git: 0745d837c2e9 - main - unionfs: prevent upperrootvp from being recycled during mount

The branch main has been updated by jah:

URL: https://cgit.FreeBSD.org/src/commit/?id=0745d837c2e9e37bd069bd7c05c8f9892fba0752

commit 0745d837c2e9e37bd069bd7c05c8f9892fba0752
Author:     Jason A. Harmening <jah@FreeBSD.org>
AuthorDate: 2023-03-26 01:41:33 +0000
Commit:     Jason A. Harmening <jah@FreeBSD.org>
CommitDate: 2023-05-07 23:30:43 +0000

    unionfs: prevent upperrootvp from being recycled during mount
    
    If upperrootvp is doomed by a concurrent unmount, unionfs_nodeget()
    may return without a reference or lock on it.  unionfs_domount() must
    prevent the vnode from being recycled for use by a different file until
    it is finished with the vnode, namely once vfs_register_upper_from_vp()
    fails.  Accomplish this by holding the reference returned by namei()
    a bit longer.
    
    Reviewed by:    kib, markj
    Tested by:      pho
    Differential Revision:  https://reviews.freebsd.org/D39767
---
 sys/fs/unionfs/union_vfsops.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/sys/fs/unionfs/union_vfsops.c b/sys/fs/unionfs/union_vfsops.c
index 8831397c8c5b..cd6c7bd90655 100644
--- a/sys/fs/unionfs/union_vfsops.c
+++ b/sys/fs/unionfs/union_vfsops.c
@@ -285,8 +285,8 @@ unionfs_domount(struct mount *mp)
 	 */
 	error = unionfs_nodeget(mp, ump->um_uppervp, ump->um_lowervp,
 	    NULLVP, &(ump->um_rootvp), NULL);
-	vrele(upperrootvp);
 	if (error != 0) {
+		vrele(upperrootvp);
 		free(ump, M_UNIONFSMNT);
 		mp->mnt_data = NULL;
 		return (error);
@@ -295,11 +295,24 @@ unionfs_domount(struct mount *mp)
 	KASSERT((ump->um_rootvp->v_vflag & VV_ROOT) != 0,
 	    ("%s: rootvp without VV_ROOT", __func__));
 
+	/*
+	 * Do not release the namei() reference on upperrootvp until after
+	 * we attempt to register the upper mounts.  A concurrent unmount
+	 * of the upper or lower FS may have caused unionfs_nodeget() to
+	 * create a unionfs node with a NULL upper or lower vp and with
+	 * no reference held on upperrootvp or lowerrootvp.
+	 * vfs_register_upper() should subsequently fail, which is what
+	 * we want, but we must ensure neither underlying vnode can be
+	 * reused until that happens.  We assume the caller holds a reference
+	 * to lowerrootvp as it is the mount's covered vnode.
+	 */
 	lowermp = vfs_register_upper_from_vp(ump->um_lowervp, mp,
 	    &ump->um_lower_link);
 	uppermp = vfs_register_upper_from_vp(ump->um_uppervp, mp,
 	    &ump->um_upper_link);
 
+	vrele(upperrootvp);
+
 	if (lowermp == NULL || uppermp == NULL) {
 		if (lowermp != NULL)
 			vfs_unregister_upper(lowermp, &ump->um_lower_link);