riscv: Disallow PR_GET_TAGGED_ADDR_CTRL without Supm

When the prctl() interface for pointer masking was added, it did not
check that the pointer masking ISA extension was supported, only the
individual submodes. Userspace could still attempt to disable pointer
masking and query the pointer masking state. commit 81de1af
("riscv: Fix kernel crash due to PR_SET_TAGGED_ADDR_CTRL") disallowed
the former, as the senvcfg write could crash on older systems.
PR_GET_TAGGED_ADDR_CTRL state does not crash, because it reads only
kernel-internal state and not senvcfg, but it should still be disallowed
for consistency.

Fixes: 09d6775 ("riscv: Add support for userspace pointer masking")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/r/20250507145230.2272871-1-samuel.holland@sifive.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Author: SiFiveHolland

arch/riscv/kernel/process.c

@@ -333,6 +333,9 @@ long get_tagged_addr_ctrl(struct task_struct *task)
 	struct thread_info *ti = task_thread_info(task);
 	long ret = 0;
 
+	if (!riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM))
+		return -EINVAL;
+
 	if (is_compat_thread(ti))
 		return -EINVAL;