Hellwig <hch@....de>, Marek Szyprowski <m.szyprowski@...sung.com>,
 Petr Tesarik <petr.tesarik1@...wei-partners.com>,
 Dexuan Cui <decui@...rosoft.com>, Nicolin Chen <nicolinc@...dia.com>,
 Michael Kelley <mhklinux@...look.com>
Subject: Re: [PATCH v4 5/5] iommu/dma: Force swiotlb_max_mapping_size on an
 untrusted device

On 21/02/2024 11:35 am, Will Deacon wrote:
> From: Nicolin Chen <nicolinc@...dia.com>
> 
> The swiotlb does not support a mapping size > swiotlb_max_mapping_size().
> On the other hand, with a 64KB PAGE_SIZE configuration, it's observed that
> an NVME device can map a size between 300KB~512KB, which certainly failed
> the swiotlb mappings, though the default pool of swiotlb has many slots:
>      systemd[1]: Started Journal Service.
>   => nvme 0000:00:01.0: swiotlb buffer is full (sz: 327680 bytes), total 32768 (slots), used 32 (slots)
>      note: journal-offline[392] exited with irqs disabled
>      note: journal-offline[392] exited with preempt_count 1
> 
> Call trace:
> [    3.099918]  swiotlb_tbl_map_single+0x214/0x240
> [    3.099921]  iommu_dma_map_page+0x218/0x328
> [    3.099928]  dma_map_page_attrs+0x2e8/0x3a0
> [    3.101985]  nvme_prep_rq.part.0+0x408/0x878 [nvme]
> [    3.102308]  nvme_queue_rqs+0xc0/0x300 [nvme]
> [    3.102313]  blk_mq_flush_plug_list.part.0+0x57c/0x600
> [    3.102321]  blk_add_rq_to_plug+0x180/0x2a0
> [    3.102323]  blk_mq_submit_bio+0x4c8/0x6b8
> [    3.103463]  __submit_bio+0x44/0x220
> [    3.103468]  submit_bio_noacct_nocheck+0x2b8/0x360
> [    3.103470]  submit_bio_noacct+0x180/0x6c8
> [    3.103471]  submit_bio+0x34/0x130
> [    3.103473]  ext4_bio_write_folio+0x5a4/0x8c8
> [    3.104766]  mpage_submit_folio+0xa0/0x100
> [    3.104769]  mpage_map_and_submit_buffers+0x1a4/0x400
> [    3.104771]  ext4_do_writepages+0x6a0/0xd78
> [    3.105615]  ext4_writepages+0x80/0x118
> [    3.105616]  do_writepages+0x90/0x1e8
> [    3.105619]  filemap_fdatawrite_wbc+0x94/0xe0
> [    3.105622]  __filemap_fdatawrite_range+0x68/0xb8
> [    3.106656]  file_write_and_wait_range+0x84/0x120
> [    3.106658]  ext4_sync_file+0x7c/0x4c0
> [    3.106660]  vfs_fsync_range+0x3c/0xa8
> [    3.106663]  do_fsync+0x44/0xc0
> 
> Since untrusted devices might go down the swiotlb pathway with dma-iommu,
> these devices should not map a size larger than swiotlb_max_mapping_size.
> 
> To fix this bug, add iommu_dma_max_mapping_size() for untrusted devices to
> take into account swiotlb_max_mapping_size() v.s. iova_rcache_range() from
> the iommu_dma_opt_mapping_size().

On the basis that this is at least far closer to correct than doing nothing,

Acked-by: Robin Murphy <robin.murphy@....com>

TBH I'm scared to think about theoretical correctness for all the 
interactions between the IOVA granule and min_align_mask, since just the 
SWIOTLB stuff is bad enough, even before you realise the ways that the 
IOVA allocation isn't necessarily right either. However I reckon as long 
as we don't ever see a granule smaller than IO_TLB_SIZE, and/or a 
min_align_mask larger than a granule, then this should probably work 
well enough as-is.

Cheers,
Robin.

> Fixes: 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers")
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> Link: https://lore.kernel.org/r/ee51a3a5c32cf885b18f6416171802669f4a718a.1707851466.git.nicolinc@nvidia.com
> Signed-off-by: Will Deacon <will@...nel.org>
> ---
>   drivers/iommu/dma-iommu.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 50ccc4f1ef81..7d1a20da6d94 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1706,6 +1706,13 @@ static size_t iommu_dma_opt_mapping_size(void)
>   	return iova_rcache_range();
>   }
>   
> +static size_t iommu_dma_max_mapping_size(struct device *dev)
> +{
> +	if (is_swiotlb_active(dev) && dev_is_untrusted(dev))
> +		return swiotlb_max_mapping_size(dev);
> +	return SIZE_MAX;
> +}
> +
>   static const struct dma_map_ops iommu_dma_ops = {
>   	.flags			= DMA_F_PCI_P2PDMA_SUPPORTED,
>   	.alloc			= iommu_dma_alloc,
> @@ -1728,6 +1735,7 @@ static const struct dma_map_ops iommu_dma_ops = {
>   	.unmap_resource		= iommu_dma_unmap_resource,
>   	.get_merge_boundary	= iommu_dma_get_merge_boundary,
>   	.opt_mapping_size	= iommu_dma_opt_mapping_size,
> +	.max_mapping_size       = iommu_dma_max_mapping_size,
>   };
>   
>   /*