um: defer killing userspace on page table update failures

In some cases we can get to fix_range_common() with mmap_sem held,
and in others we get there without it being held. For example, we
get there with it held from sys_mprotect(), and without it held
from fork_handler().

Avoid any issues in this and simply defer killing the task until
it runs the next time. Do it on the mm so that another task that
shares the same mm can't continue running afterwards.

Cc: [e-mail]
Fixes: 468f65976a8d ("um: Fix hung task in fix_range_common()")
Signed-off-by: Johannes Berg <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/include/shared/skas/mm_id.h

···
 		int pid;
 	} u;
 	unsigned long stack;
+	int kill;
 };
 
 #endif

arch/um/kernel/tlb.c

···
 
 	/* This is not an else because ret is modified above */
 	if (ret) {
+		struct mm_id *mm_idp = &current->mm->context.id;
+
 		printk(KERN_ERR "fix_range_common: failed, killing current "
 		       "process: %d\n", task_tgid_vnr(current));
-		/* We are under mmap_lock, release it such that current can terminate */
-		mmap_write_unlock(current->mm);
-		force_sig(SIGKILL);
-		do_signal(&current->thread.regs);
+		mm_idp->kill = 1;
 	}
 }
 

arch/um/os-Linux/skas/process.c

···
 }
 
 int userspace_pid[NR_CPUS];
+int kill_userspace_mm[NR_CPUS];
 
 /**
  * start_userspace() - prepare a new userspace process
···
 	interrupt_end();
 
 	while (1) {
+		if (kill_userspace_mm[0])
+			fatal_sigsegv();
 
 		/*
 		 * This can legitimately fail if the process loads a
···
 void __switch_mm(struct mm_id *mm_idp)
 {
 	userspace_pid[0] = mm_idp->u.pid;
+	kill_userspace_mm[0] = mm_idp->kill;
 }