pdch: Fix heap-use-after-free in pdch->ulc

In existing previous code, pdch->ulc would be freed in
gprs_rlcmac_pdch::free_resources() when  it became disabled as per PCUIF
info_ind (for instance, when a DYN TS is switched PDCH->SDCCH8).
However, pdch->ulc was so far only allocated during pdch_init, which is
only called during bts_alloc() time.
Hence, after first info_ind disabling it, if it became again enabled
(again by info_ind re-enabling it after SDCCH8 was not longer in use),
the pdch->ulc would be used again but it would point to freed memory.

Let's rearrange how/when resources are freed to make it more logical.
With this patch, pdch internal resources are freed upon ->disable(), and
re-allocated upon ->enable().

Change-Id: Id51f5f6a54ac9f24b784c17bc360ac38f5726fc7

4 files changed, 22 insertions, 13 deletions

diff --git a/src/osmobts_sock.c b/src/osmobts_sock.c
index 5c6415fb..91b62a00 100644
--- a/src/osmobts_sock.c
+++ b/src/osmobts_sock.c
@@ -121,7 +121,8 @@ static void pcu_sock_close(int lost)
 			}
 #endif
 			for (ts = 0; ts < 8; ts++)
-				pdch_disable(&bts->trx[trx].pdch[ts]);
+				if (pdch_is_enabled(&bts->trx[trx].pdch[ts]))
+					pdch_disable(&bts->trx[trx].pdch[ts]);
 	/* FIXME: NOT ALL RESOURCES are freed in this case... inconsistent with the other code. Share the code with pcu_l1if.c
 	for the reset. */
 			bts_trx_free_all_tbf(&bts->trx[trx]);
diff --git a/src/pcu_l1_if.cpp b/src/pcu_l1_if.cpp
index 2373f60c..5aa88494 100644
--- a/src/pcu_l1_if.cpp
+++ b/src/pcu_l1_if.cpp
@@ -650,7 +650,7 @@ bssgp_failed:
 		for (trx_nr = 0; trx_nr < ARRAY_SIZE(bts->trx); trx_nr++) {
 			bts->trx[trx_nr].arfcn = info_ind->trx[trx_nr].arfcn;
 			for (ts_nr = 0; ts_nr < ARRAY_SIZE(bts->trx[0].pdch); ts_nr++)
-				bts->trx[trx_nr].pdch[ts_nr].free_resources();
+				bts->trx[trx_nr].pdch[ts_nr].disable();
 		}
 		gprs_bssgp_destroy(bts);
 		exit(0);
@@ -818,7 +818,6 @@ bssgp_failed:
 			} else {
 				if (pdch->is_enabled()) {
 					pcu_tx_act_req(bts, pdch, 0);
-					pdch->free_resources();
 					pdch->disable();
 				}
 			}
diff --git a/src/pdch.cpp b/src/pdch.cpp
index 9321384b..2ec40ce1 100644
--- a/src/pdch.cpp
+++ b/src/pdch.cpp
@@ -139,19 +139,24 @@ void pdch_init(struct gprs_rlcmac_pdch *pdch, struct gprs_rlcmac_trx *trx, uint8
 	/*  Initialize the PTCCH/D message (Packet Timing Advance Control Channel) */
 	memset(pdch->ptcch_msg, PTCCH_TAI_FREE, PTCCH_TAI_NUM);
 	memset(pdch->ptcch_msg + PTCCH_TAI_NUM, PTCCH_PADDING, 7);
-	pdch->ulc = pdch_ulc_alloc(pdch, trx->bts);
 }
 
 void gprs_rlcmac_pdch::enable()
 {
-	/* TODO: Check if there are still allocated resources.. */
+	OSMO_ASSERT(m_is_enabled == 0);
 	INIT_LLIST_HEAD(&paging_list);
+
+	OSMO_ASSERT(!this->ulc);
+	this->ulc = pdch_ulc_alloc(this, trx->bts);
+
 	m_is_enabled = 1;
 }
 
 void gprs_rlcmac_pdch::disable()
 {
-	/* TODO.. kick free_resources once we know the TRX/TS we are on */
+	OSMO_ASSERT(m_is_enabled == 1);
+	this->free_resources();
+
 	m_is_enabled = 0;
 }
 
@@ -159,10 +164,6 @@ void gprs_rlcmac_pdch::free_resources()
 {
 	struct gprs_rlcmac_paging *pag;
 
-	/* we are not enabled. there should be no resources */
-	if (!is_enabled())
-		return;
-
 	/* kick all TBF on slot */
 	pdch_free_all_tbf(this);
 
@@ -171,6 +172,7 @@ void gprs_rlcmac_pdch::free_resources()
 		talloc_free(pag);
 
 	talloc_free(this->ulc);
+	this->ulc = NULL;
 }
 
 struct gprs_rlcmac_paging *gprs_rlcmac_pdch::dequeue_paging()
@@ -1174,6 +1176,12 @@ void pdch_free_all_tbf(struct gprs_rlcmac_pdch *pdch)
 	}
 }
 
-void pdch_disable(struct gprs_rlcmac_pdch *pdch) {
+void pdch_disable(struct gprs_rlcmac_pdch *pdch)
+{
 	pdch->disable();
 }
+
+bool pdch_is_enabled(const struct gprs_rlcmac_pdch *pdch)
+{
+	return pdch->is_enabled();
+}
diff --git a/src/pdch.h b/src/pdch.h
index cfa0195f..00f0b9d6 100644
--- a/src/pdch.h
+++ b/src/pdch.h
@@ -55,8 +55,6 @@ struct gprs_rlcmac_pdch {
 
 	bool add_paging(uint8_t chan_needed, const struct osmo_mobile_identity *mi);
 
-	void free_resources();
-
 	bool is_enabled() const;
 
 	void enable();
@@ -145,6 +143,8 @@ private:
 		enum gprs_rlcmac_tbf_direction dir);
 	gprs_rlcmac_tbf *tbf_by_tfi(uint8_t tfi,
 		enum gprs_rlcmac_tbf_direction dir);
+
+	void free_resources();
 #endif
 
 	uint8_t m_num_tbfs[2];
@@ -191,6 +191,7 @@ extern "C" {
 void pdch_init(struct gprs_rlcmac_pdch *pdch, struct gprs_rlcmac_trx *trx, uint8_t ts_nr);
 void pdch_free_all_tbf(struct gprs_rlcmac_pdch *pdch);
 void pdch_disable(struct gprs_rlcmac_pdch *pdch);
+bool pdch_is_enabled(const struct gprs_rlcmac_pdch *pdch);
 #ifdef __cplusplus
 }
 #endif