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    Subject[PATCH AUTOSEL for 4.9 130/281] VFS: close race between getcwd() and d_move()
    Date
    From: NeilBrown <neilb@suse.com>

    [ Upstream commit 61647823aa920e395afcce4b57c32afb51456cab ]

    d_move() will call __d_drop() and then __d_rehash()
    on the dentry being moved. This creates a small window
    when the dentry appears to be unhashed. Many tests
    of d_unhashed() are made under ->d_lock and so are safe
    from racing with this window, but some aren't.
    In particular, getcwd() calls d_unlinked() (which calls
    d_unhashed()) without d_lock protection, so it can race.

    This races has been seen in practice with lustre, which uses d_move() as
    part of name lookup. See:
    https://jira.hpdd.intel.com/browse/LU-9735
    It could race with a regular rename(), and result in ENOENT instead
    of either the 'before' or 'after' name.

    The race can be demonstrated with a simple program which
    has two threads, one renaming a directory back and forth
    while another calls getcwd() within that directory: it should never
    fail, but does. See:
    https://patchwork.kernel.org/patch/9455345/

    We could fix this race by taking d_lock and rechecking when
    d_unhashed() reports true. Alternately when can remove the window,
    which is the approach this patch takes.

    ___d_drop() is introduce which does *not* clear d_hash.pprev
    so the dentry still appears to be hashed. __d_drop() calls
    ___d_drop(), then clears d_hash.pprev.
    __d_move() now uses ___d_drop() and only clears d_hash.pprev
    when not rehashing.

    Signed-off-by: NeilBrown <neilb@suse.com>
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    ---
    fs/dcache.c | 23 ++++++++++++++++-------
    1 file changed, 16 insertions(+), 7 deletions(-)

    diff --git a/fs/dcache.c b/fs/dcache.c
    index 67957f5b325c..c697589fddda 100644
    --- a/fs/dcache.c
    +++ b/fs/dcache.c
    @@ -461,9 +461,11 @@ static void dentry_lru_add(struct dentry *dentry)
    * d_drop() is used mainly for stuff that wants to invalidate a dentry for some
    * reason (NFS timeouts or autofs deletes).
    *
    - * __d_drop requires dentry->d_lock.
    + * __d_drop requires dentry->d_lock
    + * ___d_drop doesn't mark dentry as "unhashed"
    + * (dentry->d_hash.pprev will be LIST_POISON2, not NULL).
    */
    -void __d_drop(struct dentry *dentry)
    +static void ___d_drop(struct dentry *dentry)
    {
    if (!d_unhashed(dentry)) {
    struct hlist_bl_head *b;
    @@ -479,12 +481,17 @@ void __d_drop(struct dentry *dentry)

    hlist_bl_lock(b);
    __hlist_bl_del(&dentry->d_hash);
    - dentry->d_hash.pprev = NULL;
    hlist_bl_unlock(b);
    /* After this call, in-progress rcu-walk path lookup will fail. */
    write_seqcount_invalidate(&dentry->d_seq);
    }
    }
    +
    +void __d_drop(struct dentry *dentry)
    +{
    + ___d_drop(dentry);
    + dentry->d_hash.pprev = NULL;
    +}
    EXPORT_SYMBOL(__d_drop);

    void d_drop(struct dentry *dentry)
    @@ -2373,7 +2380,7 @@ EXPORT_SYMBOL(d_delete);
    static void __d_rehash(struct dentry *entry)
    {
    struct hlist_bl_head *b = d_hash(entry->d_name.hash);
    - BUG_ON(!d_unhashed(entry));
    +
    hlist_bl_lock(b);
    hlist_bl_add_head_rcu(&entry->d_hash, b);
    hlist_bl_unlock(b);
    @@ -2810,9 +2817,9 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
    write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);

    /* unhash both */
    - /* __d_drop does write_seqcount_barrier, but they're OK to nest. */
    - __d_drop(dentry);
    - __d_drop(target);
    + /* ___d_drop does write_seqcount_barrier, but they're OK to nest. */
    + ___d_drop(dentry);
    + ___d_drop(target);

    /* Switch the names.. */
    if (exchange)
    @@ -2824,6 +2831,8 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
    __d_rehash(dentry);
    if (exchange)
    __d_rehash(target);
    + else
    + target->d_hash.pprev = NULL;

    /* ... and switch them in the tree */
    if (IS_ROOT(dentry)) {
    --
    2.14.1
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