neels has uploaded this change for review.
fix inter-BSC-in handover encryption
In the field we saw Handover Requests without any Chosen Encryption
Algorithm IE, and osmo-bsc completely failed on those. This made me
understand my mistake from when I wrote this handover code.
So far, from a BSSMAP Handover Request, we (I) used only the Chosen
Encryption Algorithm IE to pick the encryption to use on the target
lchan. That is very wrong.
Instead, figure out the intersection of permitted algorithms MSC & BSC,
and pick the best of those. Which means, actually, completely ignore the
Chosen Encryption Algorithm IE.
In the message, the permitted algorithms are passed as a bitmask. The
current code using gsm0808_dec_encrypt_info() passes this on as an
array. In order to select_best_cipher(), I could convert that array back
to a bitmask. Instead pass the bitmask on from message decoding
alongside the struct gsm0808_encrypt_info in req->ei_as_bitmask.
Related: SYS#5539
Change-Id: Iffedc981b60d309ed2e5decd5efedee07a757b53
---
M include/osmocom/bsc/gsm_data.h
M src/osmo-bsc/handover_fsm.c
M src/osmo-bsc/osmo_bsc_bssap.c
3 files changed, 25 insertions(+), 7 deletions(-)
git pull ssh://gerrit.osmocom.org:29418/osmo-bsc refs/changes/49/27249/1
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 50c7e24..3391e9e 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -270,6 +270,8 @@
struct gsm0808_channel_type ct;
struct gsm0808_speech_codec_list scl;
struct gsm0808_encrypt_info ei;
+ /* The same information as in 'ei' but as the handy bitmask as on the wire. */
+ uint8_t ei_as_bitmask;
bool kc128_present;
uint8_t kc128[16];
struct gsm_classmark classmark;
@@ -1457,4 +1459,6 @@
enum rsl_cmod_spd chan_mode_to_rsl_cmod_spd(enum gsm48_chan_mode chan_mode);
+int select_best_cipher(uint8_t msc_mask, uint8_t bsc_mask);
+
#endif /* _GSM_DATA_H */
diff --git a/src/osmo-bsc/handover_fsm.c b/src/osmo-bsc/handover_fsm.c
index 37e7417..5cd2f8d 100644
--- a/src/osmo-bsc/handover_fsm.c
+++ b/src/osmo-bsc/handover_fsm.c
@@ -490,6 +490,7 @@
LOG_HO(conn, LOGL_ERROR, "Failed to parse Encryption Information IE\n");
return false;
}
+ req->ei_as_bitmask = *e->val;
if ((e = TLVP_GET(tp, GSM0808_IE_KC_128))) {
if (e->len != 16) {
@@ -630,6 +631,7 @@
int match_idx;
struct osmo_fsm_inst *fi;
struct channel_mode_and_rate ch_mode_rate = {};
+ int chosen_a5_n;
handover_fsm_alloc(conn);
@@ -717,16 +719,28 @@
.msc_assigned_cic = req->msc_assigned_cic,
};
- if (req->chosen_encr_alg) {
- info.encr.alg_id = req->chosen_encr_alg;
- if (info.encr.alg_id > 1 && !req->ei.key_len) {
- ho_fail(HO_RESULT_ERROR, "Chosen Encryption Algorithm (Serving) reflects A5/%u"
- " but there is no key (Encryption Information)", info.encr.alg_id - 1);
+ /* Figure out the encryption algorithm */
+ chosen_a5_n = select_best_cipher(req->ei_as_bitmask, bsc_gsmnet->a5_encryption_mask);
+ if (chosen_a5_n < 0) {
+ ho_fail(HO_RESULT_ERROR,
+ "There is no A5 encryption mode that both BSC and MSC permit: MSC 0x%x & BSC 0x%x = 0\n",
+ req->ei_as_bitmask, bsc_gsmnet->a5_encryption_mask);
+ return;
+ }
+ if (chosen_a5_n > 0 && !req->ei.key_len) {
+ /* There is no key. Is A5/0 permitted? */
+ if ((req->ei_as_bitmask & bsc_gsmnet->a5_encryption_mask & 0x1) == 0x1) {
+ chosen_a5_n = 0;
+ } else {
+ ho_fail(HO_RESULT_ERROR,
+ "Encryption is required, but there is no key (Encryption Information)");
return;
}
}
- if (req->ei.key_len) {
+ /* Put encryption info in the chan activation info */
+ info.encr.alg_id = ALG_A5_NR_TO_RSL(chosen_a5_n);
+ if (chosen_a5_n > 0) {
if (req->ei.key_len > sizeof(info.encr.key)) {
ho_fail(HO_RESULT_ERROR, "Encryption Information IE key length is too large: %u\n",
req->ei.key_len);
diff --git a/src/osmo-bsc/osmo_bsc_bssap.c b/src/osmo-bsc/osmo_bsc_bssap.c
index b18627c..8acf293 100644
--- a/src/osmo-bsc/osmo_bsc_bssap.c
+++ b/src/osmo-bsc/osmo_bsc_bssap.c
@@ -394,7 +394,7 @@
}
/* select the best cipher permitted by the intersection of both masks */
-static int select_best_cipher(uint8_t msc_mask, uint8_t bsc_mask)
+int select_best_cipher(uint8_t msc_mask, uint8_t bsc_mask)
{
/* A5/7 ... A5/3: We assume higher is better,
* but: A5/1 is better than A5/2, which is better than A5/0 */
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