Re: [PATCH] net: sched: fix possible OOB write in fl_set_geneve_opt()

From: Hangyu Hua
Date: Wed May 31 2023 - 01:39:04 EST

On 30/5/2023 19:36, Simon Horman wrote:
[Updated Pieter's email address, dropped old email address of mine]

On Mon, May 29, 2023 at 12:36:15PM +0800, Hangyu Hua wrote:
If we send two TCA_FLOWER_KEY_ENC_OPTS_GENEVE packets and their total
size is 252 bytes(key->enc_opts.len = 252) then
key->enc_opts.len = opt->length = data_len / 4 = 0 when the third
TCA_FLOWER_KEY_ENC_OPTS_GENEVE packet enters fl_set_geneve_opt. This
bypasses the next bounds check and results in an out-of-bounds.

Fixes: 0a6e77784f49 ("net/sched: allow flower to match tunnel options")
Signed-off-by: Hangyu Hua <hbh25y@xxxxxxxxx>

Hi Hangyu Hua,

Thanks. I think I see the problem too.
But I do wonder, is this more general than Geneve options?
That is, can this occur with any sequence of options, that
consume space in enc_opts (configured in fl_set_key()) that
in total are more than 256 bytes?

I think you are right. It is a good idea to add check in fl_set_vxlan_opt and fl_set_erspan_opt and fl_set_gtp_opt too.
But they should be submitted as other patches. fl_set_geneve_opt has already check this with the following code:

static int fl_set_geneve_opt(const struct nlattr *nla, struct fl_flow_key *key,
int depth, int option_len,
struct netlink_ext_ack *extack)
if (new_len > FLOW_DIS_TUN_OPTS_MAX) {
NL_SET_ERR_MSG(extack, "Tunnel options exceeds max size");
return -ERANGE;

This bug will only be triggered under this special condition(key->enc_opts.len = 252). So I think it will be better understood by submitting this patch independently.

By the way, I think memset's third param should be option_len in fl_set_vxlan_opt and fl_set_erspan_opt. Do I need to submit another patch to fix all these issues?


net/sched/cls_flower.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
index e960a46b0520..a326fbfe4339 100644
--- a/net/sched/cls_flower.c
+++ b/net/sched/cls_flower.c
@@ -1153,6 +1153,9 @@ static int fl_set_geneve_opt(const struct nlattr *nla, struct fl_flow_key *key,
if (option_len > sizeof(struct geneve_opt))
data_len = option_len - sizeof(struct geneve_opt);
+ if (key->enc_opts.len > FLOW_DIS_TUN_OPTS_MAX - 4)
+ return -ERANGE;
opt = (struct geneve_opt *)&key->[key->enc_opts.len];
memset(opt, 0xff, option_len);
opt->length = data_len / 4;