In various error scenarios (for example, if the server closes the
TCP connection before sending the full response), send_vc can return
without resetting the *resplen2 value. This can pass uninitialized
or unexpected data to the caller.
Since commit 44d20bca52 (Implement
second fallback mode for DNS requests), there is a code path which
returns early, before *resplen2 is initialized. This happens if the
name server address is immediately recognized as invalid (because of
lack of protocol support, or if it is a broadcast address such
255.255.255.255, or another invalid address).
If this happens and *resplen2 was non-zero (which is the case if a
previous query resulted in a failure), __libc_res_nquery would reuse
an existing second answer buffer. This answer has been previously
identified as unusable (for example, it could be an NXDOMAIN
response). Due to the presence of a second answer, no name server
switching will occur. The result is a name resolution failure,
although a successful resolution would have been possible if name
servers have been switched and queries had proceeded along the search
path.
The above paragraph still simplifies the situation. Before glibc
2.23, if the second answer needed malloc, the stub resolver would
still attempt to reuse the second answer, but this is not possible
because __libc_res_nsearch has freed it, after the unsuccessful call
to __libc_res_nquerydomain, and set the buffer pointer to NULL. This
eventually leads to an assertion failure in __libc_res_nquery:
/* Make sure both hp and hp2 are defined */
assert((hp != NULL) && (hp2 != NULL));
If assertions are disabled, the consequence is a NULL pointer
dereference on the next line.
Starting with glibc 2.23, as a result of commit
e9db92d3ac (CVE-2015-7547: getaddrinfo()
stack-based buffer overflow (Bug 18665)), the second answer is always
allocated with malloc. This means that the assertion failure happens
with small responses as well because there is no buffer to reuse, as
soon as there is a name resolution failure which triggers a search for
an answer along the search path.
This commit addresses the issue by ensuring that *resplen2 is
initialized before the send_dg function returns.
This commit also addresses a bug where an invalid second reply is
incorrectly returned as a valid to the caller.
* A stack-based buffer overflow was found in libresolv when invoked from
libnss_dns, allowing specially crafted DNS responses to seize control
of execution flow in the DNS client. The buffer overflow occurs in
the functions send_dg (send datagram) and send_vc (send TCP) for the
NSS module libnss_dns.so.2 when calling getaddrinfo with AF_UNSPEC
family. The use of AF_UNSPEC triggers the low-level resolver code to
send out two parallel queries for A and AAAA. A mismanagement of the
buffers used for those queries could result in the response of a query
writing beyond the alloca allocated buffer created by
_nss_dns_gethostbyname4_r. Buffer management is simplified to remove
the overflow. Thanks to the Google Security Team and Red Hat for
reporting the security impact of this issue, and Robert Holiday of
Ciena for reporting the related bug 18665. (CVE-2015-7547)
See also:
https://sourceware.org/ml/libc-alpha/2016-02/msg00416.htmlhttps://sourceware.org/ml/libc-alpha/2016-02/msg00418.html
One common case of __GNUC_PREREQ (4, 7) conditionals is use of
diagnostic control pragmas for -Wmaybe-uninitialized, an option
introduced in GCC 4.7 where older GCC needed -Wuninitialized to be
controlled instead if the warning appeared with older GCC. This patch
removes such conditionals.
(There remain several older uses of -Wno-uninitialized in makefiles
that still need to be converted to diagnostic control pragmas if the
issue is still present with current sources and supported GCC
versions, and it's likely that in most cases those pragmas also will
end up controlling -Wmaybe-uninitialized.)
Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch, except for libresolv
since res_send.c contains assertions whose line numbers are changed by
the patch).
* resolv/res_send.c (send_vc) [__GNUC_PREREQ (4, 7)]: Make code
unconditional.
* soft-fp/fmadf4.c [__GNUC_PREREQ (4, 7)]: Likewise.
[!__GNUC_PREREQ (4, 7)]: Remove conditional code.
* soft-fp/fmasf4.c [__GNUC_PREREQ (4, 7)]: Make code
unconditional.
[!__GNUC_PREREQ (4, 7)]: Remove conditional code.
* soft-fp/fmatf4.c [__GNUC_PREREQ (4, 7)]: Make code
unconditional.
[!__GNUC_PREREQ (4, 7)]: Remove conditional code.
* stdlib/setenv.c
[((__GNUC__ << 16) + __GNUC_MINOR__) >= ((4 << 16) + 7)]: Make
code unconditional.
[!(((__GNUC__ << 16) + __GNUC_MINOR__) >= ((4 << 16) + 7))]:
Remove conditional code.
* sysdeps/ieee754/dbl-64/e_lgamma_r.c
(__ieee754_lgamma_r) [__GNUC_PREREQ (4, 7)]: Make code
unconditional.
(__ieee754_lgamma_r) [!__GNUC_PREREQ (4, 7)]: Remove conditional
code.
* sysdeps/ieee754/flt-32/e_lgammaf_r.c
(__ieee754_lgammaf_r) [__GNUC_PREREQ (4, 7)]: Make code
unconditional.
(__ieee754_lgammaf_r) [!__GNUC_PREREQ (4, 7)]: Remove conditional
code.
* sysdeps/ieee754/ldbl-128/k_tanl.c
(__kernel_tanl) [__GNUC_PREREQ (4, 7)]: Make code unconditional.
(__kernel_tanl) [!__GNUC_PREREQ (4, 7)]: Remove conditional code.
* sysdeps/ieee754/ldbl-128ibm/k_tanl.c
(__kernel_tanl) [__GNUC_PREREQ (4, 7)]: Make code unconditional.
(__kernel_tanl) [!__GNUC_PREREQ (4, 7)]: Remove conditional code.
* sysdeps/ieee754/ldbl-96/e_lgammal_r.c
(__ieee754_lgammal_r) [__GNUC_PREREQ (4, 7)]: Make code
unconditional.
(__ieee754_lgammal_r) [!__GNUC_PREREQ (4, 7)]: Remove conditional
code.
* sysdeps/ieee754/ldbl-96/k_tanl.c
(__kernel_tanl) [__GNUC_PREREQ (4, 7)]: Make code unconditional.
(__kernel_tanl) [!__GNUC_PREREQ (4, 7)]: Remove conditional code.
Remove use of ext.nsmap member of struct __res_state and always use
an identity mapping betwen the nsaddr_list array and the ext.nsaddrs
array. The fact that a nameserver has an IPv6 address is signalled by
setting nsaddr_list[].sin_family to zero.
I see this warning in my build on F21 x86_64, which seems to be due to
a weak check for array bounds. Fixed by making the bounds check
stronger.
This is not an actual bug since nscount is never set to anything
greater than MAXNS. The compiler however does not know this, so we
need the stronger bounds check to quieten the compiler.
In send_vc function at resolv/res_send.c, There is the
following warning on some architectures:
'resplen' may be used uninitialized in this function
[-Wmaybe-uninitialized]
And this is a false positive. This patch suppress the
compiler warning.
[Fixes BZ #14308, #12994, #13651]
AF_UNSPEC results in sending two queries in parallel, one for the A
record and the other for the AAAA record. If one of these is a
referral, then the query fails, which is wrong. It should return at
least the one successful response.
The fix has two parts. The first part makes the referral fall back to
the SERVFAIL path, which results in using the successful response.
There is a bug in that path however, due to which the second part is
necessary. The bug here is that if the first response is a failure
and the second succeeds, __libc_res_nsearch does not detect that and
assumes a failure. The case where the first response is a success and
the second fails, works correctly.
This condition is produced by buggy routers, so here's a crude
interposable library that can simulate such a condition. The library
overrides the recvfrom syscall and modifies the header of the packet
received to reproduce this scenario. It has two key variables:
mod_packet and first_error.
The mod_packet variable when set to 0, results in odd packets being
modified to be a referral. When set to 1, even packets are modified
to be a referral.
The first_error causes the first response to be a failure so that a
domain-appended search is performed to test the second part of the
__libc_nsearch fix.
The driver for this fix is a simple getaddrinfo program that does an
AF_UNSPEC query. I have omitted this since it should be easy to
implement.
I have tested this on x86_64.
The interceptor library source:
/* Override recvfrom and modify the header of the first DNS response to make it
a referral and reproduce bz #845218. We have to resort to this ugly hack
because we cannot make bind return the buggy response of a referral for the
AAAA record and an authoritative response for the A record. */
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdbool.h>
#include <endian.h>
#include <dlfcn.h>
#include <stdlib.h>
/* Lifted from resolv/arpa/nameser_compat.h. */
typedef struct {
unsigned id :16; /*%< query identification number */
#if BYTE_ORDER == BIG_ENDIAN
/* fields in third byte */
unsigned qr: 1; /*%< response flag */
unsigned opcode: 4; /*%< purpose of message */
unsigned aa: 1; /*%< authoritive answer */
unsigned tc: 1; /*%< truncated message */
unsigned rd: 1; /*%< recursion desired */
/* fields
* in
* fourth
* byte
* */
unsigned ra: 1; /*%< recursion available */
unsigned unused :1; /*%< unused bits (MBZ as of 4.9.3a3) */
unsigned ad: 1; /*%< authentic data from named */
unsigned cd: 1; /*%< checking disabled by resolver */
unsigned rcode :4; /*%< response code */
#endif
#if BYTE_ORDER == LITTLE_ENDIAN || BYTE_ORDER == PDP_ENDIAN
/* fields
* in
* third
* byte
* */
unsigned rd :1; /*%< recursion desired */
unsigned tc :1; /*%< truncated message */
unsigned aa :1; /*%< authoritive answer */
unsigned opcode :4; /*%< purpose of message */
unsigned qr :1; /*%< response flag */
/* fields
* in
* fourth
* byte
* */
unsigned rcode :4; /*%< response code */
unsigned cd: 1; /*%< checking disabled by resolver */
unsigned ad: 1; /*%< authentic data from named */
unsigned unused :1; /*%< unused bits (MBZ as of 4.9.3a3) */
unsigned ra :1; /*%< recursion available */
#endif
/* remaining
* bytes
* */
unsigned qdcount :16; /*%< number of question entries */
unsigned ancount :16; /*%< number of answer entries */
unsigned nscount :16; /*%< number of authority entries */
unsigned arcount :16; /*%< number of resource entries */
} HEADER;
static int done = 0;
/* Packets to modify. 0 for the odd packets and 1 for even packets. */
static const int mod_packet = 0;
/* Set to true if the first request should result in an error, resulting in a
search query. */
static bool first_error = true;
static ssize_t (*real_recvfrom) (int sockfd, void *buf, size_t len, int flags,
struct sockaddr *src_addr, socklen_t *addrlen);
void
__attribute__ ((constructor))
init (void)
{
real_recvfrom = dlsym (RTLD_NEXT, "recvfrom");
if (real_recvfrom == NULL)
{
printf ("Failed to get reference to recvfrom: %s\n", dlerror ());
printf ("Cannot simulate test\n");
abort ();
}
}
/* Modify the second packet that we receive to set the header in a manner as to
reproduce BZ #845218. */
static void
mod_buf (HEADER *h, int port)
{
if (done % 2 == mod_packet || (first_error && done == 1))
{
printf ("(Modifying header)");
if (first_error && done == 1)
h->rcode = 3;
else
h->rcode = 0; /* NOERROR == 0. */
h->ancount = 0;
h->aa = 0;
h->ra = 0;
h->arcount = 0;
}
done++;
}
ssize_t
recvfrom (int sockfd, void *buf, size_t len, int flags,
struct sockaddr *src_addr, socklen_t *addrlen)
{
ssize_t ret = real_recvfrom (sockfd, buf, len, flags, src_addr, addrlen);
int port = htons (((struct sockaddr_in *) src_addr)->sin_port);
struct in_addr addr = ((struct sockaddr_in *) src_addr)->sin_addr;
const char *host = inet_ntoa (addr);
printf ("\n*** From %s:%d: ", host, port);
mod_buf (buf, port);
printf ("returned %zd\n", ret);
return ret;
}
This patch defines _STRING_ARCH_unaligned to 0 on default bits/string.h
header to avoid undefined compiler warnings on platforms that do not
define it. It also make adjustments in code where tests checked if macro
existed or not.
There is some more shardware/software out there which has problems
if two DNS requests are sent using the same tuple
(source addr, source port, dest addr, dest port)
This can range from firewalls to load balancers. Some of the vendors
already fixed it in response to this problem. Still, we need a way
to make glibc work with broken environments. The single-request-reopen
flag can be used or we fall back automatically to this mode.
* resolv/res_init.c (res_setoptions): Recognize single-request option.
* resolv/res_send.c (send_dg): If we sent two requests at once and
only get one reply before timeout switch to mode where we send the
second request only after the first answer has been received.
Use it instead of locally defined resplen2 variable.
(res_nsend): Adjust for __libc_res_nsend interface change.
(send_vc): Initialize *resplen2 if necessary. Read length of
package into an appropriately aligned variable. Store converted length
in new variable and use it appropriately.
Add branch prediction help.
* resolv/res_query.c (__libc_res_nquery): Take additional parameter
and pass it on to __libc_res_nsend. Adjust all callers.
(__libc_res_nsearch): Likewise.
(__libc_res_nqeurydomain): Likewise.
* resolv/nss_dns/dns-host.c: Adjust for __libc_res_nsearch interface
change.
(_nss_dns_gethostbyname4): Don't unconditionally allocate tmp array.
Define resplen2 variable and pass it to __libc_res_nsearch and then
to gaih_getanswer.
(getanswer_r): In case of incorrect DNS data don't overread buffer.
Add branch prediction.
(gaih_getanswer_slice): Likewise. Check for invalid data types.
(gaih_getanswer): Don't decode second slice if first one failed due
to a too small buffer. Don't let not found status of second
decoder shadow results of the first.
* resolv/gethnamaddr.c (gethostbyname2): Adjust for __libc_res_nsearch
and __libc_res_nquery interface changes
(gethostbyaddr): Adjust for __libc_res_nquery interface change.
* include/resolv.h: Adjust prototypes for __libc_res_nquery,
__libc_res_nsearch, and __libc_res_nsend.
* resolv/nss_dns/dns-canon.c: Adjust for __libc_res_nquery interface
change.
* resolv/nss_dns/dns-network.c: Adjust for __libc_res_nquery and
__libc_res_nsearch interface changes.
answer was too short don't try to read that answer's header.
* resolv/res_send.c (send_dg): In case of timeout and there are
two queries and one has been answered, return value indicating
success.
prototypes.
* include/arpa/nameser_compat.h: Define T_UNSPEC.
* nis/Versions (libnss_nis): Export _nss_nis_gethostbyname4_r.
(libnss_nisplus): Export _nss_nisplus_gethostbyname4_r.
* nis/nss_nis/nis-hosts.c (LINE_PARSER): Change to also handle
af==AF_UNSPEC.
(_nss_nis_gethostbyname4_r): New function.
* nis/nss_nisplus/nisplus-hosts.c (_nss_nisplus_parse_hostent):
Change to also handle af==AF_UNSPEC.
(get_tablename): New function. Use it to avoid duplication.
(_nss_nisplus_gethostbyname4_r): New function.
* nscd/aicache.c (addhstaiX): Use gethostbyname4_r function is
available.
* nss/Versions (libnss_files): Export _nss_files_gethostbyname4_r.
* nss/nss.h: Define struct gaih_addrtuple.
* nss/nss_files/files-hosts.c (LINE_PARSER): Change to also handle
af==AF_UNSPEC.
(_nss_files_gethostbyname4_r): New function.
* resolv/Versions (libnss_dns): Export _nss_dns_gethostbyname4_r.
* resolv/gethnmaddr.c: Adjust __libc_res_nsearch and __libc_res_nquery
calls.
* resolv/res_query.c (__libc_res_nquery): Take two additional
parameters for second answer buffer. Handle type=T_UNSPEC to mean
look up IPv4 and IPv6.
Change all callers.
* resolv/res_send.c (__libc_res_nsend): Take five aditional parameters
for an additional query and answer buffer. Pass to send_vc and
send_dg.
(send_vc): Send possibly two requests and receive two answers.
(send_dg): Likewise.
* resolv/nss_dns/dns-host.c: Adjust calls to __libc_res_nsearch and
__libc_res_nquery.
(_nss_dns_gethostbyname4_r): New function.
(gaih_getanswer_slice): Likewise.
(gaih_getanswer): Likewise.
* resolv/nss_dns/dns-canon.c (_nss_dns_getcanonname_r): Adjust
__libc_res_nquery call.
* resolv/nss_dns/dns-network.c (_nss_dns_getnetbyaddr_r): Likewise.
(_nss_dns_getnetbyname_r): Adjust __libc_res_nsearch call.
* sysdeps/posix/getaddrinfo.c: Use gethostbyname4_r function is
available.
Tomas Janousek <tjanouse@redhat.com>
Ulrich Drepper <drepper@redhat.com>
[BZ #4647]
* resolv/res_send.c (send_dg): Remove socket_pf. Use ipv6_unavail
member in __res_state, only convaddr4to6 if nssocks[ns] is a PF_INET6
socket.
* resolv/resolv.h (__res_state): Add ipv6_unavail member. Make
unused member a bitmap.
* resolv/res_init.c (__res_vinit): Reset ipv6_unavail if IPv6
servers are configured.
2007-06-18 Jakub Jelinek <jakub@redhat.com>
