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Date:      Wed, 15 Jun 2016 15:56:03 +0000 (UTC)
From:      Konstantin Belousov <kib@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r301929 - head/sys/kern
Message-ID:  <201606151556.u5FFu3pt032257@repo.freebsd.org>
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Author: kib
Date: Wed Jun 15 15:56:03 2016
New Revision: 301929
URL: https://svnweb.freebsd.org/changeset/base/301929

Log:
  Do not assume that we own the use reference on the covered vnode until
  we set MNTK_UNMOUNT flag on the mp.  Otherwise parallel unmount which
  wins race with us could dereference the covered vnode, and we are
  left with the locked freed memory.
  
  Reported and tested by:	pho
  Sponsored by:	The FreeBSD Foundation
  Approved by:	re (gjb)
  MFC after:	1 week

Modified:
  head/sys/kern/vfs_mount.c

Modified: head/sys/kern/vfs_mount.c
==============================================================================
--- head/sys/kern/vfs_mount.c	Wed Jun 15 15:55:14 2016	(r301928)
+++ head/sys/kern/vfs_mount.c	Wed Jun 15 15:56:03 2016	(r301929)
@@ -1220,7 +1220,6 @@ dounmount(struct mount *mp, int flags, s
 		VI_LOCK(coveredvp);
 		vholdl(coveredvp);
 		vn_lock(coveredvp, LK_EXCLUSIVE | LK_INTERLOCK | LK_RETRY);
-		vdrop(coveredvp);
 		/*
 		 * Check for mp being unmounted while waiting for the
 		 * covered vnode lock.
@@ -1228,18 +1227,22 @@ dounmount(struct mount *mp, int flags, s
 		if (coveredvp->v_mountedhere != mp ||
 		    coveredvp->v_mountedhere->mnt_gen != mnt_gen_r) {
 			VOP_UNLOCK(coveredvp, 0);
+			vdrop(coveredvp);
 			vfs_rel(mp);
 			return (EBUSY);
 		}
 	}
+
 	/*
 	 * Only privileged root, or (if MNT_USER is set) the user that did the
 	 * original mount is permitted to unmount this filesystem.
 	 */
 	error = vfs_suser(mp, td);
 	if (error != 0) {
-		if (coveredvp)
+		if (coveredvp != NULL) {
 			VOP_UNLOCK(coveredvp, 0);
+			vdrop(coveredvp);
+		}
 		vfs_rel(mp);
 		return (error);
 	}
@@ -1249,8 +1252,10 @@ dounmount(struct mount *mp, int flags, s
 	if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0 ||
 	    !TAILQ_EMPTY(&mp->mnt_uppers)) {
 		MNT_IUNLOCK(mp);
-		if (coveredvp)
+		if (coveredvp != NULL) {
 			VOP_UNLOCK(coveredvp, 0);
+			vdrop(coveredvp);
+		}
 		vn_finished_write(mp);
 		return (EBUSY);
 	}
@@ -1283,6 +1288,16 @@ dounmount(struct mount *mp, int flags, s
 	if (mp->mnt_flag & MNT_EXPUBLIC)
 		vfs_setpublicfs(NULL, NULL, NULL);
 
+	/*
+	 * From now, we can claim that the use reference on the
+	 * coveredvp is ours, and the ref can be released only by
+	 * successfull unmount by us, or left for later unmount
+	 * attempt.  The previously acquired hold reference is no
+	 * longer needed to protect the vnode from reuse.
+	 */
+	if (coveredvp != NULL)
+		vdrop(coveredvp);
+
 	vfs_msync(mp, MNT_WAIT);
 	MNT_ILOCK(mp);
 	async_flag = mp->mnt_flag & MNT_ASYNC;

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