[PATCH v3 20/46] instrumented.h: add KMSAN support

[Alexander Potapenko <glider@google.com>]

To avoid false positives, KMSAN needs to unpoison the data copied from
the userspace. To detect infoleaks - check the memory buffer passed to
copy_to_user().

Signed-off-by: Alexander Potapenko <glider@google.com>
---
v2:
 -- move implementation of kmsan_copy_to_user() here

Link: https://linux-review.googlesource.com/id/I43e93b9c02709e6be8d222342f1b044ac8bdbaaf
---
 include/linux/instrumented.h |  5 ++++-
 include/linux/kmsan-checks.h | 19 ++++++++++++++++++
 mm/kmsan/hooks.c             | 38 ++++++++++++++++++++++++++++++++++++
 3 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
index ee8f7d17d34f5..c73c1b19e9227 100644
--- a/include/linux/instrumented.h
+++ b/include/linux/instrumented.h
@@ -2,7 +2,7 @@
 
 /*
  * This header provides generic wrappers for memory access instrumentation that
- * the compiler cannot emit for: KASAN, KCSAN.
+ * the compiler cannot emit for: KASAN, KCSAN, KMSAN.
  */
 #ifndef _LINUX_INSTRUMENTED_H
 #define _LINUX_INSTRUMENTED_H
@@ -10,6 +10,7 @@
 #include <linux/compiler.h>
 #include <linux/kasan-checks.h>
 #include <linux/kcsan-checks.h>
+#include <linux/kmsan-checks.h>
 #include <linux/types.h>
 
 /**
@@ -117,6 +118,7 @@ instrument_copy_to_user(void __user *to, const void *from, unsigned long n)
 {
 	kasan_check_read(from, n);
 	kcsan_check_read(from, n);
+	kmsan_copy_to_user(to, from, n, 0);
 }
 
 /**
@@ -151,6 +153,7 @@ static __always_inline void
 instrument_copy_from_user_after(const void *to, const void __user *from,
 				unsigned long n, unsigned long left)
 {
+	kmsan_unpoison_memory(to, n - left);
 }
 
 #endif /* _LINUX_INSTRUMENTED_H */
diff --git a/include/linux/kmsan-checks.h b/include/linux/kmsan-checks.h
index ecd8336190fc0..aabaf1ba7c251 100644
--- a/include/linux/kmsan-checks.h
+++ b/include/linux/kmsan-checks.h
@@ -84,6 +84,21 @@ void kmsan_unpoison_memory(const void *address, size_t size);
  */
 void kmsan_check_memory(const void *address, size_t size);
 
+/**
+ * kmsan_copy_to_user() - Notify KMSAN about a data transfer to userspace.
+ * @to:      destination address in the userspace.
+ * @from:    source address in the kernel.
+ * @to_copy: number of bytes to copy.
+ * @left:    number of bytes not copied.
+ *
+ * If this is a real userspace data transfer, KMSAN checks the bytes that were
+ * actually copied to ensure there was no information leak. If @to belongs to
+ * the kernel space (which is possible for compat syscalls), KMSAN just copies
+ * the metadata.
+ */
+void kmsan_copy_to_user(void __user *to, const void *from, size_t to_copy,
+			size_t left);
+
 #else
 
 #define kmsan_init(value) (value)
@@ -98,6 +113,10 @@ static inline void kmsan_unpoison_memory(const void *address, size_t size)
 static inline void kmsan_check_memory(const void *address, size_t size)
 {
 }
+static inline void kmsan_copy_to_user(void __user *to, const void *from,
+				      size_t to_copy, size_t left)
+{
+}
 
 #endif
 
diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c
index 43a529569053d..1cdb4420977f1 100644
--- a/mm/kmsan/hooks.c
+++ b/mm/kmsan/hooks.c
@@ -212,6 +212,44 @@ void kmsan_iounmap_page_range(unsigned long start, unsigned long end)
 }
 EXPORT_SYMBOL(kmsan_iounmap_page_range);
 
+void kmsan_copy_to_user(void __user *to, const void *from, size_t to_copy,
+			size_t left)
+{
+	unsigned long ua_flags;
+
+	if (!kmsan_enabled || kmsan_in_runtime())
+		return;
+	/*
+	 * At this point we've copied the memory already. It's hard to check it
+	 * before copying, as the size of actually copied buffer is unknown.
+	 */
+
+	/* copy_to_user() may copy zero bytes. No need to check. */
+	if (!to_copy)
+		return;
+	/* Or maybe copy_to_user() failed to copy anything. */
+	if (to_copy <= left)
+		return;
+
+	ua_flags = user_access_save();
+	if ((u64)to < TASK_SIZE) {
+		/* This is a user memory access, check it. */
+		kmsan_internal_check_memory((void *)from, to_copy - left, to,
+					    REASON_COPY_TO_USER);
+		user_access_restore(ua_flags);
+		return;
+	}
+	/* Otherwise this is a kernel memory access. This happens when a compat
+	 * syscall passes an argument allocated on the kernel stack to a real
+	 * syscall.
+	 * Don't check anything, just copy the shadow of the copied bytes.
+	 */
+	kmsan_internal_memmove_metadata((void *)to, (void *)from,
+					to_copy - left);
+	user_access_restore(ua_flags);
+}
+EXPORT_SYMBOL(kmsan_copy_to_user);
+
 /* Functions from kmsan-checks.h follow. */
 void kmsan_poison_memory(const void *address, size_t size, gfp_t flags)
 {
-- 
2.36.0.rc2.479.g8af0fa9b8e-goog