From 05d57f1793fb250c85028c9952c3720010baa853 Mon Sep 17 00:00:00 2001 From: Alexei Starovoitov Date: Mon, 20 Jan 2020 19:22:31 -0800 Subject: [PATCH] bpf: Fix trampoline usage in preempt 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 Signed-off-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann Acked-by: Paul E. McKenney Link: https://lore.kernel.org/bpf/20200121032231.3292185-1-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(); + 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);