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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id s16-20020a17090a1c1000b0020a81cf4a9asm11077191pjs.14.2022.10.18.01.32.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 18 Oct 2022 01:32:56 -0700 (PDT) From: Kees Cook To: linux-hardening@vger.kernel.org Cc: Kees Cook , Nick Desaulniers , llvm@lists.linux.dev, linux-kernel@vger.kernel.org Subject: [PATCH] fortify: Short-circuit known-safe calls to strscpy() Date: Tue, 18 Oct 2022 01:32:51 -0700 Message-Id: <20221018083051.never.939-kees@kernel.org> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=2706; h=from:subject:message-id; bh=TuV6DsCHn4YM1pzSIZxk5kOcXo+TKcB3VVIFFMdDAmw=; b=owEBbQKS/ZANAwAKAYly9N/cbcAmAcsmYgBjTmSy9TMK14Z6uFIMcZ/T8ujj2hXR4Xr/JigD6sMv 42vJH2iJAjMEAAEKAB0WIQSlw/aPIp3WD3I+bhOJcvTf3G3AJgUCY05ksgAKCRCJcvTf3G3AJsPHD/ 9PrjfaHojf06MfJOXnvTmaDU+LsTURln+B94F70PVDLDd2Dav7dKtEGqNxJuG3bEbC6c5oEREbdQXK RrYPH2Wx5O8CtmmUwkaSiYIjjZc5y7Y3CfO2dbjGBNiFYD2jWESj5dkgWJLbibCkVHtiSGIoBO63oE fICz4Gy2XDEfxp+a2fPArCNgKzXW7QU7OBNE/2VdFn6mbVrCJhSZdhOKlS6HOh+IE+ma4n6dYL0IB7 e/MHsdfllNhhxqZa36zBSWiq6fz4DJjUJ4V7z/WQ4Agsj2mjiGVDSTM8MD8SII9Jz4Uh9TJmvnZExd SjY8asnB9rEAS3FXN+DQbm87gFf2jtHZOwOWs6klYMkR6cBtBsJ+OUM+FTzqOtfmlv0IQd47QraSFx Vr8GiqaOY+Pn9G/Rjpb+rjIFeJ4YMiY/lEia9QPmxU36+ML52uBrhAlNe/QsAVCXdBzZ500FRIIu/d uXFSZfQfoYkf8+mTlUo30HujO84nZ/0JVpYarc/JbJLynPuBQPsG0PGCoGJyA2/M3ZBiL8DF20TdXg x5JsNVPwhpcJOklcCegGs7A+kGhW6kvKCxOpEa99xZYzhSy3ozTqvxOY023eMoIcEhAwCeDxVVojFf NnVntHm6jMPdmeR5EH0vHa0d1lMBerG4BgLsbTXcTGmmSVYjBZWvusnKSSKQ== X-Developer-Key: i=keescook@chromium.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org 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 Tested-by: Nathan Chancellor --- 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[] = {