CVE-2024-35814: swiotlb: Fix double-allocation of slots due to broken alignment handling

In the Linux kernel, the following vulnerability has been resolved:

swiotlb: Fix double-allocation of slots due to broken alignment handling

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 dmaalloccoherent(), 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: origaddr 0x0 allocsize 0x2000, iotlbalignmask 0x800 stride 0x2: got slot 1645-1649/7168 (0x98326800) | virtio-pci 0000:00:07.0: origaddr 0x0 allocsize 0x2000, iotlbalignmask 0x800 stride 0x2: got slot 1649-1653/7168 (0x98328800) | virtio-pci 0000:00:07.0: origaddr 0x0 allocsize 0x2000, iotlbalignmask 0x800 stride 0x2: got slot 1653-1657/7168 (0x9832a800)

This ends badly (typically buffer corruption and/or a hang) because swiotlballoc() 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 ~PAGEMASK in 'iotlbalignmask' for large allocations and resulting in alignment requirements exceeding swiotlbmaxmappingsize().