string: Introduce strtomem() and strtomem_pad()

One of the "legitimate" uses of strncpy() is copying a NUL-terminated
string into a fixed-size non-NUL-terminated character array. To avoid
the weaknesses and ambiguity of intent when using strncpy(), provide
replacement functions that explicitly distinguish between trailing
padding and not, and require the destination buffer size be discoverable
by the compiler.

For example:

struct obj {
	int foo;
	char small[4] __nonstring;
	char big[8] __nonstring;
	int bar;
};

struct obj p;

/* This will truncate to 4 chars with no trailing NUL */
strncpy(p.small, "hello", sizeof(p.small));
/* p.small contains 'h', 'e', 'l', 'l' */

/* This will NUL pad to 8 chars. */
strncpy(p.big, "hello", sizeof(p.big));
/* p.big contains 'h', 'e', 'l', 'l', 'o', '\0', '\0', '\0' */

When the "__nonstring" attributes are missing, the intent of the
programmer becomes ambiguous for whether the lack of a trailing NUL
in the p.small copy is a bug. Additionally, it's not clear whether
the trailing padding in the p.big copy is _needed_. Both cases
become unambiguous with:

strtomem(p.small, "hello");
strtomem_pad(p.big, "hello", 0);

See also https://github.com/KSPP/linux/issues/90

Expand the memcpy KUnit tests to include these functions.

Cc: Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index a6e36d9c3d14bb5647db46986a2fe444106ac00c..c8fd53a11a207285577696e5b3d53a6710f5806e 100644 (file)
--- a/Documentation/process/deprecated.rst
+++ b/Documentation/process/deprecated.rst
@@ -138,17 +138,20 @@ be NUL terminated. This can lead to various linear read overflows and
 other misbehavior due to the missing termination. It also NUL-pads
 the destination buffer if the source contents are shorter than the
 destination buffer size, which may be a needless performance penalty
-for callers using only NUL-terminated strings. The safe replacement is
+for callers using only NUL-terminated strings.
+
+When the destination is required to be NUL-terminated, the replacement is
 strscpy(), though care must be given to any cases where the return value
 of strncpy() was used, since strscpy() does not return a pointer to the
 destination, but rather a count of non-NUL bytes copied (or negative
 errno when it truncates). Any cases still needing NUL-padding should
 instead use strscpy_pad().
 
-If a caller is using non-NUL-terminated strings, strncpy() can
-still be used, but destinations should be marked with the `__nonstring
+If a caller is using non-NUL-terminated strings, strtomem() should be
+used, and the destinations should be marked with the `__nonstring
 <https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_
-attribute to avoid future compiler warnings.
+attribute to avoid future compiler warnings. For cases still needing
+NUL-padding, strtomem_pad() can be used.
 
 strlcpy()
 ---------

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index 3b401fa0f37462825af2c1d9d371998d52de03ef..8e8c2c87b1d5082c1ca1a43dbacd0189d11e0cf0 100644 (file)
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -77,6 +77,38 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
 #define POS    __pass_object_size(1)
 #define POS0   __pass_object_size(0)
 
+/**
+ * strncpy - Copy a string to memory with non-guaranteed NUL padding
+ *
+ * @p: pointer to destination of copy
+ * @q: pointer to NUL-terminated source string to copy
+ * @size: bytes to write at @p
+ *
+ * If strlen(@q) >= @size, the copy of @q will stop after @size bytes,
+ * and @p will NOT be NUL-terminated
+ *
+ * If strlen(@q) < @size, following the copy of @q, trailing NUL bytes
+ * will be written to @p until @size total bytes have been written.
+ *
+ * Do not use this function. While FORTIFY_SOURCE tries to avoid
+ * over-reads of @q, it cannot defend against writing unterminated
+ * results to @p. Using strncpy() remains ambiguous and fragile.
+ * Instead, please choose an alternative, so that the expectation
+ * of @p's contents is unambiguous:
+ *
+ * +--------------------+-----------------+------------+
+ * | @p needs to be:    | padded to @size | not padded |
+ * +====================+=================+============+
+ * |     NUL-terminated | strscpy_pad()   | strscpy()  |
+ * +--------------------+-----------------+------------+
+ * | not NUL-terminated | strtomem_pad()  | strtomem() |
+ * +--------------------+-----------------+------------+
+ *
+ * Note strscpy*()'s differing return values for detecting truncation,
+ * and strtomem*()'s expectation that the destination is marked with
+ * __nonstring when it is a character array.
+ *
+ */
 __FORTIFY_INLINE __diagnose_as(__builtin_strncpy, 1, 2, 3)
 char *strncpy(char * const POS p, const char *q, __kernel_size_t size)
 {

diff --git a/include/linux/string.h b/include/linux/string.h
index 61ec7e4f6311a8c87b2793eb3e8cdc25a7bf5f6e..cf7607b321027a7c108dacbbf5cb7bf19a6c4978 100644 (file)
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -260,6 +260,49 @@ static inline const char *kbasename(const char *path)
 void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
                    int pad);
 
+/**
+ * strtomem_pad - Copy NUL-terminated string to non-NUL-terminated buffer
+ *
+ * @dest: Pointer of destination character array (marked as __nonstring)
+ * @src: Pointer to NUL-terminated string
+ * @pad: Padding character to fill any remaining bytes of @dest after copy
+ *
+ * This is a replacement for strncpy() uses where the destination is not
+ * a NUL-terminated string, but with bounds checking on the source size, and
+ * an explicit padding character. If padding is not required, use strtomem().
+ *
+ * Note that the size of @dest is not an argument, as the length of @dest
+ * must be discoverable by the compiler.
+ */
+#define strtomem_pad(dest, src, pad)   do {                            \
+       const size_t _dest_len = __builtin_object_size(dest, 1);        \
+                                                                       \
+       BUILD_BUG_ON(!__builtin_constant_p(_dest_len) ||                \
+                    _dest_len == (size_t)-1);                          \
+       memcpy_and_pad(dest, _dest_len, src, strnlen(src, _dest_len), pad); \
+} while (0)
+
+/**
+ * strtomem - Copy NUL-terminated string to non-NUL-terminated buffer
+ *
+ * @dest: Pointer of destination character array (marked as __nonstring)
+ * @src: Pointer to NUL-terminated string
+ *
+ * This is a replacement for strncpy() uses where the destination is not
+ * a NUL-terminated string, but with bounds checking on the source size, and
+ * without trailing padding. If padding is required, use strtomem_pad().
+ *
+ * Note that the size of @dest is not an argument, as the length of @dest
+ * must be discoverable by the compiler.
+ */
+#define strtomem(dest, src)    do {                                    \
+       const size_t _dest_len = __builtin_object_size(dest, 1);        \
+                                                                       \
+       BUILD_BUG_ON(!__builtin_constant_p(_dest_len) ||                \
+                    _dest_len == (size_t)-1);                          \
+       memcpy(dest, src, min(_dest_len, strnlen(src, _dest_len)));     \
+} while (0)
+
 /**
  * memset_after - Set a value after a struct member to the end of a struct
  *

diff --git a/lib/memcpy_kunit.c b/lib/memcpy_kunit.c
index 62f8ffcbbaa352cd99812f2e38f27b56e6a542f9..d22fa3838ee978bb4be58c20784206d283ad0475 100644 (file)
--- a/lib/memcpy_kunit.c
+++ b/lib/memcpy_kunit.c
@@ -29,9 +29,8 @@ struct some_bytes {
 };
 
 #define check(instance, v) do {        \
-       int i;  \
        BUILD_BUG_ON(sizeof(instance.data) != 32);      \
-       for (i = 0; i < sizeof(instance.data); i++) {   \
+       for (size_t i = 0; i < sizeof(instance.data); i++) {    \
                KUNIT_ASSERT_EQ_MSG(test, instance.data[i], v, \
                        "line %d: '%s' not initialized to 0x%02x @ %d (saw 0x%02x)\n", \
                        __LINE__, #instance, v, i, instance.data[i]);   \
@@ -39,9 +38,8 @@ struct some_bytes {
 } while (0)
 
 #define compare(name, one, two) do { \
-       int i; \
        BUILD_BUG_ON(sizeof(one) != sizeof(two)); \
-       for (i = 0; i < sizeof(one); i++) {     \
+       for (size_t i = 0; i < sizeof(one); i++) {      \
                KUNIT_EXPECT_EQ_MSG(test, one.data[i], two.data[i], \
                        "line %d: %s.data[%d] (0x%02x) != %s.data[%d] (0x%02x)\n", \
                        __LINE__, #one, i, one.data[i], #two, i, two.data[i]); \
@@ -272,10 +270,63 @@ static void memset_test(struct kunit *test)
 #undef TEST_OP
 }
 
+static void strtomem_test(struct kunit *test)
+{
+       static const char input[] = "hi";
+       static const char truncate[] = "this is too long";
+       struct {
+               unsigned long canary1;
+               unsigned char output[sizeof(unsigned long)] __nonstring;
+               unsigned long canary2;
+       } wrap;
+
+       memset(&wrap, 0xFF, sizeof(wrap));
+       KUNIT_EXPECT_EQ_MSG(test, wrap.canary1, ULONG_MAX,
+                           "bad initial canary value");
+       KUNIT_EXPECT_EQ_MSG(test, wrap.canary2, ULONG_MAX,
+                           "bad initial canary value");
+
+       /* Check unpadded copy leaves surroundings untouched. */
+       strtomem(wrap.output, input);
+       KUNIT_EXPECT_EQ(test, wrap.canary1, ULONG_MAX);
+       KUNIT_EXPECT_EQ(test, wrap.output[0], input[0]);
+       KUNIT_EXPECT_EQ(test, wrap.output[1], input[1]);
+       for (size_t i = 2; i < sizeof(wrap.output); i++)
+               KUNIT_EXPECT_EQ(test, wrap.output[i], 0xFF);
+       KUNIT_EXPECT_EQ(test, wrap.canary2, ULONG_MAX);
+
+       /* Check truncated copy leaves surroundings untouched. */
+       memset(&wrap, 0xFF, sizeof(wrap));
+       strtomem(wrap.output, truncate);
+       KUNIT_EXPECT_EQ(test, wrap.canary1, ULONG_MAX);
+       for (size_t i = 0; i < sizeof(wrap.output); i++)
+               KUNIT_EXPECT_EQ(test, wrap.output[i], truncate[i]);
+       KUNIT_EXPECT_EQ(test, wrap.canary2, ULONG_MAX);
+
+       /* Check padded copy leaves only string padded. */
+       memset(&wrap, 0xFF, sizeof(wrap));
+       strtomem_pad(wrap.output, input, 0xAA);
+       KUNIT_EXPECT_EQ(test, wrap.canary1, ULONG_MAX);
+       KUNIT_EXPECT_EQ(test, wrap.output[0], input[0]);
+       KUNIT_EXPECT_EQ(test, wrap.output[1], input[1]);
+       for (size_t i = 2; i < sizeof(wrap.output); i++)
+               KUNIT_EXPECT_EQ(test, wrap.output[i], 0xAA);
+       KUNIT_EXPECT_EQ(test, wrap.canary2, ULONG_MAX);
+
+       /* Check truncated padded copy has no padding. */
+       memset(&wrap, 0xFF, sizeof(wrap));
+       strtomem(wrap.output, truncate);
+       KUNIT_EXPECT_EQ(test, wrap.canary1, ULONG_MAX);
+       for (size_t i = 0; i < sizeof(wrap.output); i++)
+               KUNIT_EXPECT_EQ(test, wrap.output[i], truncate[i]);
+       KUNIT_EXPECT_EQ(test, wrap.canary2, ULONG_MAX);
+}
+
 static struct kunit_case memcpy_test_cases[] = {
        KUNIT_CASE(memset_test),
        KUNIT_CASE(memcpy_test),
        KUNIT_CASE(memmove_test),
+       KUNIT_CASE(strtomem_test),
        {}
 };