Re: [PATCH v2 1/3] swiotlb: Fix allocation alignment requirement when searching slots

[Robin Murphy]

On 31/01/2024 12:25 pm, Will Deacon wrote:
> 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 fails to take into account the
> 8KiB buffer size and returns page-unaligned allocations if 'area->index'
> was left unaligned by a previous allocation from the buffer:

Hmm, but isn't this fundamentally swiotlb_alloc()'s fault for assuming 
it's going to get a page-aligned address back despite asking for 0 
alignment in the first place? I'm not sure SWIOTLB has ever promised 
implicit size-alignment, so it feels somewhat misplaced to be messing 
with the algorithm before fixing the obvious issue in the caller :/

Cheers,
Robin.

>   # 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 in tears (typically buffer corruption and/or a hang) because
> swiotlb_alloc() blindly returns a pointer to the 'struct page'
> corresponding to the allocation and therefore the first half of the page
> ends up being allocated twice.
>
> 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.
>
> Fixes: bbb73a103fbb ("swiotlb: fix a braino in the alignment check fix")
> Fixes: 0eee5ae10256 ("swiotlb: fix slot alignment checks")
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Cc: Robin Murphy <robin.murphy@xxxxxxx>
> Cc: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
> Cc: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> ---
>   kernel/dma/swiotlb.c | 29 +++++++++++++++--------------
>   1 file changed, 15 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index b079a9a8e087..56cc08b1fbd6 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -982,7 +982,7 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
>   		phys_to_dma_unencrypted(dev, pool->start) & boundary_mask;
>   	unsigned long max_slots = get_max_slots(boundary_mask);
>   	unsigned int iotlb_align_mask =
> -		dma_get_min_align_mask(dev) | alloc_align_mask;
> +		dma_get_min_align_mask(dev) & ~(IO_TLB_SIZE - 1);
>   	unsigned int nslots = nr_slots(alloc_size), stride;
>   	unsigned int offset = swiotlb_align_offset(dev, orig_addr);
>   	unsigned int index, slots_checked, count = 0, i;
> @@ -993,19 +993,18 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
>   	BUG_ON(!nslots);
>   	BUG_ON(area_index >= pool->nareas);
>   
> +	/*
> +	 * For mappings with an alignment requirement don't bother looping to
> +	 * unaligned slots once we found an aligned one.
> +	 */
> +	stride = get_max_slots(max(alloc_align_mask, iotlb_align_mask));
> +
>   	/*
>   	 * For allocations of PAGE_SIZE or larger only look for page aligned
>   	 * allocations.
>   	 */
>   	if (alloc_size >= PAGE_SIZE)
> -		iotlb_align_mask |= ~PAGE_MASK;
> -	iotlb_align_mask &= ~(IO_TLB_SIZE - 1);
> -
> -	/*
> -	 * For mappings with an alignment requirement don't bother looping to
> -	 * unaligned slots once we found an aligned one.
> -	 */
> -	stride = (iotlb_align_mask >> IO_TLB_SHIFT) + 1;
> +		stride = max(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);
>   
>   	spin_lock_irqsave(&area->lock, flags);
>   	if (unlikely(nslots > pool->area_nslabs - area->used))
> @@ -1015,14 +1014,16 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
>   	index = area->index;
>   
>   	for (slots_checked = 0; slots_checked < pool->area_nslabs; ) {
> -		slot_index = slot_base + index;
> +		phys_addr_t tlb_addr;
>   
> -		if (orig_addr &&
> -		    (slot_addr(tbl_dma_addr, slot_index) &
> -		     iotlb_align_mask) != (orig_addr & iotlb_align_mask)) {
> +		slot_index = slot_base + index;
> +		tlb_addr = slot_addr(tbl_dma_addr, slot_index);
> +
> +		if ((tlb_addr & alloc_align_mask) ||
> +		    (orig_addr && (tlb_addr & iotlb_align_mask) !=
> +				  (orig_addr & iotlb_align_mask))) {
>   			index = wrap_area_index(pool, index + 1);
>   			slots_checked++;
> -			continue;
>   		}
>   
>   		if (!iommu_is_span_boundary(slot_index, nslots,