scsi: megaraid_sas: Limit NVMe request size to the PRP chain frame

Summary

In the Linux kernel, the following vulnerability has been resolved: scsi: megaraid_sas: Limit NVMe request size to the PRP chain frame megasas_make_prp_nvme() builds a command's PRP list in cmd->sg_frame, a DMA pool buffer of instance->max_chain_frame_sz bytes, spending one entry per NVMe page of the transfer plus one per page of the buffer for the chain pointer. The loop runs until the transfer is described and never checks the buffer bound. max_hw_sectors comes straight from the MDTS the firmware reports for the drive. On drives with a large MDTS the only thing keeping the list inside the buffer was the block layer default of 1280 KiB, which needs 320 entries, which fit into a 4 KiB frame as that holds 512. But since commit 9b8b84879d4a ("block: Increase BLK_DEF_MAX_SECTORS_CAP") that default is 4 MiB, and such a transfer needs 1025 entries, so the list runs a full page past the end of the frame: sd 1:0:1:0: [sdb] tag#630 page boundary ptr_sgl: 0x00000000ba62d13f BUG: unable to handle page fault for address: ff663bcb81e7c000 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page RIP: 0010:megasas_build_and_issue_cmd_fusion+0xeaa/0x1870 [megaraid_sas] If the page after the frame happens to be mapped, the overrun does not fault but silently corrupts the neighbouring pool entry, which is another in-flight command's PRP list. Cap max_hw_sectors at what the chain frame can describe, less one page for transfers that do not start on a page boundary and so need one entry more. This is the megaraid_sas counterpart of commit 04631f55afc5 ("scsi: mpt3sas: Limit NVMe request size to 2 MiB"), but derives the limit from max_chain_frame_sz rather than hardcoding it.