linux-kernel - RE: [PATCH v5 1/6] swiotlb: Fix double-allocation of slots due to broken alignment handling

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Date: Thu, 29 Feb 2024 05:44:41 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Will Deacon <will@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: "kernel-team@...roid.com" <kernel-team@...roid.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, Christoph Hellwig
	<hch@....de>, Marek Szyprowski <m.szyprowski@...sung.com>, Robin Murphy
	<robin.murphy@....com>, Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	Dexuan Cui <decui@...rosoft.com>, Nicolin Chen <nicolinc@...dia.com>
Subject: RE: [PATCH v5 1/6] swiotlb: Fix double-allocation of slots due to
 broken alignment handling



From: Will Deacon <will@...nel.org> Sent: Wednesday, February 28, 2024 5:39 AM
> 
> Commit bbb73a103fbb ("swiotlb: fix a braino in the alignment check fix"),
> which was a fix for commit 0eee5ae10256 ("swiotlb: fix slot alignment
> checks"), causes a functional regression with vsock in a virtual machine
> using bouncing via a restricted DMA SWIOTLB pool.
> 
> When virtio allocates the virtqueues for the vsock device using
> dma_alloc_coherent(), the SWIOTLB search can return page-unaligned
> allocations if 'area->index' was left unaligned by a previous allocation
> from the buffer:
> 
>  # Final address in brackets is the SWIOTLB address returned to the caller
>  | virtio-pci 0000:00:07.0: orig_addr 0x0 alloc_size 0x2000, iotlb_align_mask
> 0x800 stride 0x2: got slot 1645-1649/7168 (0x98326800)
>  | virtio-pci 0000:00:07.0: orig_addr 0x0 alloc_size 0x2000, iotlb_align_mask
> 0x800 stride 0x2: got slot 1649-1653/7168 (0x98328800)
>  | virtio-pci 0000:00:07.0: orig_addr 0x0 alloc_size 0x2000, iotlb_align_mask
> 0x800 stride 0x2: got slot 1653-1657/7168 (0x9832a800)
> 
> This ends badly (typically buffer corruption and/or a hang) because
> swiotlb_alloc() is expecting a page-aligned allocation and so blindly
> returns a pointer to the 'struct page' corresponding to the allocation,
> therefore double-allocating the first half (2KiB slot) of the 4KiB page.
> 
> Fix the problem by treating the allocation alignment separately to any
> additional alignment requirements from the device, using the maximum
> of the two as the stride to search the buffer slots and taking care
> to ensure a minimum of page-alignment for buffers larger than a page.
> 
> This also resolves swiotlb allocation failures occuring due to the
> inclusion of ~PAGE_MASK in 'iotlb_align_mask' for large allocations and
> resulting in alignment requirements exceeding swiotlb_max_mapping_size().
> 
> Fixes: bbb73a103fbb ("swiotlb: fix a braino in the alignment check fix")
> Fixes: 0eee5ae10256 ("swiotlb: fix slot alignment checks")
> Cc: Christoph Hellwig <hch@....de>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Dexuan Cui <decui@...rosoft.com>
> Reviewed-by: Petr Tesarik <petr.tesarik1@...wei-partners.com>
> Tested-by: Nicolin Chen <nicolinc@...dia.com>
> Signed-off-by: Will Deacon <will@...nel.org>
> ---
>  kernel/dma/swiotlb.c | 28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)
> 

Reviewed-by: Michael Kelley <mhklinux@...look.com>