block DmaAlignment Corruption

Vulnerability analysis

When a process issues direct I/O, the kernel builds a block-I/O request from the user-supplied memory buffer without checking that the buffer address and length meet the target device's DMA alignment requirements. An earlier change removed the early alignment check and deferred it to the block layer, but device drivers and stacking drivers do not always catch misalignment after the request is submitted, so a misaligned request can reach the hardware and cause incorrect DMA transfers, memory corruption, or crashes. The fix restores alignment validation while the request is being assembled: the device alignment requirement is checked as pages are extracted from the user's buffer, and any misaligned buffer is rejected before the request is sent to the driver. Any local user with access to a block device or a filesystem that supports direct I/O can trigger this; no special privileges or hardware configuration are needed beyond opening the device or file with the direct-I/O flag.