linux-kernel - [PATCH] fix console change race exposed by CFS

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Message-Id: <1191084471.24185.11.camel@polaris>
Date:	Sat, 29 Sep 2007 18:47:51 +0200
From:	Jan Luebbe <jluebbe@...net.de>
To:	linux-kernel@...r.kernel.org
Cc:	torvalds@...ux-foundation.org
Subject: [PATCH] fix console change race exposed by CFS

From: Jan Lübbe <jluebbe@...net.de>

The new behaviour of CFS exposes a race which occurs if a switch is
requested when vt_mode.mode is VT_PROCESS.

The process with vc->vt_pid is signaled before vc->vt_newvt is set. This
causes the switch to fail when triggered by the monitoing process
because the target is still -1.

Signed-off-by: Jan Lübbe <jluebbe@...net.de>
---
Index: linux-2.6.22/drivers/char/vt_ioctl.c
===================================================================
--- linux-2.6.22.orig/drivers/char/vt_ioctl.c
+++ linux-2.6.22/drivers/char/vt_ioctl.c
@@ -1208,15 +1208,18 @@
 		/*
 		 * Send the signal as privileged - kill_pid() will
 		 * tell us if the process has gone or something else
-		 * is awry
+		 * is awry.
+		 *
+		 * We need to set vt_newvt *before* sending the signal or we
+		 * have a race.
 		 */
+		vc->vt_newvt = new_vc->vc_num;
 		if (kill_pid(vc->vt_pid, vc->vt_mode.relsig, 1) == 0) {
 			/*
 			 * It worked. Mark the vt to switch to and
 			 * return. The process needs to send us a
 			 * VT_RELDISP ioctl to complete the switch.
 			 */
-			vc->vt_newvt = new_vc->vc_num;
 			return;
 		}


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