Re: [syzbot] [kernel?] KASAN: slab-use-after-free Read in futex_unqueue

[Hillf Danton]

> Date: Tue, 10 Feb 2026 13:43:32 -0800	[thread overview]
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    72c395024dac Merge tag 'docs-7.0' of git://git.kernel.org/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=137d9b22580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=bf83be18dae53c93
> dashboard link: https://syzkaller.appspot.com/bug?extid=6c1861115b4253e45969
> compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=169f8ae6580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17f657fa580000

#syz test

--- x/kernel/futex/core.c
+++ y/kernel/futex/core.c
@@ -1735,8 +1735,7 @@ void futex_hash_free(struct mm_struct *m
 	free_percpu(mm->futex_ref);
 	kvfree(mm->futex_phash_new);
 	fph = rcu_dereference_raw(mm->futex_phash);
-	if (fph)
-		kvfree(fph);
+	kvfree_rcu(fph, rcu);
 }
 
 static bool futex_pivot_pending(struct mm_struct *mm)
--- x/kernel/fork.c
+++ y/kernel/fork.c
@@ -1192,6 +1192,7 @@ static inline void __mmput(struct mm_str
 void mmput(struct mm_struct *mm)
 {
 	might_sleep();
+	VM_BUG_ON(!atomic_read(&mm->mm_users));
 
 	if (atomic_dec_and_test(&mm->mm_users))
 		__mmput(mm);
--