ata: pata_macio: Fix DMA table overflow

Summary

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

ata: pata_macio: Fix DMA table overflow

Kolbjørn and Jonáš reported that their 32-bit PowerMacs were crashing in pata-macio since commit 09fe2bfa6b83 ("ata: pata_macio: Fix max_segment_size with PAGE_SIZE == 64K").

For example:

kernel BUG at drivers/ata/pata_macio.c:544! Oops: Exception in kernel mode, sig: 5 [#1] BE PAGE_SIZE=4K MMU=Hash SMP NR_CPUS=2 DEBUG_PAGEALLOC PowerMac ... NIP pata_macio_qc_prep+0xf4/0x190 LR pata_macio_qc_prep+0xfc/0x190 Call Trace: 0xc1421660 (unreliable) ata_qc_issue+0x14c/0x2d4 __ata_scsi_queuecmd+0x200/0x53c ata_scsi_queuecmd+0x50/0xe0 scsi_queue_rq+0x788/0xb1c __blk_mq_issue_directly+0x58/0xf4 blk_mq_plug_issue_direct+0x8c/0x1b4 blk_mq_flush_plug_list.part.0+0x584/0x5e0 __blk_flush_plug+0xf8/0x194 __submit_bio+0x1b8/0x2e0 submit_bio_noacct_nocheck+0x230/0x304 btrfs_work_helper+0x200/0x338 process_one_work+0x1a8/0x338 worker_thread+0x364/0x4c0 kthread+0x100/0x104 start_kernel_thread+0x10/0x14

That commit increased max_segment_size to 64KB, with the justification that the SCSI core was already using that size when PAGE_SIZE == 64KB, and that there was existing logic to split over-sized requests.

However with a sufficiently large request, the splitting logic causes each sg to be split into two commands in the DMA table, leading to overflow of the DMA table, triggering the BUG_ON().

With default settings the bug doesn't trigger, because the request size is limited by max_sectors_kb == 1280, however max_sectors_kb can be increased, and apparently some distros do that by default using udev rules.

Fix the bug for 4KB kernels by reverting to the old max_segment_size.

For 64KB kernels the sg_tablesize needs to be halved, to allow for the possibility that each sg will be split into two.