/dev/mem: Bail out upon SIGKILL.

syzbot found that a thread can stall for minutes inside read_mem() or
write_mem() after that thread was killed by SIGKILL [1]. Reading from
iomem areas of /dev/mem can be slow, depending on the hardware.
While reading 2GB at one read() is legal, delaying termination of killed
thread for minutes is bad. Thus, allow reading/writing /dev/mem and
/dev/kmem to be preemptible and killable.

[ 1335.912419][T20577] read_mem: sz=4096 count=2134565632
[ 1335.943194][T20577] read_mem: sz=4096 count=2134561536
[ 1335.978280][T20577] read_mem: sz=4096 count=2134557440
[ 1336.011147][T20577] read_mem: sz=4096 count=2134553344
[ 1336.041897][T20577] read_mem: sz=4096 count=2134549248

Theoretically, reading/writing /dev/mem and /dev/kmem can become
"interruptible". But this patch chose "killable". Future patch will make
them "interruptible" so that we can revert to "killable" if some program
regressed.

[1] https://syzkaller.appspot.com/bug?id=a0e3436829698d5824231251fad9d8e998f94f5e

Signed-off-by: Tetsuo Handa <[e-mail]>
Cc: stable <[e-mail]>
Reported-by: syzbot <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/char/mem.c

···
 }
 #endif
 
+static inline bool should_stop_iteration(void)
+{
+	if (need_resched())
+		cond_resched();
+	return fatal_signal_pending(current);
+}
+
 /*
  * This funcion reads the *physical* memory. The f_pos points directly to the
  * memory location.
···
 		p += sz;
 		count -= sz;
 		read += sz;
+		if (should_stop_iteration())
+			break;
 	}
 	kfree(bounce);
 
···
 		p += sz;
 		count -= sz;
 		written += sz;
+		if (should_stop_iteration())
+			break;
 	}
 
 	*ppos += written;
···
 			read += sz;
 			low_count -= sz;
 			count -= sz;
+			if (should_stop_iteration()) {
+				count = 0;
+				break;
+			}
 		}
 	}
 
···
 			buf += sz;
 			read += sz;
 			p += sz;
+			if (should_stop_iteration())
+				break;
 		}
 		free_page((unsigned long)kbuf);
 	}
···
 		p += sz;
 		count -= sz;
 		written += sz;
+		if (should_stop_iteration())
+			break;
 	}
 
 	*ppos += written;
···
 			buf += sz;
 			virtr += sz;
 			p += sz;
+			if (should_stop_iteration())
+				break;
 		}
 		free_page((unsigned long)kbuf);
 	}