This fix the bad fault reported by KUAP when io_wqe_worker access userspace.

 Bug: Read fault blocked by KUAP!
 WARNING: CPU: 1 PID: 101841 at arch/powerpc/mm/fault.c:229 __do_page_fault+0x6b4/0xcd0
 NIP [c00000000009e7e4] __do_page_fault+0x6b4/0xcd0
 LR [c00000000009e7e0] __do_page_fault+0x6b0/0xcd0
..........
 Call Trace:
 [c000000016367330] [c00000000009e7e0] __do_page_fault+0x6b0/0xcd0 (unreliable)
 [c0000000163673e0] [c00000000009ee3c] do_page_fault+0x3c/0x120
 [c000000016367430] [c00000000000c848] handle_page_fault+0x10/0x2c
 --- interrupt: 300 at iov_iter_fault_in_readable+0x148/0x6f0
..........
 NIP [c0000000008e8228] iov_iter_fault_in_readable+0x148/0x6f0
 LR [c0000000008e834c] iov_iter_fault_in_readable+0x26c/0x6f0
 interrupt: 300
 [c0000000163677e0] [c0000000007154a0] iomap_write_actor+0xc0/0x280
 [c000000016367880] [c00000000070fc94] iomap_apply+0x1c4/0x780
 [c000000016367990] [c000000000710330] iomap_file_buffered_write+0xa0/0x120
 [c0000000163679e0] [c00800000040791c] xfs_file_buffered_aio_write+0x314/0x5e0 [xfs]
 [c000000016367a90] [c0000000006d74bc] io_write+0x10c/0x460
 [c000000016367bb0] [c0000000006d80e4] io_issue_sqe+0x8d4/0x1200
 [c000000016367c70] [c0000000006d8ad0] io_wq_submit_work+0xc0/0x250
 [c000000016367cb0] [c0000000006e2578] io_worker_handle_work+0x498/0x800
 [c000000016367d40] [c0000000006e2cdc] io_wqe_worker+0x3fc/0x4f0
 [c000000016367da0] [c0000000001cb0a4] kthread+0x1c4/0x1d0
 [c000000016367e10] [c00000000000dbf0] ret_from_kernel_thread+0x5c/0x6c

The kernel consider thread AMR value for kernel thread to be
AMR_KUAP_BLOCKED. Hence access to userspace is denied. This
of course not correct and we should allow userspace access after
kthread_use_mm(). To be precise, kthread_use_mm() should inherit the
AMR value of the operating address space. But, the AMR value is
thread-specific and we inherit the address space and not thread
access restrictions. Because of this ignore AMR value when accessing
userspace via kernel thread.

current_thread_amr/iamr() are updated, because we use them in the
below stack.
....
[  530.710838] CPU: 13 PID: 5587 Comm: io_wqe_worker-0 Tainted: G      D           5.11.0-rc6+ #3
....

 NIP [c0000000000aa0c8] pkey_access_permitted+0x28/0x90
 LR [c0000000004b9278] gup_pte_range+0x188/0x420
 --- interrupt: 700
 [c00000001c4ef3f0] [0000000000000000] 0x0 (unreliable)
 [c00000001c4ef490] [c0000000004bd39c] gup_pgd_range+0x3ac/0xa20
 [c00000001c4ef5a0] [c0000000004bdd44] internal_get_user_pages_fast+0x334/0x410
 [c00000001c4ef620] [c000000000852028] iov_iter_get_pages+0xf8/0x5c0
 [c00000001c4ef6a0] [c0000000007da44c] bio_iov_iter_get_pages+0xec/0x700
 [c00000001c4ef770] [c0000000006a325c] iomap_dio_bio_actor+0x2ac/0x4f0
 [c00000001c4ef810] [c00000000069cd94] iomap_apply+0x2b4/0x740
 [c00000001c4ef920] [c0000000006a38b8] __iomap_dio_rw+0x238/0x5c0
 [c00000001c4ef9d0] [c0000000006a3c60] iomap_dio_rw+0x20/0x80
 [c00000001c4ef9f0] [c008000001927a30] xfs_file_dio_aio_write+0x1f8/0x650 [xfs]
 [c00000001c4efa60] [c0080000019284dc] xfs_file_write_iter+0xc4/0x130 [xfs]
 [c00000001c4efa90] [c000000000669984] io_write+0x104/0x4b0
 [c00000001c4efbb0] [c00000000066cea4] io_issue_sqe+0x3d4/0xf50
 [c00000001c4efc60] [c000000000670200] io_wq_submit_work+0xb0/0x2f0
 [c00000001c4efcb0] [c000000000674268] io_worker_handle_work+0x248/0x4a0
 [c00000001c4efd30] [c0000000006746e8] io_wqe_worker+0x228/0x2a0
 [c00000001c4efda0] [c00000000019d994] kthread+0x1b4/0x1c0

Fixes: 48a8ab4e ("powerpc/book3s64/pkeys: Don't update SPRN_AMR when in kernel mode.")
Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210206025634.521979-1-aneesh.kumar@linux.ibm.com