From: Marian Cingel <mcingel@tachyum.com>
Date: Tue, 31 Aug 2021 12:33:20 +0000 (+0100)
Subject: privsep: Enforce proper alignment of serialized struct cmsghdr
X-Git-Tag: v9.4.1~8
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3edf2b11831fde2253867414b75690a9b292b67d;p=thirdparty%2Fdhcpcd.git

privsep: Enforce proper alignment of serialized struct cmsghdr

Fixes #43.
---

diff --git a/src/privsep.c b/src/privsep.c
index e939fb50..d574a2bc 100644
--- a/src/privsep.c
+++ b/src/privsep.c
@@ -81,6 +81,15 @@
 #include <util.h>
 #endif
 
+/* CMSG_ALIGN is a Linux extension */
+#ifndef CMSG_ALIGN
+#define CMSG_ALIGN(n)	(CMSG_SPACE((n)) - CMSG_SPACE(0))
+#endif
+
+/* Calculate number of padding bytes to achieve 'struct cmsghdr' alignment */
+#define CALC_CMSG_PADLEN(has_cmsg, pos) \
+    ((has_cmsg) ? (socklen_t)(CMSG_ALIGN((pos)) - (pos)) : 0)
+
 int
 ps_init(struct dhcpcd_ctx *ctx)
 {
@@ -655,9 +664,11 @@ ps_unrollmsg(struct msghdr *msg, struct ps_msghdr *psm,
     const void *data, size_t len)
 {
 	uint8_t *datap, *namep, *controlp;
+	socklen_t cmsg_padlen =
+	    CALC_CMSG_PADLEN(psm->ps_controllen, psm->ps_namelen);
 
 	namep = UNCONST(data);
-	controlp = namep + psm->ps_namelen;
+	controlp = namep + psm->ps_namelen + cmsg_padlen;
 	datap = controlp + psm->ps_controllen;
 
 	if (psm->ps_namelen != 0) {
@@ -677,7 +688,7 @@ ps_unrollmsg(struct msghdr *msg, struct ps_msghdr *psm,
 			return -1;
 		}
 		msg->msg_control = controlp;
-		len -= psm->ps_controllen;
+		len -= psm->ps_controllen + cmsg_padlen;
 	} else
 		msg->msg_control = NULL;
 	msg->msg_controllen = psm->ps_controllen;
@@ -698,9 +709,11 @@ ssize_t
 ps_sendpsmmsg(struct dhcpcd_ctx *ctx, int fd,
     struct ps_msghdr *psm, const struct msghdr *msg)
 {
+	long padding[1] = { 0 };
 	struct iovec iov[] = {
 		{ .iov_base = UNCONST(psm), .iov_len = sizeof(*psm) },
 		{ .iov_base = NULL, },	/* name */
+		{ .iov_base = NULL, },	/* control padding */
 		{ .iov_base = NULL, },	/* control */
 		{ .iov_base = NULL, },	/* payload 1 */
 		{ .iov_base = NULL, },	/* payload 2 */
@@ -712,6 +725,7 @@ ps_sendpsmmsg(struct dhcpcd_ctx *ctx, int fd,
 	if (msg != NULL) {
 		struct iovec *iovp = &iov[1];
 		int i;
+		socklen_t cmsg_padlen;
 
 		psm->ps_namelen = msg->msg_namelen;
 		psm->ps_controllen = (socklen_t)msg->msg_controllen;
@@ -719,9 +733,17 @@ ps_sendpsmmsg(struct dhcpcd_ctx *ctx, int fd,
 		iovp->iov_base = msg->msg_name;
 		iovp->iov_len = msg->msg_namelen;
 		iovp++;
+
+		cmsg_padlen =
+		    CALC_CMSG_PADLEN(msg->msg_controllen, msg->msg_namelen);
+		assert(cmsg_padlen <= sizeof(padding));
+		iovp->iov_len = cmsg_padlen;
+		iovp->iov_base = cmsg_padlen != 0 ? padding : NULL;
+		iovp++;
+
 		iovp->iov_base = msg->msg_control;
 		iovp->iov_len = msg->msg_controllen;
-		iovlen = 3;
+		iovlen = 4;
 
 		for (i = 0; i < (int)msg->msg_iovlen; i++) {
 			if ((size_t)(iovlen + i) > __arraycount(iov)) {
@@ -832,6 +854,8 @@ ps_sendcmdmsg(int fd, uint16_t cmd, const struct msghdr *msg)
 		{ .iov_base = data, .iov_len = 0 },
 	};
 	size_t dl = sizeof(data);
+	socklen_t cmsg_padlen =
+	    CALC_CMSG_PADLEN(msg->msg_controllen, msg->msg_namelen);
 
 	if (msg->msg_namelen != 0) {
 		if (msg->msg_namelen > dl)
@@ -843,8 +867,13 @@ ps_sendcmdmsg(int fd, uint16_t cmd, const struct msghdr *msg)
 	}
 
 	if (msg->msg_controllen != 0) {
-		if (msg->msg_controllen > dl)
+		if (msg->msg_controllen + cmsg_padlen > dl)
 			goto nobufs;
+		if (cmsg_padlen != 0) {
+			memset(p, 0, cmsg_padlen);
+			p += cmsg_padlen;
+			dl -= cmsg_padlen;
+		}
 		psm.ps_controllen = (socklen_t)msg->msg_controllen;
 		memcpy(p, msg->msg_control, msg->msg_controllen);
 		p += msg->msg_controllen;
@@ -855,7 +884,8 @@ ps_sendcmdmsg(int fd, uint16_t cmd, const struct msghdr *msg)
 	if (psm.ps_datalen > dl)
 		goto nobufs;
 
-	iov[1].iov_len = psm.ps_namelen + psm.ps_controllen + psm.ps_datalen;
+	iov[1].iov_len =
+	    psm.ps_namelen + psm.ps_controllen + psm.ps_datalen + cmsg_padlen;
 	if (psm.ps_datalen != 0)
 		memcpy(p, msg->msg_iov[0].iov_base, psm.ps_datalen);
 	return writev(fd, iov, __arraycount(iov));