CVE-2026-98262: ata: libahci: clear PxCLBU and PxFBU for AHCI_HFLAG_32BIT_ONLY

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

ata: libahci: clear PxCLBU and PxFBU for AHCIHFLAG32BITONLY

A user reported that commit 105c42566a55 ("ata: ahci: force 32-bit DMA for JMicron JMB582/JMB585") made the JMicron JMB585 unusable on his board.

The failure is seen as soon as the ahci driver is probed, and booting with iommu=off does not solve the problem.

Looking at the AHCI specification, PxCLBU and PxFBU are both read only '0' for HBAs that do not support 64-bit addressing.

For HBAs that do support 64-bit addressing, the registers are read write, with a reset value that is Implementation Specific.

When using the AHCIHFLAG32BITONLY flag, the HBA does support 64-bit addressing, and a 32-bit DMA mask is set by simply clearing HOSTCAP64. Thus, in this case, we need to explicitly clear the registers to 0.