[PATCH v3 27/46] kmsan: instrumentation.h: add instrumentation_begin_with_regs()

[Alexander Potapenko <glider@google.com>]

When calling KMSAN-instrumented functions from non-instrumented
functions, function parameters may not be initialized properly, leading
to false positive reports. In particular, this happens all the time when
calling interrupt handlers from `noinstr` IDT entries.

We introduce instrumentation_begin_with_regs(), which calls
instrumentation_begin() and notifies KMSAN about the beginning of the
potentially instrumented region by calling
kmsan_instrumentation_begin(), which:
 - wipes the current KMSAN state at the beginning of the region, ensuring
   that the first call of an instrumented function receives initialized
   parameters (this is a pretty good approximation of having all other
   instrumented functions receive initialized parameters);
 - unpoisons the `struct pt_regs` set up by the non-instrumented assembly
   code.

Signed-off-by: Alexander Potapenko <glider@google.com>
---
Link: https://linux-review.googlesource.com/id/I0f5e3372e00bd5fe25ddbf286f7260aae9011858
---
 include/linux/instrumentation.h |  6 ++++++
 include/linux/kmsan.h           | 11 +++++++++++
 mm/kmsan/hooks.c                | 16 ++++++++++++++++
 3 files changed, 33 insertions(+)

diff --git a/include/linux/instrumentation.h b/include/linux/instrumentation.h
index 24359b4a96053..3bbce9d556381 100644
--- a/include/linux/instrumentation.h
+++ b/include/linux/instrumentation.h
@@ -15,6 +15,11 @@
 })
 #define instrumentation_begin() __instrumentation_begin(__COUNTER__)
 
+#define instrumentation_begin_with_regs(regs) do {			\
+	__instrumentation_begin(__COUNTER__);				\
+	kmsan_instrumentation_begin(regs);				\
+} while (0)
+
 /*
  * Because instrumentation_{begin,end}() can nest, objtool validation considers
  * _begin() a +1 and _end() a -1 and computes a sum over the instructions.
@@ -55,6 +60,7 @@
 #define instrumentation_end() __instrumentation_end(__COUNTER__)
 #else
 # define instrumentation_begin()	do { } while(0)
+# define instrumentation_begin_with_regs(regs) kmsan_instrumentation_begin(regs)
 # define instrumentation_end()		do { } while(0)
 #endif
 
diff --git a/include/linux/kmsan.h b/include/linux/kmsan.h
index 55f976b721566..209a5a2192e22 100644
--- a/include/linux/kmsan.h
+++ b/include/linux/kmsan.h
@@ -247,6 +247,13 @@ void kmsan_handle_dma_sg(struct scatterlist *sg, int nents,
  */
 void kmsan_handle_urb(const struct urb *urb, bool is_out);
 
+/**
+ * kmsan_instrumentation_begin() - handle instrumentation_begin().
+ * @regs: pointer to struct pt_regs that non-instrumented code passes to
+ *        instrumented code.
+ */
+void kmsan_instrumentation_begin(struct pt_regs *regs);
+
 #else
 
 static inline void kmsan_init_shadow(void)
@@ -343,6 +350,10 @@ static inline void kmsan_handle_urb(const struct urb *urb, bool is_out)
 {
 }
 
+static inline void kmsan_instrumentation_begin(struct pt_regs *regs)
+{
+}
+
 #endif
 
 #endif /* _LINUX_KMSAN_H */
diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c
index 9aecbf2825837..c20d105c143c1 100644
--- a/mm/kmsan/hooks.c
+++ b/mm/kmsan/hooks.c
@@ -366,3 +366,19 @@ void kmsan_check_memory(const void *addr, size_t size)
 					   REASON_ANY);
 }
 EXPORT_SYMBOL(kmsan_check_memory);
+
+void kmsan_instrumentation_begin(struct pt_regs *regs)
+{
+	struct kmsan_context_state *state = &kmsan_get_context()->cstate;
+
+	if (state)
+		__memset(state, 0, sizeof(struct kmsan_context_state));
+	if (!kmsan_enabled || !regs)
+		return;
+	/*
+	 * @regs may reside in cpu_entry_area, for which KMSAN does not allocate
+	 * metadata. Do not force an error in that case.
+	 */
+	kmsan_internal_unpoison_memory(regs, sizeof(*regs), /*checked*/ false);
+}
+EXPORT_SYMBOL(kmsan_instrumentation_begin);
-- 
2.36.0.rc2.479.g8af0fa9b8e-goog