| Message ID | 20221018083051.never.939-kees@kernel.org (mailing list archive) |
|---|---|
| State | Mainlined |
| Commit | 62e1cbfc5d795381a0f237ae7ee229a92d51cf9e |
| Headers | show |
| Series | fortify: Short-circuit known-safe calls to strscpy() | expand |
On Tue, Oct 18, 2022 at 01:32:51AM -0700, Kees Cook wrote: > 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 > Signed-off-by: Kees Cook <keescook@chromium.org> I ran the following commands and the tests all passed: $ tools/testing/kunit/kunit.py run --arch arm64 --cross_compile aarch64-linux-gnu- strscpy $ tools/testing/kunit/kunit.py run --arch arm64 --make_options LLVM=1 strscpy $ tools/testing/kunit/kunit.py run --arch x86_64 strscpy $ tools/testing/kunit/kunit.py run --arch x86_64 --make_options LLVM=1 strscpy $ tools/testing/kunit/kunit.py run --arch arm64 --cross_compile aarch64-linux-gnu- --kconfig_add CONFIG_FORTIFY_SOURCE=y strscpy $ tools/testing/kunit/kunit.py run --arch arm64 --kconfig_add CONFIG_FORTIFY_SOURCE=y --make_options LLVM=1 strscpy $ tools/testing/kunit/kunit.py run --arch x86_64 --kconfig_add CONFIG_FORTIFY_SOURCE=y strscpy $ tools/testing/kunit/kunit.py run --arch x86_64 --kconfig_add CONFIG_FORTIFY_SOURCE=y --make_options LLVM=1 strscpy Tested-by: Nathan Chancellor <nathan@kernel.org> > --- > include/linux/fortify-string.h | 10 ++++++++++ > lib/strscpy_kunit.c | 13 +++++++++++++ > 2 files changed, 23 insertions(+) > > diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h > index aa1a50009632..c473adb55cf5 100644 > --- a/include/linux/fortify-string.h > +++ b/include/linux/fortify-string.h > @@ -321,6 +321,16 @@ __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. > diff --git a/lib/strscpy_kunit.c b/lib/strscpy_kunit.c > index 98523f828d3a..a6b6344354ed 100644 > --- a/lib/strscpy_kunit.c > +++ b/lib/strscpy_kunit.c > @@ -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[] = { > -- > 2.34.1 > >
diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h index aa1a50009632..c473adb55cf5 100644 --- a/include/linux/fortify-string.h +++ b/include/linux/fortify-string.h @@ -321,6 +321,16 @@ __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. diff --git a/lib/strscpy_kunit.c b/lib/strscpy_kunit.c index 98523f828d3a..a6b6344354ed 100644 --- a/lib/strscpy_kunit.c +++ b/lib/strscpy_kunit.c @@ -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[] = {
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 Signed-off-by: Kees Cook <keescook@chromium.org> --- include/linux/fortify-string.h | 10 ++++++++++ lib/strscpy_kunit.c | 13 +++++++++++++ 2 files changed, 23 insertions(+)