In the Linux kernel, the following vulnerability has been resolved: ata: libahci: clear PxCLBU and PxFBU for AHCI_HFLAG_32BIT_ONLY A user

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

ata: libahci: clear PxCLBU and PxFBU for AHCI_HFLAG_32BIT_ONLY

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 AHCI_HFLAG_32BIT_ONLY flag, the HBA does support 64-bit
addressing, and a 32-bit DMA mask is set by simply clearing HOST_CAP_64.
Thus, in this case, we need to explicitly clear the registers to 0.