socket: Check lengths before advancing pointer in CMSG_NXTHDR

The inline and library functions that the CMSG_NXTHDR macro may expand
to increment the pointer to the header before checking the stride of
the increment against available space.  Since C only allows incrementing
pointers to one past the end of an array, the increment must be done
after a length check.  This commit fixes that and includes a regression
test for CMSG_FIRSTHDR and CMSG_NXTHDR.

The Linux, Hurd, and generic headers are all changed.

Tested on Linux on armv7hl, i686, x86_64, aarch64, ppc64le, and s390x.

[BZ #28846]

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

3 files changed, 149 insertions, 0 deletions

diff --git a/socket/Makefile b/socket/Makefile
index 156eec6c85..2bde78387f 100644
--- a/socket/Makefile
+++ b/socket/Makefile
@@ -34,6 +34,7 @@ routines := accept bind connect getpeername getsockname getsockopt	\
 tests := \
   tst-accept4 \
   tst-sockopt \
+  tst-cmsghdr \
   # tests
 
 tests-internal := \
diff --git a/socket/tst-cmsghdr-skeleton.c b/socket/tst-cmsghdr-skeleton.c
new file mode 100644
index 0000000000..4c6898569b
--- /dev/null
+++ b/socket/tst-cmsghdr-skeleton.c
@@ -0,0 +1,92 @@
+/* Test ancillary data header creation.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* We use the preprocessor to generate the function/macro tests instead of
+   using indirection because having all the macro expansions alongside
+   each other lets the compiler warn us about suspicious pointer
+   arithmetic across subsequent CMSG_{FIRST,NXT}HDR expansions.  */
+
+#include <stdint.h>
+
+#define RUN_TEST_CONCAT(suffix) run_test_##suffix
+#define RUN_TEST_FUNCNAME(suffix) RUN_TEST_CONCAT (suffix)
+
+static void
+RUN_TEST_FUNCNAME (CMSG_NXTHDR_IMPL) (void)
+{
+  struct msghdr m = {0};
+  struct cmsghdr *cmsg;
+  char cmsgbuf[3 * CMSG_SPACE (sizeof (PAYLOAD))] = {0};
+
+  m.msg_control = cmsgbuf;
+  m.msg_controllen = sizeof (cmsgbuf);
+
+  /* First header should point to the start of the buffer.  */
+  cmsg = CMSG_FIRSTHDR (&m);
+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
+
+  /* If the first header length consumes the entire buffer, there is no
+     space remaining for additional headers.  */
+  cmsg->cmsg_len = sizeof (cmsgbuf);
+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
+  TEST_VERIFY_EXIT (cmsg == NULL);
+
+  /* The first header length is so big, using it would cause an overflow.  */
+  cmsg = CMSG_FIRSTHDR (&m);
+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
+  cmsg->cmsg_len = SIZE_MAX;
+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
+  TEST_VERIFY_EXIT (cmsg == NULL);
+
+  /* The first header leaves just enough space to hold another header.  */
+  cmsg = CMSG_FIRSTHDR (&m);
+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
+  cmsg->cmsg_len = sizeof (cmsgbuf) - sizeof (struct cmsghdr);
+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
+  TEST_VERIFY_EXIT (cmsg != NULL);
+
+  /* The first header leaves space but not enough for another header.  */
+  cmsg = CMSG_FIRSTHDR (&m);
+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
+  cmsg->cmsg_len ++;
+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
+  TEST_VERIFY_EXIT (cmsg == NULL);
+
+  /* The second header leaves just enough space to hold another header.  */
+  cmsg = CMSG_FIRSTHDR (&m);
+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
+  cmsg->cmsg_len = CMSG_LEN (sizeof (PAYLOAD));
+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
+  TEST_VERIFY_EXIT (cmsg != NULL);
+  cmsg->cmsg_len = sizeof (cmsgbuf)
+                   - CMSG_SPACE (sizeof (PAYLOAD)) /* First header.  */
+                   - sizeof (struct cmsghdr);
+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
+  TEST_VERIFY_EXIT (cmsg != NULL);
+
+  /* The second header leaves space but not enough for another header.  */
+  cmsg = CMSG_FIRSTHDR (&m);
+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
+  TEST_VERIFY_EXIT (cmsg != NULL);
+  cmsg->cmsg_len ++;
+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
+  TEST_VERIFY_EXIT (cmsg == NULL);
+
+  return;
+}
diff --git a/socket/tst-cmsghdr.c b/socket/tst-cmsghdr.c
new file mode 100644
index 0000000000..68c96d3c9d
--- /dev/null
+++ b/socket/tst-cmsghdr.c
@@ -0,0 +1,56 @@
+/* Test ancillary data header creation.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/socket.h>
+#include <gnu/lib-names.h>
+#include <support/xdlfcn.h>
+#include <support/check.h>
+
+#define PAYLOAD "Hello, World!"
+
+/* CMSG_NXTHDR is a macro that calls an inline function defined in
+   bits/socket.h.  In case the function cannot be inlined, libc.so carries
+   a copy.  Both versions need to be tested.  */
+
+#define CMSG_NXTHDR_IMPL CMSG_NXTHDR
+#include "tst-cmsghdr-skeleton.c"
+#undef CMSG_NXTHDR_IMPL
+
+static struct cmsghdr * (* cmsg_nxthdr) (struct msghdr *, struct cmsghdr *);
+
+#define CMSG_NXTHDR_IMPL cmsg_nxthdr
+#include "tst-cmsghdr-skeleton.c"
+#undef CMSG_NXTHDR_IMPL
+
+static int
+do_test (void)
+{
+  static void *handle;
+
+  run_test_CMSG_NXTHDR ();
+
+  handle = xdlopen (LIBC_SO, RTLD_LAZY);
+  cmsg_nxthdr = (struct cmsghdr * (*) (struct msghdr *, struct cmsghdr *))
+                  xdlsym (handle, "__cmsg_nxthdr");
+
+  run_test_cmsg_nxthdr ();
+
+  return 0;
+}
+
+#include <support/test-driver.c>