[PATCH AUTOSEL 5.12 05/43] bpf: Forbid trampoline attach for functions with variable arguments

[Sasha Levin]

From: Jiri Olsa <jolsa@xxxxxxxxxx>

[ Upstream commit 31379397dcc364a59ce764fabb131b645c43e340 ]

We can't currently allow to attach functions with variable arguments.
The problem is that we should save all the registers for arguments,
which is probably doable, but if caller uses more than 6 arguments,
we need stack data, which will be wrong, because of the extra stack
frame we do in bpf trampoline, so we could crash.

Also currently there's malformed trampoline code generated for such
functions at the moment as described in:

  https://lore.kernel.org/bpf/20210429212834.82621-1-jolsa@xxxxxxxxxx/

Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Link: https://lore.kernel.org/bpf/20210505132529.401047-1-jolsa@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 kernel/bpf/btf.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index b1a76fe046cb..6bd003568fa5 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5126,6 +5126,12 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
 	m->ret_size = ret;
 
 	for (i = 0; i < nargs; i++) {
+		if (i == nargs - 1 && args[i].type == 0) {
+			bpf_log(log,
+				"The function %s with variable args is unsupported.\n",
+				tname);
+			return -EINVAL;
+		}
 		ret = __get_type_size(btf, args[i].type, &t);
 		if (ret < 0) {
 			bpf_log(log,
@@ -5133,6 +5139,12 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
 				tname, i, btf_kind_str[BTF_INFO_KIND(t->info)]);
 			return -EINVAL;
 		}
+		if (ret == 0) {
+			bpf_log(log,
+				"The function %s has malformed void argument.\n",
+				tname);
+			return -EINVAL;
+		}
 		m->arg_size[i] = ret;
 	}
 	m->nr_args = nargs;
-- 
2.30.2