Fix sget() race with failing mount

If sget() finds a matching superblock being set up, it'll
grab an active reference to it and grab s_umount.  That's
fine - we'll wait for completion of foofs_get_sb() that way.
However, if said foofs_get_sb() fails we'll end up holding
the halfway-created superblock.  deactivate_locked_super()
called by foofs_get_sb() will just unlock the sucker since
we are holding another active reference to it.

What we need is a way to tell if superblock has been successfully
set up.  Unfortunately, neither ->s_root nor the check for
MS_ACTIVE quite fit.  Cheap and easy way, suitable for backport:
new flag set by the (only) caller of ->get_sb().  If that flag
isn't present by the time sget() grabbed s_umount on preexisting
superblock it has found, it's seeing a stillborn and should
just bury it with deactivate_locked_super() (and repeat the search).

Longer term we want to set that flag in ->get_sb() instances (and
check for it to distinguish between "sget() found us a live sb"
and "sget() has allocated an sb, we need to set it up" in there,
instead of checking ->s_root as we do now).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@kernel.org

fs/namespace.c

@@ -1984,7 +1984,7 @@ long do_mount(char *dev_name, char *dir_name, char *type_page,
 	if (flags & MS_RDONLY)
 		mnt_flags |= MNT_READONLY;
 
-	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
+	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
 		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
 		   MS_STRICTATIME);
 

fs/super.c

@@ -305,8 +305,13 @@ retry:
 			if (s) {
 				up_write(&s->s_umount);
 				destroy_super(s);
+				s = NULL;
 			}
 			down_write(&old->s_umount);
+			if (unlikely(!(old->s_flags & MS_BORN))) {
+				deactivate_locked_super(old);
+				goto retry;
+			}
 			return old;
 		}
 	}
@@ -918,6 +923,7 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
 		goto out_free_secdata;
 	BUG_ON(!mnt->mnt_sb);
 	WARN_ON(!mnt->mnt_sb->s_bdi);
+	mnt->mnt_sb->s_flags |= MS_BORN;
 
 	error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata);
 	if (error)

include/linux/fs.h

@@ -209,6 +209,7 @@ struct inodes_stat_t {
 #define MS_KERNMOUNT	(1<<22) /* this is a kern_mount call */
 #define MS_I_VERSION	(1<<23) /* Update inode I_version field */
 #define MS_STRICTATIME	(1<<24) /* Always perform atime updates */
+#define MS_BORN		(1<<29)
 #define MS_ACTIVE	(1<<30)
 #define MS_NOUSER	(1<<31)