x86/mm/iommu/sva: Do not allow to set FORCE_TAGGED_SVA bit from outside

arch_prctl(ARCH_FORCE_TAGGED_SVA) overrides the default and allows LAM
and SVA to co-exist in the process. It is expected by called by the
process when it knows what it is doing.

arch_prctl() operates on the current process, but the same code is
reachable from ptrace where it can be called on arbitrary task.

Make it strict and only allow to set MM_CONTEXT_FORCE_TAGGED_SVA for the
current process.

Fixes: 23e5d9ec ("x86/mm/iommu/sva: Make LAM and SVA mutually exclusive")
Suggested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/all/20230403111020.3136-3-kirill.shutemov%40linux.intel.com

arch/x86/kernel/process_64.c

@@ -883,6 +883,8 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 	case ARCH_ENABLE_TAGGED_ADDR:
 		return prctl_enable_tagged_addr(task->mm, arg2);
 	case ARCH_FORCE_TAGGED_SVA:
+		if (current != task)
+			return -EINVAL;
 		set_bit(MM_CONTEXT_FORCE_TAGGED_SVA, &task->mm->context.flags);
 		return 0;
 	case ARCH_GET_MAX_TAG_BITS: