[PATCH v2 2/2] resolv: Check hostname for validity (CVE-2026-4438)
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Wed Mar 25 14:32:40 GMT 2026
On 20/03/26 18:52, Carlos O'Donell wrote:
> The processed hostname in getanswer_ptr should be correctly checked to
> avoid invalid characters from being allowed, including shell
> metacharacters. It is a security issue to fail to check the returned
> hostname for validity.
>
> A regression test is added for invalid metacharacters and other cases
> of invalid or valid characters.
>
> No regressions on x86_64-linux-gnu.
LGTM, thanks. Just minor suggestions below.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> ---
> v1 -> v2
> - Split out changes for CVE-2026-4438
> - Incorporate Florian's suggestions.
> - Remove not-needed xmemstream.h included.
> - Drop superflous printf in test loop (too verbose).
> - A/B tested changes again to review error messages on failure.
>
>
> resolv/Makefile | 3 +
> resolv/nss_dns/dns-host.c | 2 +-
> resolv/tst-resolv-invalid-ptr.c | 254 ++++++++++++++++++++++++++++++++
> 3 files changed, 258 insertions(+), 1 deletion(-)
> create mode 100644 resolv/tst-resolv-invalid-ptr.c
>
> diff --git a/resolv/Makefile b/resolv/Makefile
> index 95bad5df27..971608eff5 100644
> --- a/resolv/Makefile
> +++ b/resolv/Makefile
> @@ -117,6 +117,7 @@ tests += \
> tst-resolv-dns-section \
> tst-resolv-edns \
> tst-resolv-invalid-cname \
> + tst-resolv-invalid-ptr \
> tst-resolv-network \
> tst-resolv-noaaaa \
> tst-resolv-noaaaa-vc \
> @@ -312,6 +313,8 @@ $(objpfx)tst-resolv-res_init-thread: $(objpfx)libresolv.so \
> $(shared-thread-library)
> $(objpfx)tst-resolv-invalid-cname: $(objpfx)libresolv.so \
> $(shared-thread-library)
> +$(objpfx)tst-resolv-invalid-ptr: $(objpfx)libresolv.so \
> + $(shared-thread-library)
> $(objpfx)tst-resolv-no-search: $(objpfx)libresolv.so $(shared-thread-library)
> $(objpfx)tst-resolv-noaaaa: $(objpfx)libresolv.so $(shared-thread-library)
> $(objpfx)tst-resolv-noaaaa-vc: $(objpfx)libresolv.so $(shared-thread-library)
> diff --git a/resolv/nss_dns/dns-host.c b/resolv/nss_dns/dns-host.c
> index 893137027e..728dae615d 100644
> --- a/resolv/nss_dns/dns-host.c
> +++ b/resolv/nss_dns/dns-host.c
> @@ -866,7 +866,7 @@ getanswer_ptr (unsigned char *packet, size_t packetlen,
> char hname[MAXHOSTNAMELEN + 1];
> if (__ns_name_unpack (c.begin, c.end, rr.rdata,
> name_buffer, sizeof (name_buffer)) < 0
> - || !__res_binary_hnok (expected_name)
> + || !__res_binary_hnok (name_buffer)
> || __ns_name_ntop (name_buffer, hname, sizeof (hname)) < 0)
> {
> *h_errnop = NO_RECOVERY;
Ok.
> diff --git a/resolv/tst-resolv-invalid-ptr.c b/resolv/tst-resolv-invalid-ptr.c
> new file mode 100644
> index 0000000000..4671935795
> --- /dev/null
> +++ b/resolv/tst-resolv-invalid-ptr.c
> @@ -0,0 +1,254 @@
> +/* Test handling of invalid T_PTR results (bug 34015).
> + Copyright (C) 2022-2026 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 <array_length.h>
> +#include <errno.h>
> +#include <netdb.h>
> +#include <resolv.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <support/check.h>
> +#include <support/format_nss.h>
> +#include <support/resolv_test.h>
> +#include <support/support.h>
> +
> +/* Name of test, the answer, the expected error return, and if we
> + expect the call to fail. */
> +struct item {
> + const char *test;
> + const char *answer;
> + int expected;
> + bool fail;
> +};
> +
> +static const struct item test_items[] =
> + {
> + /* Test for invalid characters. */
> + { "Invalid use of \"|\"",
> + "test.|.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"&\"",
> + "test.&.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \";\"",
> + "test.;.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"<\"",
> + "test.<.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \">\"",
> + "test.>.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"(\"",
> + "test.(.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \")\"",
> + "test.).ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"$\"",
> + "test.$.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"`\"",
> + "test.`.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"\\\"",
> + "test.\\.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"\'\"",
> + "test.'.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"\"\"",
> + "test.\".ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \" \"",
> + "test. .ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"\\t\"",
> + "test.\t.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"\\n\"",
> + "test.\n.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"\\r\"",
> + "test.\r.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"*\"",
> + "test.*.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"?\"",
> + "test.?.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"[\"",
> + "test.[.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"]\"",
> + "test.].ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \",\"",
> + "test.,.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"~\"",
> + "test.~.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \":\"",
> + "test.:.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"!\"",
> + "test.!.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"@\"",
> + "test.@.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"#\"",
> + "test.#.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"%\"",
> + "test.%%.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of \"^\"",
> + "test.^.ptr.example", NO_RECOVERY, true },
> +
> + /* Test for invalid UTF-8 characters (2-byte, 4-byte, 6-byte). */
> + { "Invalid use of UTF-8 (2-byte, U+00C0-U+00C2)",
> + "ÁÂÃ.test.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of UTF-8 (4-byte, U+0750-U+0752)",
> + "ݐݑݒ.test.ptr.example", NO_RECOVERY, true },
> + { "Invalid use of UTF-8 (6-byte, U+0904-U+0906)",
> + "ऄअआ.test.ptr.example", NO_RECOVERY, true },
> +
> + /* Test for "-" which may be valid depending on position. */
> + { "Invalid leading \"-\"",
> + "-test.ptr.example", NO_RECOVERY, true },
> + { "Valid trailing \"-\"",
> + "test-.ptr.example", 0, false },
> + { "Valid mid-label use of \"-\"",
> + "te-st.ptr.example", 0, false },
> +
> + /* Test for "_" which is always valid in any position. */
> + { "Valid leading use of \"_\"",
> + "_test.ptr.example", 0, false },
> + { "Valid mid-label use of \"_\"",
> + "te_st.ptr.example", 0, false },
> + { "Valid trailing use of \"_\"",
> + "test_.ptr.example", 0, false },
> +
> + /* Sanity test the broader set [A-Za-z0-9_-] of valid characters. */
> + { "Valid \"[A-Z]\"",
> + "test.ABCDEFGHIJKLMNOPQRSTUVWXYZ.ptr.example", 0, false },
> + { "Valid \"[a-z]\"",
> + "test.abcdefghijklmnopqrstuvwxyz.ptr.example", 0, false },
> + { "Valid \"[0-9]\"",
> + "test.0123456789.ptr.example", 0, false },
> + { "Valid mixed use of \"[A-Za-z0-9_-]\"",
> + "test.012abcABZ_-.ptr.example", 0, false },
> +
> + { NULL, 0, 0 },
You don't really this sentinel value.
> + };
> +
> +static void
> +response (const struct resolv_response_context *ctx,
> + struct resolv_response_builder *b,
> + const char *qname, uint16_t qclass, uint16_t qtype)
> +{
> + TEST_COMPARE (qclass, C_IN);
> +
> + /* We only test PTR. */
> + TEST_COMPARE (qtype, T_PTR);
> +
> + unsigned int count, count1;
> + char *tail = NULL;
> +
> + if (strstr (qname, "in-addr.arpa") != NULL
> + && sscanf (qname, "%u.%ms", &count, &tail) == 2)
> + TEST_COMPARE_STRING (tail, "0.168.192.in-addr.arpa");
> + else if (sscanf (qname, "%x.%x.%ms", &count, &count1, &tail) == 3)
> + {
> + TEST_COMPARE_STRING (tail, "\
> +0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa");
> + count |= count1 << 4;
> + }
> + else
> + FAIL_EXIT1 ("invalid QNAME: %s\n", qname);
> + free (tail);
> +
> + /* We have a bounded number of possible tests. */
> + TEST_VERIFY (count >= 0);
> + TEST_VERIFY (count <= 255);
Maybe use array_length (test_items).
> +
> + struct resolv_response_flags flags = {};
> + resolv_response_init (b, flags);
> + resolv_response_add_question (b, qname, qclass, qtype);
> + resolv_response_section (b, ns_s_an);
> +
> + /* Actual answer record. */
> + resolv_response_open_record (b, qname, qclass, qtype, 60);
> +
> + /* Record the answer. */
> + resolv_response_add_name (b, test_items[count].answer);
> + resolv_response_close_record (b);
> +}
> +
> +/* Perform one check using a reverse lookup. */
> +static void
> +check_reverse (int af, int count)
> +{
> + TEST_VERIFY (af == AF_INET || af == AF_INET6);
> + TEST_VERIFY (count < array_length (test_items));
> +
> + /* Generate an address to query for each test. */
> + char addr[sizeof (struct in6_addr)] = { 0 };
> + socklen_t addrlen;
> + if (af == AF_INET)
> + {
> + addr[0] = (char) 192;
> + addr[1] = (char) 168;
> + addr[2] = (char) 0;
> + addr[3] = (char) count;
> + addrlen = 4;
> + }
> + else
> + {
> + addr[0] = 0x20;
> + addr[1] = 0x01;
> + addr[2] = 0x0d;
> + addr[3] = 0xb8;
> + addr[4] = addr[5] = addr[6] = addr[7] = 0x0;
> + addr[8] = addr[9] = addr[10] = addr[11] = 0x0;
> + addr[12] = 0x0;
> + addr[13] = 0x0;
> + addr[14] = 0x0;
> + addr[15] = (char) count;
> + addrlen = 16;
> + }
> +
> + h_errno = 0;
> + struct hostent *answer = gethostbyaddr (addr, addrlen, af);
> +
> + /* Verify h_errno is as expected. */
> + TEST_COMPARE (h_errno, test_items[count].expected);
> + if (h_errno != test_items[count].expected)
> + /* And print more information if it's not. */
> + printf ("INFO: %s\n", test_items[count].test);
> +
> + if (test_items[count].fail)
> + {
> + /* We expected a failure so verify answer is NULL. */
> + TEST_VERIFY (answer == NULL);
> + /* If it's not NULL we should print out what we received. */
> + if (answer != NULL)
> + printf ("error: unexpected success: %s\n",
> + support_format_hostent (answer));
> + }
> + else
> + /* We don't expect a failure so answer must be valid. */
> + TEST_COMPARE_STRING (answer->h_name, test_items[count].answer);
> +}
> +
> +static int
> +do_test (void)
> +{
> + struct resolv_test *obj = resolv_test_start
> + ((struct resolv_redirect_config)
> + {
> + .response_callback = response
> + });
> +
> + for (int i = 0; test_items[i].test != NULL ; i++)
Maybe use
for (int i = 0; i < array_length (test_items); i++)
> + {
> + check_reverse (AF_INET, i);
> + check_reverse (AF_INET6, i);
> + }
> + resolv_test_end (obj);
> +
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
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