From: Andrea Arcangeli aarcange@redhat.com Subject: userfaultfd: use RCU to free the task struct when fork fails
The task structure is freed while get_mem_cgroup_from_mm() holds rcu_read_lock() and dereferences mm->owner.
get_mem_cgroup_from_mm() failing fork() ---- --- task = mm->owner mm->owner = NULL; free(task) if (task) *task; /* use after free */
The fix consists in freeing the task with RCU also in the fork failure case, exactly like it always happens for the regular exit(2) path. That is enough to make the rcu_read_lock hold in get_mem_cgroup_from_mm() (left side above) effective to avoid a use after free when dereferencing the task structure.
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.
[aarcange@redhat.com: improve changelog, reduce #ifdefs per Michal] Link: http://lkml.kernel.org/r/20190429035752.4508-1-aarcange@redhat.com Link: http://lkml.kernel.org/r/20190325225636.11635-2-aarcange@redhat.com Fixes: 893e26e61d04 ("userfaultfd: non-cooperative: Add fork() event") Signed-off-by: Andrea Arcangeli aarcange@redhat.com Tested-by: zhong jiang zhongjiang@huawei.com Reported-by: syzbot+cbb52e396df3e565ab02@syzkaller.appspotmail.com Cc: Oleg Nesterov oleg@redhat.com Cc: Jann Horn jannh@google.com Cc: Hugh Dickins hughd@google.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Peter Xu peterx@redhat.com Cc: Jason Gunthorpe jgg@mellanox.com Cc: "Kirill A . Shutemov" kirill.shutemov@linux.intel.com Cc: Michal Hocko mhocko@suse.com Cc: zhong jiang zhongjiang@huawei.com Cc: syzbot+cbb52e396df3e565ab02@syzkaller.appspotmail.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
kernel/fork.c | 31 +++++++++++++++++++++++++++++-- 1 file changed, 29 insertions(+), 2 deletions(-)
--- a/kernel/fork.c~userfaultfd-use-rcu-to-free-the-task-struct-when-fork-fails +++ a/kernel/fork.c @@ -955,6 +955,15 @@ 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 @@ -1343,6 +1352,7 @@ 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: @@ -1726,6 +1736,21 @@ static int pidfd_create(struct pid *pid) return fd; }
+static void __delayed_free_task(struct rcu_head *rhp) +{ + struct task_struct *tsk = container_of(rhp, struct task_struct, rcu); + + free_task(tsk); +} + +static __always_inline void delayed_free_task(struct task_struct *tsk) +{ + if (IS_ENABLED(CONFIG_MEMCG)) + call_rcu(&tsk->rcu, __delayed_free_task); + else + free_task(tsk); +} + /* * This creates a new process as a copy of the old one, * but does not actually start it yet. @@ -2233,8 +2258,10 @@ 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); @@ -2265,7 +2292,7 @@ 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); _