arm64: io: Extract user memory type in ioremap_prot()

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

arm64: io: Extract user memory type in ioremap_prot()

The only caller of ioremap_prot() outside of the generic ioremap()

implementation is generic_access_phys(), which passes a 'pgprot_t' value

determined from the user mapping of the target 'pfn' being accessed by

the kernel. On arm64, the 'pgprot_t' contains all of the non-address

bits from the pte, including the permission controls, and so we end up

returning a new user mapping from ioremap_prot() which faults when

accessed from the kernel on systems with PAN:

| Unable to handle kernel read from unreadable memory at virtual address ffff80008ea89000

| ...

| Call trace:

| __memcpy_fromio+0x80/0xf8

| generic_access_phys+0x20c/0x2b8

| __access_remote_vm+0x46c/0x5b8

| access_remote_vm+0x18/0x30

| environ_read+0x238/0x3e8

| vfs_read+0xe4/0x2b0

| ksys_read+0xcc/0x178

| __arm64_sys_read+0x4c/0x68

Extract only the memory type from the user 'pgprot_t' in ioremap_prot()

and assert that we're being passed a user mapping, to protect us against

any changes in future that may require additional handling. To avoid

falsely flagging users of ioremap(), provide our own ioremap() macro

which simply wraps __ioremap_prot().