Commit 7d01ef7585c0 for kernel

commit 7d01ef7585c07afaf487759a48486228cd065726
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Tue Apr 6 12:33:07 2021 -0400

    Make sure nd->path.mnt and nd->path.dentry are always valid pointers

    Initialize them in set_nameidata() and make sure that terminate_walk() clears them
    once the pointers become potentially invalid (i.e. we leave RCU mode or drop them
    in non-RCU one).  Currently we have "path_init() always initializes them and nobody
    accesses them outside of path_init()/terminate_walk() segments", which is asking
    for trouble.

    With that change we would have nd->path.{mnt,dentry}
            1) always valid - NULL or pointing to currently allocated objects.
            2) non-NULL while we are successfully walking
            3) NULL when we are not walking at all
            4) contributing to refcounts whenever non-NULL outside of RCU mode.

    Fixes: 6c6ec2b0a3e0 ("fs: add support for LOOKUP_CACHED")
    Reported-by: syzbot+c88a7030da47945a3cc3@syzkaller.appspotmail.com
    Tested-by: Christian Brauner <christian.brauner@ubuntu.com>
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 216f16e74351..fc8760d4314e 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -579,6 +579,8 @@ static void set_nameidata(struct nameidata *p, int dfd, struct filename *name)
 	p->stack = p->internal;
 	p->dfd = dfd;
 	p->name = name;
+	p->path.mnt = NULL;
+	p->path.dentry = NULL;
 	p->total_link_count = old ? old->total_link_count : 0;
 	p->saved = old;
 	current->nameidata = p;
@@ -652,6 +654,8 @@ static void terminate_walk(struct nameidata *nd)
 		rcu_read_unlock();
 	}
 	nd->depth = 0;
+	nd->path.mnt = NULL;
+	nd->path.dentry = NULL;
 }

 /* path_put is needed afterwards regardless of success or failure */
@@ -2322,8 +2326,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 	}

 	nd->root.mnt = NULL;
-	nd->path.mnt = NULL;
-	nd->path.dentry = NULL;

 	/* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */
 	if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) {