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fortify: Short-circuit known-safe calls to strscpy()

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Replacing compile-time safe calls of strcpy()-related functions with
strscpy() was always calling the full strscpy() logic when a builtin
would be better. For example:

	char buf[16];
	strcpy(buf, "yes");

would reduce to __builtin_memcpy(buf, "yes", 4), but not if it was:

	strscpy(buf, yes, sizeof(buf));

Fix this by checking if all sizes are known at compile-time.

Cc: linux-hardening@vger.kernel.org
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
This commit is contained in:
Kees Cook 2022-10-02 09:17:03 -07:00
parent 41eefc46a3
commit 62e1cbfc5d
2 changed files with 23 additions and 0 deletions
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10
include/linux/fortify-string.h
View File

@ -314,6 +314,16 @@ __FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, s
if (__compiletime_lessthan(p_size, size))
__write_overflow();
/* Short-circuit for compile-time known-safe lengths. */
if (__compiletime_lessthan(p_size, SIZE_MAX)) {
len = __compiletime_strlen(q);
if (len < SIZE_MAX && __compiletime_lessthan(len, size)) {
__underlying_memcpy(p, q, len + 1);
return len;
}
}
/*
* This call protects from read overflow, because len will default to q
* length if it smaller than size.

13
lib/strscpy_kunit.c
View File

@ -81,6 +81,8 @@ static void tc(struct kunit *test, char *src, int count, int expected,
static void strscpy_test(struct kunit *test)
{
char dest[8];
/*
* tc() uses a destination buffer of size 6 and needs at
* least 2 characters spare (one for null and one to check for
@ -111,6 +113,17 @@ static void strscpy_test(struct kunit *test)
tc(test, "ab", 4, 2, 2, 1, 1);
tc(test, "a", 4, 1, 1, 1, 2);
tc(test, "", 4, 0, 0, 1, 3);
/* Compile-time-known source strings. */
KUNIT_EXPECT_EQ(test, strscpy(dest, "", ARRAY_SIZE(dest)), 0);
KUNIT_EXPECT_EQ(test, strscpy(dest, "", 3), 0);
KUNIT_EXPECT_EQ(test, strscpy(dest, "", 1), 0);
KUNIT_EXPECT_EQ(test, strscpy(dest, "", 0), -E2BIG);
KUNIT_EXPECT_EQ(test, strscpy(dest, "Fixed", ARRAY_SIZE(dest)), 5);
KUNIT_EXPECT_EQ(test, strscpy(dest, "Fixed", 3), -E2BIG);
KUNIT_EXPECT_EQ(test, strscpy(dest, "Fixed", 1), -E2BIG);
KUNIT_EXPECT_EQ(test, strscpy(dest, "Fixed", 0), -E2BIG);
KUNIT_EXPECT_EQ(test, strscpy(dest, "This is too long", ARRAY_SIZE(dest)), -E2BIG);
}
static struct kunit_case strscpy_test_cases[] = {