Messages in this thread | | | Date | Wed, 18 Oct 2023 18:52:59 +0100 | Subject | Re: [PATCH] swiotlb: Rewrite comment explaining why the source is preserved on DMA_FROM_DEVICE | From | Robin Murphy <> |
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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@arm.com>
> Cc: Yan Zhao <yan.y.zhao@intel.com> > Cc: Robin Murphy <robin.murphy@arm.com> > Cc: Linus Torvalds <torvalds@linux-foundation.org> > Link: https://lore.kernel.org/all/ZN5elYQ5szQndN8n@google.com > Signed-off-by: Sean Christopherson <seanjc@google.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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