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/tools/include/asm-generic/bitops/atomic.h

#ifndef _TOOLS_LINUX_ASM_GENERIC_BITOPS_ATOMIC_H_
#define _TOOLS_LINUX_ASM_GENERIC_BITOPS_ATOMIC_H_

#include <asm/types.h>
#include <asm/bitsperlong.h>

static inline void set_bit(int nr, unsigned long *addr)
{
	addr[nr / __BITS_PER_LONG] |= 1UL << (nr % __BITS_PER_LONG);
}

static inline void clear_bit(int nr, unsigned long *addr)
{
	addr[nr / __BITS_PER_LONG] &= ~(1UL << (nr % __BITS_PER_LONG));
}

static __always_inline int test_bit(unsigned int nr, const unsigned long *addr)
{
	return ((1UL << (nr % __BITS_PER_LONG)) &
		(((unsigned long *)addr)[nr / __BITS_PER_LONG])) != 0;
}

#endif /* _TOOLS_LINUX_ASM_GENERIC_BITOPS_ATOMIC_H_ */