[PATCH 5.5 313/367] bpf: Fix trampoline usage in preempt

[Greg Kroah-Hartman]

From: Alexei Starovoitov <ast@xxxxxxxxxx>

commit 05d57f1793fb250c85028c9952c3720010baa853 upstream.

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@xxxxxxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Link: https://lore.kernel.org/bpf/20200121032231.3292185-1-ast@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/bpf/trampoline.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -150,6 +150,14 @@ static int bpf_trampoline_update(struct
 	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, &tr->func.model, flags,
 					  fentry, fentry_cnt,
 					  fexit, fexit_cnt,
@@ -240,6 +248,8 @@ void bpf_trampoline_put(struct bpf_tramp
 		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);