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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id y62sm11996200pfg.88.2021.08.22.00.56.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 22 Aug 2021 00:56:55 -0700 (PDT) From: Kees Cook To: linux-kernel@vger.kernel.org Cc: Kees Cook , Rasmus Villemoes , Daniel Micay , Francis Laniel , Bart Van Assche , David Gow , linux-mm@kvack.org, clang-built-linux@googlegroups.com, linux-hardening@vger.kernel.org Subject: [PATCH for-next 22/25] string.h: Introduce memset_after() for wiping trailing members/padding Date: Sun, 22 Aug 2021 00:51:19 -0700 Message-Id: <20210822075122.864511-23-keescook@chromium.org> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210822075122.864511-1-keescook@chromium.org> References: <20210822075122.864511-1-keescook@chromium.org> MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=2741; h=from:subject; bh=E6gFtKPMaqY94NYGTcjgqzMn4Zg8efYnWFddeRIDocU=; b=owEBbQKS/ZANAwAKAYly9N/cbcAmAcsmYgBhIgH50IyFEulz6vf4MWlQ/Ezu82EvPp7pmwWKKNPw FQi+vpGJAjMEAAEKAB0WIQSlw/aPIp3WD3I+bhOJcvTf3G3AJgUCYSIB+QAKCRCJcvTf3G3AJh3rEA CrFF/z6kOuE4E1nPr3TROjLRJGQjTLaabDuXR/aIm4UQKeKkoTwGA2+A4SqjMTq/+P5Ys7g2X4LT/W Z2l1Vk9U0SrRQPJl7vZYRJ+9O4qUs3Kf8Ny2bIVgxeZT2N0blFHjxagjdD8vAzrY3b1LssVIU5HLiQ CkTZwHf1BnUGyXWqlHau1Vtb3xdCMueRk/f0+dyoPkbCq9ZiKc0XgtZ5MOlzVE66BQLezYe3VZxDUH aDeSYCo7PVgr9pV36o86mUKSUXJWTQ74tdMd1P2VvIhlNEI+vIzBXIjvq2l/IC+Ia2/oqyD/PBLJJk RoXa16m2Lr5ydwZuU+5qMNSo8Cz5OcHc0iQS5x/zK4L2Y6i84HtrS2Ja/RoBQ4SawOGvwkqQMDWUhM tML3SJfr/6qBoVbM7j7oTKiWGwFiZkpavW0iPpwrFqTjTonEi3vdUzO/ahwHLs5uU8gEPP+nPovcfo wZ9AAvOWdDZeSU8i0x0zCI2Hiu5fGcmVETpSSG/F1WqZuQHz8KRI4ndkMy5COuwjY5DADDoBVXuei2 GDhIxOMqkO2jtycMsYYhPX4LOTHskAA/ikwrRY9YVN753cwclPR1h9ULCg29KZIvJfiQ4oFMPNQ49S SyWtCljmqAu2E43Fu6xl6RJf6H3Wn1D9Ei2FNzhed+0om1vGjS5eu9TtBUHg== X-Developer-Key: i=keescook@chromium.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=chromium.org header.s=google header.b=fg7ZhYLv; spf=pass (imf06.hostedemail.com: domain of keescook@chromium.org designates 209.85.216.42 as permitted sender) smtp.mailfrom=keescook@chromium.org; dmarc=pass (policy=none) header.from=chromium.org X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: F3782801A89F X-Stat-Signature: 19joqi78zjarjq43s37qhube7f59zqdd X-HE-Tag: 1629619016-252591 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. Signed-off-by: Kees Cook --- include/linux/string.h | 17 +++++++++++++++++ lib/test_memcpy.c | 13 +++++++++++++ 2 files changed, 30 insertions(+) diff --git a/include/linux/string.h b/include/linux/string.h index 9473f81b9db2..d593de2635ba 100644 --- a/include/linux/string.h +++ b/include/linux/string.h @@ -271,6 +271,23 @@ static inline void memcpy_and_pad(void *dest, size_t= dest_len, memcpy(dest, src, dest_len); } =20 +/** + * memset_after - Set a value after a struct member to the end of a stru= ct + * + * @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) \ +({ \ + u8 *__ptr =3D (u8 *)(obj); \ + typeof(v) __val =3D (v); \ + memset(__ptr + offsetofend(typeof(*(obj)), member), __val, \ + sizeof(*(obj)) - offsetofend(typeof(*(obj)), member)); \ +}) + /** * 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 ec546cec883e..3b485de8c885 100644 --- a/lib/test_memcpy.c +++ b/lib/test_memcpy.c @@ -215,6 +215,13 @@ static void memset_test(struct kunit *test) 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, }, }; + struct some_bytes after =3D { + .data =3D { 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 dest =3D { }; int count, value; u8 *ptr; @@ -245,6 +252,12 @@ static void memset_test(struct kunit *test) ptr +=3D 8; memset(ptr++, value++, count++); compare("argument side-effects", dest, three); + + /* Verify memset_after() */ + dest =3D control; + memset_after(&dest, 0x72, three); + compare("memset_after()", dest, after); + #undef TEST_OP } =20 --=20 2.30.2