vt: selection, handle pending signals in paste_selection

mainline inclusion
from mainline-v5.6-rc2
commit 687bff0cd08f790d540cfb7b2349f0d876cdddec
category: bugfix
bugzilla: 13690
CVE: CVE-2020-8648

-------------------------------------------------

When pasting a selection to a vt, the task is set as INTERRUPTIBLE while
waiting for a tty to unthrottle. But signals are not handled at all.
Normally, this is not a problem as tty_ldisc_receive_buf receives all
the goods and a user has no reason to interrupt the task.

There are two scenarios where this matters:
1) when the tty is throttled and a signal is sent to the process, it
   spins on a CPU until the tty is unthrottled. schedule() does not
   really echedule, but returns immediately, of course.
2) when the sel_buffer becomes invalid, KASAN prevents any reads from it
   and the loop simply does not proceed and spins forever (causing the
   tty to throttle, but the code never sleeps, the same as above). This
   sometimes happens as there is a race in the sel_buffer handling code.

So add signal handling to this ioctl (TIOCL_PASTESEL) and return -EINTR
in case a signal is pending.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200210081131.23572-1-jslaby@suse.czSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NHanjun Guo <guohanjun@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

drivers/tty/vt/selection.c

@@ -27,6 +27,8 @@
 #include <linux/console.h>
 #include <linux/tty_flip.h>
 
+#include <linux/sched/signal.h>
+
 /* Don't take this from <ctype.h>: 011-015 on the screen aren't spaces */
 #define isspace(c)	((c) == ' ')
 
@@ -337,6 +339,7 @@ int paste_selection(struct tty_struct *tty)
 	unsigned int count;
 	struct  tty_ldisc *ld;
 	DECLARE_WAITQUEUE(wait, current);
+	int ret = 0;
 
 	console_lock();
 	poke_blanked_console();
@@ -350,6 +353,10 @@ int paste_selection(struct tty_struct *tty)
 	add_wait_queue(&vc->paste_wait, &wait);
 	while (sel_buffer && sel_buffer_lth > pasted) {
 		set_current_state(TASK_INTERRUPTIBLE);
+		if (signal_pending(current)) {
+			ret = -EINTR;
+			break;
+		}
 		if (tty_throttled(tty)) {
 			schedule();
 			continue;
@@ -365,5 +372,5 @@ int paste_selection(struct tty_struct *tty)
 
 	tty_buffer_unlock_exclusive(&vc->port);
 	tty_ldisc_deref(ld);
-	return 0;
+	return ret;
 }