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Message-ID: <85e332bf-decf-427c-a2e5-95ab872d4ea6@arm.com>
Date: Wed, 18 Oct 2023 18:52:59 +0100
From: Robin Murphy <robin.murphy@....com>
To: Sean Christopherson <seanjc@...gle.com>,
Christoph Hellwig <hch@....de>,
Marek Szyprowski <m.szyprowski@...sung.com>
Cc: iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
Yan Zhao <yan.y.zhao@...el.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] swiotlb: Rewrite comment explaining why the source is
preserved on DMA_FROM_DEVICE
On 2023-10-18 18:34, Sean Christopherson wrote:
> Rewrite the comment explaining why swiotlb copies the original buffer to
> the TLB buffer before initiating DMA *from* the device, i.e. before the
> device DMAs into the TLB buffer. The existing comment's argument that
> preserving the original data can prevent a kernel memory leak is bogus.
>
> If the driver that triggered the mapping _knows_ that the device will
> overwrite the entire mapping, or the driver will consume only the written
> parts, then copying from the original memory is completely pointless.
>
> If neither of the above holds true, then copying from the original adds
> value only if preserving the data is necessary for functional correctness,
> or the driver explicitly initialized the original memory. If the driver
> didn't initialize the memory, then copying the original buffer to the TLB
> buffer simply changes what kernel data is leaked to userspace.
>
> Writing the entire TLB buffer _does_ prevent leaking stale TLB buffer data
> from a previous bounce, but that can be achieved by simply zeroing the TLB
> buffer when grabbing a slot.
>
> The real reason swiotlb ended up initializing the TLB buffer with the
> original buffer is that it's necessary to make swiotlb operate as
> transparently as possible, i.e. to behave as closely as possible to
> hardware, and to avoid corrupting the original buffer, e.g. if the driver
> knows the device will do partial writes and is relying on the unwritten
> data to be preserved.
Thanks Sean, I like this :)
Reviewed-by: Robin Murphy <robin.murphy@....com>
> Cc: Yan Zhao <yan.y.zhao@...el.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Link: https://lore.kernel.org/all/ZN5elYQ5szQndN8n@google.com
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> kernel/dma/swiotlb.c | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 01637677736f..e071415a75dc 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1296,11 +1296,13 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
> pool->slots[index + i].orig_addr = slot_addr(orig_addr, i);
> tlb_addr = slot_addr(pool->start, index) + offset;
> /*
> - * When dir == DMA_FROM_DEVICE we could omit the copy from the orig
> - * to the tlb buffer, if we knew for sure the device will
> - * overwrite the entire current content. But we don't. Thus
> - * unconditional bounce may prevent leaking swiotlb content (i.e.
> - * kernel memory) to user-space.
> + * When the device is writing memory, i.e. dir == DMA_FROM_DEVICE, copy
> + * the original buffer to the TLB buffer before initiating DMA in order
> + * to preserve the original's data if the device does a partial write,
> + * i.e. if the device doesn't overwrite the entire buffer. Preserving
> + * the original data, even if it's garbage, is necessary to match
> + * hardware behavior (use of swiotlb is supposed to be transparent) and
Super-nit: I think that last "and" is superfluous (i.e. unwritten memory
not magically corrupting itself *is* the aforementioned hardware behaviour).
> + * so that swiotlb doesn't corrupt bytes that the device does NOT write.
> */
> swiotlb_bounce(dev, tlb_addr, mapping_size, DMA_TO_DEVICE);
> return tlb_addr;
>
> base-commit: 213f891525c222e8ed145ce1ce7ae1f47921cb9c
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