This patch is presently stuck. AFAICT we just need a changelog update to reflect Michal's observations? From: Andrea Arcangeli <aarcange@xxxxxxxxxx> Subject: userfaultfd: use RCU to free the task struct when fork fails MEMCG depends on the task structure not to be freed under rcu_read_lock() in get_mem_cgroup_from_mm() after it dereferences mm->owner. An alternate possible fix would be to defer the delivery of the userfaultfd contexts to the monitor until after fork() is guaranteed to succeed. Such a change would require more changes because it would create a strict ordering dependency where the uffd methods would need to be called beyond the last potentially failing branch in order to be safe. This solution as opposed only adds the dependency to common code to set mm->owner to NULL and to free the task struct that was pointed by mm->owner with RCU, if fork ends up failing. The userfaultfd methods can still be called anywhere during the fork runtime and the monitor will keep discarding orphaned "mm" coming from failed forks in userland. This race condition couldn't trigger if CONFIG_MEMCG was set =n at build time. Link: http://lkml.kernel.org/r/20190325225636.11635-2-aarcange@xxxxxxxxxx Fixes: 893e26e61d04 ("userfaultfd: non-cooperative: Add fork() event") Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> Tested-by: zhong jiang <zhongjiang@xxxxxxxxxx> Reported-by: syzbot+cbb52e396df3e565ab02@xxxxxxxxxxxxxxxxxxxxxxxxx Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Jann Horn <jannh@xxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: Peter Xu <peterx@xxxxxxxxxx> Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx> Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Cc: syzbot+cbb52e396df3e565ab02@xxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- kernel/fork.c | 34 ++++++++++++++++++++++++++++++++-- 1 file changed, 32 insertions(+), 2 deletions(-) --- a/kernel/fork.c~userfaultfd-use-rcu-to-free-the-task-struct-when-fork-fails +++ a/kernel/fork.c @@ -952,6 +952,15 @@ static void mm_init_aio(struct mm_struct #endif } +static __always_inline void mm_clear_owner(struct mm_struct *mm, + struct task_struct *p) +{ +#ifdef CONFIG_MEMCG + if (mm->owner == p) + WRITE_ONCE(mm->owner, NULL); +#endif +} + static void mm_init_owner(struct mm_struct *mm, struct task_struct *p) { #ifdef CONFIG_MEMCG @@ -1331,6 +1340,7 @@ static struct mm_struct *dup_mm(struct t free_pt: /* don't put binfmt in mmput, we haven't got module yet */ mm->binfmt = NULL; + mm_init_owner(mm, NULL); mmput(mm); fail_nomem: @@ -1662,6 +1672,24 @@ static inline void rcu_copy_process(stru #endif /* #ifdef CONFIG_TASKS_RCU */ } +#ifdef CONFIG_MEMCG +static void __delayed_free_task(struct rcu_head *rhp) +{ + struct task_struct *tsk = container_of(rhp, struct task_struct, rcu); + + free_task(tsk); +} +#endif /* CONFIG_MEMCG */ + +static __always_inline void delayed_free_task(struct task_struct *tsk) +{ +#ifdef CONFIG_MEMCG + call_rcu(&tsk->rcu, __delayed_free_task); +#else /* CONFIG_MEMCG */ + free_task(tsk); +#endif /* CONFIG_MEMCG */ +} + /* * This creates a new process as a copy of the old one, * but does not actually start it yet. @@ -2123,8 +2151,10 @@ bad_fork_cleanup_io: bad_fork_cleanup_namespaces: exit_task_namespaces(p); bad_fork_cleanup_mm: - if (p->mm) + if (p->mm) { + mm_clear_owner(p->mm, p); mmput(p->mm); + } bad_fork_cleanup_signal: if (!(clone_flags & CLONE_THREAD)) free_signal_struct(p->signal); @@ -2155,7 +2185,7 @@ bad_fork_cleanup_count: bad_fork_free: p->state = TASK_DEAD; put_task_stack(p); - free_task(p); + delayed_free_task(p); fork_out: spin_lock_irq(¤t->sighand->siglock); hlist_del_init(&delayed.node); _
