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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id mr18sm3750578pjb.39.2021.08.17.23.05.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Aug 2021 23:06:02 -0700 (PDT) From: Kees Cook To: linux-kernel@vger.kernel.org Cc: Kees Cook , Steffen Klassert , Herbert Xu , "David S. Miller" , Jakub Kicinski , Andrew Morton , Francis Laniel , Vincenzo Frascino , Daniel Axtens , netdev@vger.kernel.org, "Gustavo A. R. Silva" , Greg Kroah-Hartman , linux-wireless@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-staging@lists.linux.dev, linux-block@vger.kernel.org, linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com, Rasmus Villemoes , linux-hardening@vger.kernel.org Subject: [PATCH v2 37/63] string.h: Introduce memset_after() for wiping trailing members/padding Date: Tue, 17 Aug 2021 23:05:07 -0700 Message-Id: <20210818060533.3569517-38-keescook@chromium.org> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210818060533.3569517-1-keescook@chromium.org> References: <20210818060533.3569517-1-keescook@chromium.org> MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=4092; h=from:subject; bh=ynt+tRmRAcx7nox2wqu1NNTkTOzsqx2+YTbDvMLTbxE=; b=owEBbQKS/ZANAwAKAYly9N/cbcAmAcsmYgBhHKMmwbMjodaindknTsBVe7537WHFelKIadd+u5e3 SxHIQdeJAjMEAAEKAB0WIQSlw/aPIp3WD3I+bhOJcvTf3G3AJgUCYRyjJgAKCRCJcvTf3G3AJnBLEA CdNu7o5W+92HzRi1fko57n/mLwv/f7ERDN9lR9c1k1ZpJRD294w1BBHne+86h1ErieaOKABkA1tR9Z RPuVXQx84Ll35fJ448mje10Vlj4xMIsIO7I2+pCjPpZeZUknIkUAfgL4BIt11O1QZKIXt5gv0ApWLk F0vG/OdP1EsH7UfN0sGFW3dUPcQnV365DGpsYsXEbfYPUtv7V0sIo4u2EqY+DWjZNXgimvh1QDuwFD ASW7FG+u7LV5vdhie6SOhQtFr9ccWj/RPLaI95ILtuhSn99zvuAmxmiV9Sq50joZ8BHWNBKRMezCGO 3/YVBD1ioQyCyuoiDxeL9UMcrdPSZQt6aLznsfSXoKOqLO+Ir7tMR4XrhkEg8MfM+mA6JudX+GjyI5 HhSyIFjb1vJpco5l2tSPxCz4ioyocE52DQfc0wA9/yfi2liO4gbBZCqBmTCsIZpwdwhQkDkVBK5ZvR HOtc2ADG105U1yBtwAKKBM1Kpn6jPowZCs5D1qDOHYz1gAWljX17P6SuxaG3G/dMeo5MT0LpM97ZfJ VauEeeQUKsZsz2LSYtk7JsMuq3dNZ28R5tJc/mwglhdLai09Fi/99TDZ2wV4u2YvpwKr9UaxVWIuT5 e3kxAxaDFTyXoo70DNH9x1G7c+SuCaUbgMUIaDh6BALQ3dZqhLv+3W2zODmg== X-Developer-Key: i=keescook@chromium.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org A common idiom in kernel code is to wipe the contents of a structure after a given member. This is especially useful in places where there is trailing padding. These open-coded cases are usually difficult to read and very sensitive to struct layout changes. Introduce a new helper, memset_after() that takes the target struct instance, the byte to write, and the member name after which the zeroing should start. Additionally adds memset_startat() for wiping trailing members _starting_ at a specific member instead of after a member, which is more readable in certain circumstances, but doesn't include any preceding padding. Cc: Steffen Klassert Cc: Herbert Xu Cc: "David S. Miller" Cc: Jakub Kicinski Cc: Andrew Morton Cc: Francis Laniel Cc: Vincenzo Frascino Cc: Daniel Axtens Cc: netdev@vger.kernel.org Signed-off-by: Kees Cook --- include/linux/string.h | 29 +++++++++++++++++++++++++++++ lib/test_memcpy.c | 24 ++++++++++++++++++++++++ 2 files changed, 53 insertions(+) diff --git a/include/linux/string.h b/include/linux/string.h index cbe889e404e2..fe56a1774207 100644 --- a/include/linux/string.h +++ b/include/linux/string.h @@ -272,6 +272,35 @@ static __always_inline void memcpy_and_pad(void *dest, size_t dest_len, memcpy(dest, src, dest_len); } +/** + * memset_after - Set a value after a struct member to the end of a struct + * + * @obj: Address of target struct instance + * @v: Byte value to repeatedly write + * @member: after which struct member to start writing bytes + * + * This is good for clearing padding following the given member. + */ +#define memset_after(obj, v, member) do { \ + memset((u8 *)(obj) + offsetofend(typeof(*(obj)), member), v, \ + sizeof(*(obj)) - offsetofend(typeof(*(obj)), member)); \ +} while (0) + +/** + * memset_startat - Set a value starting at a member to the end of a struct + * + * @obj: Address of target struct instance + * @v: Byte value to repeatedly write + * @member: struct member to start writing at + * + * Note that if there is padding between the prior member and the target + * member, memset_after() should be used to clear the prior padding. + */ +#define memset_startat(obj, v, member) do { \ + memset((u8 *)(obj) + offsetof(typeof(*(obj)), member), v, \ + sizeof(*(obj)) - offsetof(typeof(*(obj)), member)); \ +} while (0) + /** * str_has_prefix - Test if a string has a given prefix * @str: The string to test diff --git a/lib/test_memcpy.c b/lib/test_memcpy.c index be192b8e82b7..50bc99552a17 100644 --- a/lib/test_memcpy.c +++ b/lib/test_memcpy.c @@ -215,6 +215,20 @@ static void memset_test(struct kunit *test) 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, }, }; + struct some_bytes after = { + .data = { 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x72, + 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, + 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, + 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, + }, + }; + struct some_bytes startat = { + .data = { 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, + 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, + 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, + 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, + }, + }; struct some_bytes dest = { }; int count, value; u8 *ptr; @@ -245,6 +259,16 @@ static void memset_test(struct kunit *test) ptr += 8; memset(ptr++, value++, count++); compare("argument side-effects", dest, three); + + /* Verify memset_after() */ + dest = control; + memset_after(&dest, 0x72, three); + compare("memset_after()", dest, after); + + /* Verify memset_startat() */ + dest = control; + memset_startat(&dest, 0x79, four); + compare("memset_startat()", dest, startat); #undef TEST_OP } -- 2.30.2