[PATCH] futex: Initialize futex_phash_new during fork().

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

During a hash resize operation the new private hash is stored in
mm_struct::futex_phash_new if the current hash can not be immediately
replaced.

The new hash must not be copied during fork() into the new task. Doing
so will lead to a double-free of the memory by the two tasks.

Initialize the mm_struct::futex_phash_new during fork().

Reported-by: Calvin Owens <calvin@wbinvd.org>
Closes: https://lore.kernel.org/all/aFBQ8CBKmRzEqIfS@mozart.vkv.me/
Tested-by: Calvin Owens <calvin@wbinvd.org>
Fixes: bd54df5ea7cad ("futex: Allow to resize the private local hash")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/futex.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 005b040c4791b..b37193653e6b5 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -89,6 +89,7 @@ void futex_hash_free(struct mm_struct *mm);
 static inline void futex_mm_init(struct mm_struct *mm)
 {
 	RCU_INIT_POINTER(mm->futex_phash, NULL);
+	mm->futex_phash_new = NULL;
 	mutex_init(&mm->futex_hash_lock);
 }
 
-- 
2.50.0