[PATCH 4/4] x86/kasan: Instrument user memory access API

[Andrey Ryabinin]

Exchange between user and kernel memory is coded in assembly language.
Which means that such accesses won't be spotted by KASAN as a compiler
instruments only C code.
Add explicit KASAN checks to user memory access API to ensure that
userspace writes to (or reads from) a valid kernel memory.

Note: Unlike others strncpy_from_user() is written mostly in C and KASAN
sees memory accesses in it. However, it makes sense to add explicit check
for all @count bytes that *potentially* could be written to the kernel.

Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
---
 arch/x86/include/asm/uaccess.h    | 5 +++++
 arch/x86/include/asm/uaccess_64.h | 7 +++++++
 lib/strncpy_from_user.c           | 2 ++
 3 files changed, 14 insertions(+)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 0b17fad..5dd6d18 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -5,6 +5,7 @@
  */
 #include <linux/errno.h>
 #include <linux/compiler.h>
+#include <linux/kasan-checks.h>
 #include <linux/thread_info.h>
 #include <linux/string.h>
 #include <asm/asm.h>
@@ -732,6 +733,8 @@ copy_from_user(void *to, const void __user *from, unsigned long n)
 
 	might_fault();
 
+	kasan_check_write(to, n);
+
 	/*
 	 * While we would like to have the compiler do the checking for us
 	 * even in the non-constant size case, any false positives there are
@@ -765,6 +768,8 @@ copy_to_user(void __user *to, const void *from, unsigned long n)
 {
 	int sz = __compiletime_object_size(from);
 
+	kasan_check_read(from, n);
+
 	might_fault();
 
 	/* See the comment in copy_from_user() above. */
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index 3076986..2eac2aa 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -7,6 +7,7 @@
 #include <linux/compiler.h>
 #include <linux/errno.h>
 #include <linux/lockdep.h>
+#include <linux/kasan-checks.h>
 #include <asm/alternative.h>
 #include <asm/cpufeatures.h>
 #include <asm/page.h>
@@ -109,6 +110,7 @@ static __always_inline __must_check
 int __copy_from_user(void *dst, const void __user *src, unsigned size)
 {
 	might_fault();
+	kasan_check_write(dst, size);
 	return __copy_from_user_nocheck(dst, src, size);
 }
 
@@ -175,6 +177,7 @@ static __always_inline __must_check
 int __copy_to_user(void __user *dst, const void *src, unsigned size)
 {
 	might_fault();
+	kasan_check_read(src, size);
 	return __copy_to_user_nocheck(dst, src, size);
 }
 
@@ -242,12 +245,14 @@ int __copy_in_user(void __user *dst, const void __user *src, unsigned size)
 static __must_check __always_inline int
 __copy_from_user_inatomic(void *dst, const void __user *src, unsigned size)
 {
+	kasan_check_write(dst, size);
 	return __copy_from_user_nocheck(dst, src, size);
 }
 
 static __must_check __always_inline int
 __copy_to_user_inatomic(void __user *dst, const void *src, unsigned size)
 {
+	kasan_check_read(src, size);
 	return __copy_to_user_nocheck(dst, src, size);
 }
 
@@ -258,6 +263,7 @@ static inline int
 __copy_from_user_nocache(void *dst, const void __user *src, unsigned size)
 {
 	might_fault();
+	kasan_check_write(dst, size);
 	return __copy_user_nocache(dst, src, size, 1);
 }
 
@@ -265,6 +271,7 @@ static inline int
 __copy_from_user_inatomic_nocache(void *dst, const void __user *src,
 				  unsigned size)
 {
+	kasan_check_write(dst, size);
 	return __copy_user_nocache(dst, src, size, 0);
 }
 
diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
index 3384032..e3472b0 100644
--- a/lib/strncpy_from_user.c
+++ b/lib/strncpy_from_user.c
@@ -1,5 +1,6 @@
 #include <linux/compiler.h>
 #include <linux/export.h>
+#include <linux/kasan-checks.h>
 #include <linux/uaccess.h>
 #include <linux/kernel.h>
 #include <linux/errno.h>
@@ -103,6 +104,7 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
 	if (unlikely(count <= 0))
 		return 0;
 
+	kasan_check_write(dst, count);
 	max_addr = user_addr_max();
 	src_addr = (unsigned long)src;
 	if (likely(src_addr < max_addr)) {
-- 
2.7.3