Re: [PATCH bpf-next] bpf: Fix trampoline usage in preempt
From: "Paul E. McKenney" <paulmck@kernel.org>
Date: 2020-01-21 04:54:44
Also in:
bpf
On Mon, Jan 20, 2020 at 07:22:31PM -0800, Alexei Starovoitov wrote:
quoted hunk ↗ jump to hunk
Though the second half of trampoline page is unused a task could be preempted in the middle of the first half of trampoline and two updates to trampoline would change the code from underneath the preempted task. Hence wait for tasks to voluntarily schedule or go to userspace. Add similar wait before freeing the trampoline. Fixes: fec56f5890d9 ("bpf: Introduce BPF trampoline") Reported-by: Jann Horn <jannh@google.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> --- kernel/bpf/trampoline.c | 10 ++++++++++ 1 file changed, 10 insertions(+)diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c index 79a04417050d..7657ede7aee2 100644 --- a/kernel/bpf/trampoline.c +++ b/kernel/bpf/trampoline.c@@ -160,6 +160,14 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr) if (fexit_cnt) flags = BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_SKIP_FRAME; + /* Though the second half of trampoline page is unused a task could be + * preempted in the middle of the first half of trampoline and two + * updates to trampoline would change the code from underneath the + * preempted task. Hence wait for tasks to voluntarily schedule or go + * to userspace. + */ + synchronize_rcu_tasks();
So in this case, although the trampoline is not freed, it is reused. And we need to clear everyone off of the trampoline before reusing it. If this states the situation correctly: Acked-by: Paul E. McKenney <paulmck@kernel.org>
quoted hunk ↗ jump to hunk
err = arch_prepare_bpf_trampoline(new_image, new_image + PAGE_SIZE / 2, &tr->func.model, flags, fentry, fentry_cnt,@@ -251,6 +259,8 @@ void bpf_trampoline_put(struct bpf_trampoline *tr) goto out; if (WARN_ON_ONCE(!hlist_empty(&tr->progs_hlist[BPF_TRAMP_FEXIT]))) goto out; + /* wait for tasks to get out of trampoline before freeing it */ + synchronize_rcu_tasks(); bpf_jit_free_exec(tr->image); hlist_del(&tr->hlist); kfree(tr);-- 2.23.0