arm64: extable: fix load_unaligned_zeropad() reg indices

In ex_handler_load_unaligned_zeropad() we erroneously extract the data and
addr register indices from ex->type rather than ex->data. As ex->type will
contain EX_TYPE_LOAD_UNALIGNED_ZEROPAD (i.e. 4):
 * We'll always treat X0 as the address register, since EX_DATA_REG_ADDR is
   extracted from bits [9:5]. Thus, we may attempt to dereference an
   arbitrary address as X0 may hold an arbitrary value.
 * We'll always treat X4 as the data register, since EX_DATA_REG_DATA is
   extracted from bits [4:0]. Thus we will corrupt X4 and cause arbitrary
   behaviour within load_unaligned_zeropad() and its caller.

Fix this by extracting both values from ex->data as originally intended.

On an MTE-enabled QEMU image we are hitting the following crash:
 Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
 Call trace:
  fixup_exception+0xc4/0x108
  __do_kernel_fault+0x3c/0x268
  do_tag_check_fault+0x3c/0x104
  do_mem_abort+0x44/0xf4
  el1_abort+0x40/0x64
  el1h_64_sync_handler+0x60/0xa0
  el1h_64_sync+0x7c/0x80
  link_path_walk+0x150/0x344
  path_openat+0xa0/0x7dc
  do_filp_open+0xb8/0x168
  do_sys_openat2+0x88/0x17c
  __arm64_sys_openat+0x74/0xa0
  invoke_syscall+0x48/0x148
  el0_svc_common+0xb8/0xf8
  do_el0_svc+0x28/0x88
  el0_svc+0x24/0x84
  el0t_64_sync_handler+0x88/0xec
  el0t_64_sync+0x1b4/0x1b8
 Code: f8695a69 71007d1f 540000e0 927df12a (f940014a)

Fixes: 753b3236 ("arm64: extable: add load_unaligned_zeropad() handler")
Cc: <stable@vger.kernel.org> # 5.16.x
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Evgenii Stepanov <eugenis@google.com>
Link: https://lore.kernel.org/r/20220125182217.2605202-1-eugenis@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/extable.c

@@ -40,8 +40,8 @@ static bool
 ex_handler_load_unaligned_zeropad(const struct exception_table_entry *ex,
 				  struct pt_regs *regs)
 {
-	int reg_data = FIELD_GET(EX_DATA_REG_DATA, ex->type);
-	int reg_addr = FIELD_GET(EX_DATA_REG_ADDR, ex->type);
+	int reg_data = FIELD_GET(EX_DATA_REG_DATA, ex->data);
+	int reg_addr = FIELD_GET(EX_DATA_REG_ADDR, ex->data);
 	unsigned long data, addr, offset;
 
 	addr = pt_regs_read_reg(regs, reg_addr);