netdev - Re: [PATCH net v3] selftests: net: fix "buffer overflow detected" for tap.c

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Message-ID: <willemdebruijn.kernel.311f094b4d393@gmail.com>
Date: Tue, 16 Dec 2025 14:54:57 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: "Alice C. Munduruca" <alice.munduruca@...onical.com>, 
 netdev@...r.kernel.org
Cc: linux-kselftest@...r.kernel.org, 
 "Alice C. Munduruca" <alice.munduruca@...onical.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>, 
 Shuah Khan <shuah@...nel.org>, 
 Willem de Bruijn <willemb@...gle.com>, 
 Cengiz Can <cengiz.can@...onical.com>, 
 cbulinaru@...il.com
Subject: Re: [PATCH net v3] selftests: net: fix "buffer overflow detected" for
 tap.c

Alice C. Munduruca wrote:
> 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@...onical.com>
> Reviewed-by: Cengiz Can <cengiz.can@...onical.com>

Reviewed-by: Willem de Bruijn <willemb@...gle.com>

Cc: original author cbulinaru@...il.com in case we're overlooking a
reason for the split between rtattr_add_str and rtattr_add_strsz.

the first avoids the \0 and is used for IFLA_NAME. Device names are
guaranteed to fit into IFNAMSIZ, including the terminating \0.
> ---
>  tools/testing/selftests/net/tap.c | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/testing/selftests/net/tap.c b/tools/testing/selftests/net/tap.c
> index 247c3b3ac1c9..51a209014f1c 100644
> --- 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);
> -- 
> 2.48.1
>