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fix d_walk()/non-delayed __d_free() race 

Ascend-to-parent logics in d_walk() depends on all encountered child
dentries not getting freed without an RCU delay.  Unfortunately, in
quite a few cases it is not true, with hard-to-hit oopsable race as
the result.

Fortunately, the fix is simiple; right now the rule is "if it ever
been hashed, freeing must be delayed" and changing it to "if it
ever had a parent, freeing must be delayed" closes that hole and
covers all cases the old rule used to cover.  Moreover, pipes and
sockets remain _not_ covered, so we do not introduce RCU delay in
the cases which are the reason for having that delay conditional
in the first place.

Cc: stable@vger.kernel.org # v3.2+ (and watch out for __d_materialise_dentry())
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2016-06-07 21:26:55 -04:00
parent e6ec03a25f
commit 3d56c25e3b
1 changed files with 2 additions and 2 deletions
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4
fs/dcache.c
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@ -1636,7 +1636,7 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
struct dentry *dentry = __d_alloc(parent->d_sb, name); struct dentry *dentry = __d_alloc(parent->d_sb, name);
if (!dentry) if (!dentry)
return NULL; return NULL;
dentry->d_flags |= DCACHE_RCUACCESS;
spin_lock(&parent->d_lock); spin_lock(&parent->d_lock);
/* /*
* don't need child lock because it is not subject * don't need child lock because it is not subject
@ -2358,7 +2358,6 @@ static void __d_rehash(struct dentry * entry, struct hlist_bl_head *b)
{ {
BUG_ON(!d_unhashed(entry)); BUG_ON(!d_unhashed(entry));
hlist_bl_lock(b); hlist_bl_lock(b);
entry->d_flags |= DCACHE_RCUACCESS;
hlist_bl_add_head_rcu(&entry->d_hash, b); hlist_bl_add_head_rcu(&entry->d_hash, b);
hlist_bl_unlock(b); hlist_bl_unlock(b);
} }
@ -2843,6 +2842,7 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
/* ... and switch them in the tree */ /* ... and switch them in the tree */
if (IS_ROOT(dentry)) { if (IS_ROOT(dentry)) {
/* splicing a tree */ /* splicing a tree */
dentry->d_flags |= DCACHE_RCUACCESS;
dentry->d_parent = target->d_parent; dentry->d_parent = target->d_parent;
target->d_parent = target; target->d_parent = target;
list_del_init(&target->d_child); list_del_init(&target->d_child);