selftests: net: fix "buffer overflow detected" for tap.c

When the selftest 'tap.c' is compiled with '-D_FORTIFY_SOURCE=3',
the strcpy() in rtattr_add_strsz() is replaced with a checked
version which causes the test to consistently fail when compiled
with toolchains for which this option is enabled by default.

 TAP version 13
 1..3
 # Starting 3 tests from 1 test cases.
 #  RUN           tap.test_packet_valid_udp_gso ...
 *** buffer overflow detected ***: terminated
 # test_packet_valid_udp_gso: Test terminated by assertion
 #          FAIL  tap.test_packet_valid_udp_gso
 not ok 1 tap.test_packet_valid_udp_gso
 #  RUN           tap.test_packet_valid_udp_csum ...
 *** buffer overflow detected ***: terminated
 # test_packet_valid_udp_csum: Test terminated by assertion
 #          FAIL  tap.test_packet_valid_udp_csum
 not ok 2 tap.test_packet_valid_udp_csum
 #  RUN           tap.test_packet_crash_tap_invalid_eth_proto ...
 *** buffer overflow detected ***: terminated
 # test_packet_crash_tap_invalid_eth_proto: Test terminated by assertion
 #          FAIL  tap.test_packet_crash_tap_invalid_eth_proto
 not ok 3 tap.test_packet_crash_tap_invalid_eth_proto
 # FAILED: 0 / 3 tests passed.
 # Totals: pass:0 fail:3 xfail:0 xpass:0 skip:0 error:0

A buffer overflow is detected by the fortified glibc __strcpy_chk()
since the __builtin_object_size() of `RTA_DATA(rta)` is incorrectly
reported as 1, even though there is ample space in its bounding
buffer `req`.

Additionally, given that IFLA_IFNAME also expects a null-terminated
string, callers of rtaddr_add_str{,sz}() could simply use the
rtaddr_add_strsz() variant. (which has been renamed to remove the
trailing `sz`) memset() has been used for this function since it
is unchecked and thus circumvents the issue discussed in the
previous paragraph.

Fixes: 2e64fe4624d1 ("selftests: add few test cases for tap driver")
Signed-off-by: Alice C. Munduruca <alice.munduruca@canonical.com>
Reviewed-by: Cengiz Can <cengiz.can@canonical.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20251216170641.250494-1-alice.munduruca@canonical.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/tools/testing/selftests/net/tap.c b/tools/testing/selftests/net/tap.c
index 9ec1c9b50e7729e6d271fff4b13720c76ee15db3..a0c9418132c82d27a325cd4bf9d78a23e97f66e0 100644 (file)
--- a/tools/testing/selftests/net/tap.c
+++ b/tools/testing/selftests/net/tap.c
@@ -56,18 +56,12 @@ static void rtattr_end(struct nlmsghdr *nh, struct rtattr *attr)
 static struct rtattr *rtattr_add_str(struct nlmsghdr *nh, unsigned short type,
                                     const char *s)
 {
-       struct rtattr *rta = rtattr_add(nh, type, strlen(s));
+       unsigned int strsz = strlen(s) + 1;
+       struct rtattr *rta;
 
-       memcpy(RTA_DATA(rta), s, strlen(s));
-       return rta;
-}
-
-static struct rtattr *rtattr_add_strsz(struct nlmsghdr *nh, unsigned short type,
-                                      const char *s)
-{
-       struct rtattr *rta = rtattr_add(nh, type, strlen(s) + 1);
+       rta = rtattr_add(nh, type, strsz);
 
-       strcpy(RTA_DATA(rta), s);
+       memcpy(RTA_DATA(rta), s, strsz);
        return rta;
 }
 
@@ -119,7 +113,7 @@ static int dev_create(const char *dev, const char *link_type,
 
        link_info = rtattr_begin(&req.nh, IFLA_LINKINFO);
 
-       rtattr_add_strsz(&req.nh, IFLA_INFO_KIND, link_type);
+       rtattr_add_str(&req.nh, IFLA_INFO_KIND, link_type);
 
        if (fill_info_data) {
                info_data = rtattr_begin(&req.nh, IFLA_INFO_DATA);