[PATCH bpf-next] bpf, seccomp: fix false positive preemption splat for cbpf->ebpf progs

[Daniel Borkmann]

In 568f196756ad ("bpf: check that BPF programs run with preemption disabled")
a check was added for BPF_PROG_RUN() that for every invocation preemption is
disabled to not break eBPF assumptions (e.g. per-cpu map). Of course this does
not count for seccomp because only cBPF -> eBPF is loaded here and it does not
make use of any functionality that would require this assertion. Fix this false
positive by adding and using __BPF_PROG_RUN() variant that does not have the
cant_sleep(); check.

Fixes: 568f196756ad ("bpf: check that BPF programs run with preemption disabled")
Reported-by: syzbot+8bf19ee2aa580de7a2a7@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
---
 include/linux/filter.h | 9 ++++++++-
 kernel/seccomp.c       | 2 +-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index f32b3ec..2f3e29a 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -533,7 +533,14 @@ struct sk_filter {
 	struct bpf_prog	*prog;
 };

-#define BPF_PROG_RUN(filter, ctx)  ({ cant_sleep(); (*(filter)->bpf_func)(ctx, (filter)->insnsi); })
+#define bpf_prog_run__non_preempt(prog, ctx)	\
+	({ cant_sleep(); __BPF_PROG_RUN(prog, ctx); })
+/* Native eBPF or cBPF -> eBPF transitions. Preemption must be disabled. */
+#define BPF_PROG_RUN(prog, ctx)			\
+	bpf_prog_run__non_preempt(prog, ctx)
+/* cBPF -> eBPF only, but not for native eBPF. */
+#define __BPF_PROG_RUN(prog, ctx)		\
+	(*(prog)->bpf_func)(ctx, (prog)->insnsi)

 #define BPF_SKB_CB_LEN QDISC_CB_PRIV_LEN

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index e815781..826d4e4 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -268,7 +268,7 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
 	 * value always takes priority (ignoring the DATA).
 	 */
 	for (; f; f = f->prev) {
-		u32 cur_ret = BPF_PROG_RUN(f->prog, sd);
+		u32 cur_ret = __BPF_PROG_RUN(f->prog, sd);

 		if (ACTION_ONLY(cur_ret) < ACTION_ONLY(ret)) {
 			ret = cur_ret;
-- 
2.9.5