[PATCH v4 18/45] instrumented.h: add KMSAN support

Alexander Potapenko
2022-07-01T14:26:16Z

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 a6522a0c28df9..c4cae333deec5 100644
--- a/include/linux/kmsan-checks.h
+++ b/include/linux/kmsan-checks.h
@@ -46,6 +46,21 @@ 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

static inline void kmsan_poison_memory(const void *address, size_t size,
@@ -58,6 +73,10 @@ 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)
{
--
2.37.0.rc0.161.g10f37bed90-goog