Merge branch 'net-mac80211-kernel-enable-kcov-remote-coverage-collection-for-802-11-frame-handling'

Aleksandr Nogikh says:

====================
net, mac80211, kernel: enable KCOV remote coverage collection for 802.11 frame handling

This patch series enables remote KCOV coverage collection during
802.11 frames processing. These changes make it possible to perform
coverage-guided fuzzing in search of remotely triggerable bugs.

Normally, KCOV collects coverage information for the code that is
executed inside the system call context. It is easy to identify where
that coverage should go and whether it should be collected at all by
looking at the current process. If KCOV was enabled on that process,
coverage will be stored in a buffer specific to that process.
Howerever, it is not always enough as handling can happen elsewhere
(e.g. in separate kernel threads).

When it is impossible to infer KCOV-related info just by looking at
the currently running process, one needs to manually pass some
information to the code that should be instrumented. The information
takes the form of 64 bit integers (KCOV remote handles). Zero is the
special value that corresponds to an empty handle. More details on
KCOV and remote coverage collection can be found in
Documentation/dev-tools/kcov.rst.

The series consists of three commits.
1. Apply a minor fix to kcov_common_handle() so that it returns a
valid handle (zero) when called in an interrupt context.
2. Take the remote handle from KCOV and attach it to newly allocated
SKBs as an skb extension. If the allocation happens inside a system
call context, the SKB will be tied to the process that issued the
syscall (if that process is interested in remote coverage collection).
3. Annotate the code that processes incoming 802.11 frames with
kcov_remote_start()/kcov_remote_stop().

v5:
* Collecting remote coverate at ieee80211_rx_list() instead of
  ieee80211_rx()

v4:
https://lkml.kernel.org/r/20201028182018.1780842-1-aleksandrnogikh@gmail.com
* CONFIG_SKB_EXTENSIONS is now automatically selected by CONFIG_KCOV.
* Elaborated on a minor optimization in skb_set_kcov_handle().

v3:
https://lkml.kernel.org/r/20201026150851.528148-1-aleksandrnogikh@gmail.com
* kcov_handle is now stored in skb extensions instead of sk_buff
  itself.
* Updated the cover letter.

v2:
https://lkml.kernel.org/r/20201009170202.103512-1-a.nogikh@gmail.com
* Moved KCOV annotations from ieee80211_tasklet_handler to
  ieee80211_rx.
* Updated kcov_common_handle() to return 0 if it is called in
  interrupt context.
* Updated the cover letter.

v1:
https://lkml.kernel.org/r/20201007101726.3149375-1-a.nogikh@gmail.com
====================

Link: https://lore.kernel.org/r/20201029173620.2121359-1-aleksandrnogikh@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -4150,6 +4150,9 @@ enum skb_ext_id {
 #endif
 #if IS_ENABLED(CONFIG_MPTCP)
 	SKB_EXT_MPTCP,
+#endif
+#if IS_ENABLED(CONFIG_KCOV)
+	SKB_EXT_KCOV_HANDLE,
 #endif
 	SKB_EXT_NUM, /* must be last */
 };
@@ -4605,5 +4608,35 @@ static inline void skb_reset_redirect(struct sk_buff *skb)
 #endif
 }
 
+#ifdef CONFIG_KCOV
+static inline void skb_set_kcov_handle(struct sk_buff *skb,
+				       const u64 kcov_handle)
+{
+	/* Do not allocate skb extensions only to set kcov_handle to zero
+	 * (as it is zero by default). However, if the extensions are
+	 * already allocated, update kcov_handle anyway since
+	 * skb_set_kcov_handle can be called to zero a previously set
+	 * value.
+	 */
+	if (skb_has_extensions(skb) || kcov_handle) {
+		u64 *kcov_handle_ptr = skb_ext_add(skb, SKB_EXT_KCOV_HANDLE);
+
+		if (kcov_handle_ptr)
+			*kcov_handle_ptr = kcov_handle;
+	}
+}
+
+static inline u64 skb_get_kcov_handle(struct sk_buff *skb)
+{
+	u64 *kcov_handle = skb_ext_find(skb, SKB_EXT_KCOV_HANDLE);
+
+	return kcov_handle ? *kcov_handle : 0;
+}
+#else
+static inline void skb_set_kcov_handle(struct sk_buff *skb,
+				       const u64 kcov_handle) { }
+static inline u64 skb_get_kcov_handle(struct sk_buff *skb) { return 0; }
+#endif /* CONFIG_KCOV */
+
 #endif	/* __KERNEL__ */
 #endif	/* _LINUX_SKBUFF_H */

kernel/kcov.c

@@ -1023,6 +1023,8 @@ EXPORT_SYMBOL(kcov_remote_stop);
 /* See the comment before kcov_remote_start() for usage details. */
 u64 kcov_common_handle(void)
 {
+	if (!in_task())
+		return 0;
 	return current->kcov_handle;
 }
 EXPORT_SYMBOL(kcov_common_handle);

lib/Kconfig.debug

@@ -1870,6 +1870,7 @@ config KCOV
 	depends on CC_HAS_SANCOV_TRACE_PC || GCC_PLUGINS
 	select DEBUG_FS
 	select GCC_PLUGIN_SANCOV if !CC_HAS_SANCOV_TRACE_PC
+	select SKB_EXTENSIONS
 	help
 	  KCOV exposes kernel code coverage information in a form suitable
 	  for coverage-guided fuzzing (randomized testing).

net/core/skbuff.c

@@ -249,6 +249,9 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 
 		fclones->skb2.fclone = SKB_FCLONE_CLONE;
 	}
+
+	skb_set_kcov_handle(skb, kcov_common_handle());
+
 out:
 	return skb;
 nodata:
@@ -282,6 +285,8 @@ static struct sk_buff *__build_skb_around(struct sk_buff *skb,
 	memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
 	atomic_set(&shinfo->dataref, 1);
 
+	skb_set_kcov_handle(skb, kcov_common_handle());
+
 	return skb;
 }
 
@@ -4203,6 +4208,9 @@ static const u8 skb_ext_type_len[] = {
 #if IS_ENABLED(CONFIG_MPTCP)
 	[SKB_EXT_MPTCP] = SKB_EXT_CHUNKSIZEOF(struct mptcp_ext),
 #endif
+#if IS_ENABLED(CONFIG_KCOV)
+	[SKB_EXT_KCOV_HANDLE] = SKB_EXT_CHUNKSIZEOF(u64),
+#endif
 };
 
 static __always_inline unsigned int skb_ext_total_length(void)
@@ -4219,6 +4227,9 @@ static __always_inline unsigned int skb_ext_total_length(void)
 #endif
 #if IS_ENABLED(CONFIG_MPTCP)
 		skb_ext_type_len[SKB_EXT_MPTCP] +
+#endif
+#if IS_ENABLED(CONFIG_KCOV)
+		skb_ext_type_len[SKB_EXT_KCOV_HANDLE] +
 #endif
 		0;
 }

net/mac80211/iface.c

@@ -1356,6 +1356,7 @@ static void ieee80211_iface_work(struct work_struct *work)
 	while ((skb = skb_dequeue(&sdata->skb_queue))) {
 		struct ieee80211_mgmt *mgmt = (void *)skb->data;
 
+		kcov_remote_start_common(skb_get_kcov_handle(skb));
 		if (ieee80211_is_action(mgmt->frame_control) &&
 		    mgmt->u.action.category == WLAN_CATEGORY_BACK) {
 			int len = skb->len;
@@ -1465,6 +1466,7 @@ static void ieee80211_iface_work(struct work_struct *work)
 		}
 
 		kfree_skb(skb);
+		kcov_remote_stop();
 	}
 
 	/* then other type-dependent work */

net/mac80211/rx.c

@@ -4742,6 +4742,8 @@ void ieee80211_rx_list(struct ieee80211_hw *hw, struct ieee80211_sta *pubsta,
 
 	status->rx_flags = 0;
 
+	kcov_remote_start_common(skb_get_kcov_handle(skb));
+
 	/*
 	 * Frames with failed FCS/PLCP checksum are not returned,
 	 * all other frames are returned without radiotap header
@@ -4749,15 +4751,15 @@ void ieee80211_rx_list(struct ieee80211_hw *hw, struct ieee80211_sta *pubsta,
 	 * Also, frames with less than 16 bytes are dropped.
 	 */
 	skb = ieee80211_rx_monitor(local, skb, rate);
-	if (!skb)
-		return;
+	if (skb) {
+		ieee80211_tpt_led_trig_rx(local,
+					  ((struct ieee80211_hdr *)skb->data)->frame_control,
+					  skb->len);
 
-	ieee80211_tpt_led_trig_rx(local,
-			((struct ieee80211_hdr *)skb->data)->frame_control,
-			skb->len);
-
-	__ieee80211_rx_handle_packet(hw, pubsta, skb, list);
+		__ieee80211_rx_handle_packet(hw, pubsta, skb, list);
+	}
 
+	kcov_remote_stop();
 	return;
  drop:
 	kfree_skb(skb);