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lock_parent() needs to recheck if dentry got __dentry_kill'ed under it

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In case when dentry passed to lock_parent() is protected from freeing only
by the fact that it's on a shrink list and trylock of parent fails, we
could get hit by __dentry_kill() (and subsequent dentry_kill(parent))
between unlocking dentry and locking presumed parent.  We need to recheck
that dentry is alive once we lock both it and parent *and* postpone
rcu_read_unlock() until after that point.  Otherwise we could return
a pointer to struct dentry that already is rcu-scheduled for freeing, with
->d_lock held on it; caller's subsequent attempt to unlock it can end
up with memory corruption.

Cc: stable@vger.kernel.org # 3.12+, counting backports
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2018-02-23 20:47:17 -05:00
parent 91ab883eb2
commit 3b82140963
1 changed files with 8 additions and 3 deletions
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11
fs/dcache.c
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@ -647,11 +647,16 @@ again:
spin_unlock(&parent->d_lock);
goto again;
}
rcu_read_unlock();
if (parent != dentry)
if (parent != dentry) {
spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
else
if (unlikely(dentry->d_lockref.count < 0)) {
spin_unlock(&parent->d_lock);
parent = NULL;
}
} else {
parent = NULL;
}
rcu_read_unlock();
return parent;
}