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[Patch resend] nsfs: mark dentry with DCACHE_RCUACCESS

Andrey reported a use-after-free in __ns_get_path():

 spin_lock include/linux/spinlock.h:299 [inline]
 lockref_get_not_dead+0x19/0x80 lib/lockref.c:179
 __ns_get_path+0x197/0x860 fs/nsfs.c:66
 open_related_ns+0xda/0x200 fs/nsfs.c:143
 sock_ioctl+0x39d/0x440 net/socket.c:1001
 vfs_ioctl fs/ioctl.c:45 [inline]
 do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:685
 SYSC_ioctl fs/ioctl.c:700 [inline]
 SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691

We are under rcu read lock protection at that point:

        d = atomic_long_read(&ns->stashed);
        if (!d)
                goto slow;
        dentry = (struct dentry *)d;
        if (!lockref_get_not_dead(&dentry->d_lockref))
                goto slow;

but don't use a proper RCU API on the free path, therefore
a parallel __d_free() could free it at the same time.
We need to mark the stashed dentry with DCACHE_RCUACCESS so
that __d_free() will be called after all readers leave RCU.

Fixes: e149ed2b805f ("take the targets of /proc/*/ns/* symlinks to separate fs")
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
 fs/nsfs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 1656843..323f492 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -91,6 +91,7 @@ static void *__ns_get_path(struct path *path, struct ns_common *ns)
 		return ERR_PTR(-ENOMEM);
 	d_instantiate(dentry, inode);
+	dentry->d_flags |= DCACHE_RCUACCESS;
 	dentry->d_fsdata = (void *)ns->ops;
 	d = atomic_long_cmpxchg(&ns->stashed, 0, (unsigned long)dentry);
 	if (d) {