RE: [PATCH v4 1/5] swiotlb: Fix double-allocation of slots due to broken alignment handling

[Michael Kelley]

From: Will Deacon <will@xxxxxxxxxx> Sent: Wednesday, February 21, 2024 3:35 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.

Could you also add some text that this patch fixes the scenario I
described in the other email thread?  Something like:

The changes to page alignment handling also fix a problem when
the alloc_align_mask is zero.  The page alignment handling added
in the two mentioned commits could force alignment to more bits
in orig_addr than specified by the device's DMA min_align_mask,
resulting in a larger offset.   Since swiotlb_max_mapping_size()
is based only on the DMA min_align_mask, that larger offset
plus the requested size could exceed IO_TLB_SEGSIZE slots, and
the mapping could fail when it shouldn't.  

> 
> 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: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Reviewed-by: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> ---
>  kernel/dma/swiotlb.c | 28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index b079a9a8e087..2ec2cc81f1a2 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 = umax(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);

Is this special handling of alloc_size >= PAGE_SIZE really needed?
I think the comment is somewhat inaccurate. If orig_addr is non-zero, and
alloc_align_mask is zero, the requirement is for the alignment to match
the DMA min_align_mask bits in orig_addr, even if the allocation is
larger than a page.   And with Patch 3 of this series, the swiotlb_alloc()
case passes in alloc_align_mask to handle page size and larger requests.
So it seems like this doesn't do anything useful unless orig_addr and
alloc_align_mask are both zero, and there aren't any cases of that
after this patch series.  If the caller wants alignment, specify
it with alloc_align_mask.

> 
>  	spin_lock_irqsave(&area->lock, flags);
>  	if (unlikely(nslots > pool->area_nslabs - area->used))
> @@ -1015,11 +1014,14 @@ 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))) {

It looks like these changes will cause a mapping failure in some
iommu_dma_map_page() cases that previously didn't fail.
Everything is made right by Patch 4 of your series, but from a
bisect standpoint, there will be a gap where things are worse.
In [1], I think Nicolin reported a crash with just this patch applied.

While the iommu_dma_map_page() case can already fail due to
"too large" requests because of not setting a max mapping size,
this patch can cause smaller requests to fail as well until Patch 4
gets applied.  That might be problem to avoid, perhaps by
merging the Patch 4 changes into this patch.

Michael

[1] https://lore.kernel.org/linux-iommu/SN6PR02MB415727E61B5295C259CCB268D4512@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#m0ec36324b17947adefc18b3ac715e1952150f89d

>  			index = wrap_area_index(pool, index + 1);
>  			slots_checked++;
>  			continue;
> --
> 2.44.0.rc0.258.g7320e95886-goog