mnt: Protect the mountpoint hashtable with mount_lock

Protecting the mountpoint hashtable with namespace_sem was sufficient
until a call to umount_mnt was added to mntput_no_expire.  At which
point it became possible for multiple calls of put_mountpoint on
the same hash chain to happen on the same time.

Kristen Johansen <kjlx@templeofstupid.com> reported:
> This can cause a panic when simultaneous callers of put_mountpoint
> attempt to free the same mountpoint.  This occurs because some callers
> hold the mount_hash_lock, while others hold the namespace lock.  Some
> even hold both.
>
> In this submitter's case, the panic manifested itself as a GP fault in
> put_mountpoint() when it called hlist_del() and attempted to dereference
> a m_hash.pprev that had been poisioned by another thread.

Al Viro observed that the simple fix is to switch from using the namespace_sem
to the mount_lock to protect the mountpoint hash table.

I have taken Al's suggested patch moved put_mountpoint in pivot_root
(instead of taking mount_lock an additional time), and have replaced
new_mountpoint with get_mountpoint a function that does the hash table
lookup and addition under the mount_lock.   The introduction of get_mounptoint
ensures that only the mount_lock is needed to manipulate the mountpoint
hashtable.

d_set_mounted is modified to only set DCACHE_MOUNTED if it is not
already set.  This allows get_mountpoint to use the setting of
DCACHE_MOUNTED to ensure adding a struct mountpoint for a dentry
happens exactly once.

Cc: stable@vger.kernel.org
Fixes: ce07d891a089 ("mnt: Honor MNT_LOCKED when detaching mounts")
Reported-by: Krister Johansen <kjlx@templeofstupid.com>
Suggested-by: Al Viro <viro@ZenIV.linux.org.uk>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

path: root/flowtop/Makefile

flowtop-libs =	-lurcu \
		-lnetfilter_conntrack \
		$(shell PKG_CONFIG_PATH=$(PKG_CONFIG_PATH) $(PKG_CONFIG) --libs ncurses 2> /dev/null \
			|| echo '-lncurses') \
		$(shell PKG_CONFIG_PATH=$(PKG_CONFIG_PATH) $(PKG_CONFIG) --libs tinfo 2> /dev/null ) \
		-lpthread

ifeq ($(CONFIG_GEOIP), 1)
flowtop-libs +=	-lGeoIP \
		-lz
endif

flowtop-objs =	xmalloc.o \
		str.o \
		sig.o \
		sock.o \
		proc.o \
		dev.o \
		link.o \
		hash.o \
		lookup.o \
		screen.o \
		die.o \
		sysctl.o \
		flowtop.o

ifeq ($(CONFIG_GEOIP), 1)
flowtop-objs +=	geoip.o \
		ioops.o
endif

flowtop-eflags = $(shell $(PKG_CONFIG) --cflags ncurses) \
                 $(shell $(PKG_CONFIG) --cflags libnetfilter_conntrack)

flowtop-confs =	tcp.conf \
		udp.conf \
		geoip.conf