Skip to content

Commit

Permalink
resolved: fix use-after-free with queries hitting the cache
Browse files Browse the repository at this point in the history 
When dns_transaction_complete() manages to resolve a query, it invalidates the
query candidate object. It shall not be accessed afterwards.

We have the following chain of calls:
dns_query_candidate_go → dns_transaction_go → dns_transaction_prepare → dns_cache_lookup (success: 1)
                                                                      → dns_transaction_complete
After returning back to dns_query_candidate_go(), we'd attempt to continue
iteration over the list of transactions attached to the query candidate,
accessing already freed (and overwritten) memory:

(gdb) bt
0  0x00007f637297cf47 in hashmap_iterate_entry (i=0x7ffe7e15cc90, h=0x706f746b73656465) at ../src/basic/hashmap.c:703
1  _hashmap_iterate (h=0x706f746b73656465, i=i@entry=0x7ffe7e15cc90, value=value@entry=0x7ffe7e15cc88,
    key=key@entry=0x0) at ../src/basic/hashmap.c:712
2  0x00007f637297d01b in set_iterate (s=<optimized out>, i=i@entry=0x7ffe7e15cc90, value=value@entry=0x7ffe7e15cc88)
    at ../src/basic/hashmap.c:733
hence we crash

3  0x0000557bc99eb80f in dns_query_candidate_go (c=c@entry=0x557bcaf86890) at ../src/resolve/resolved-dns-query.c:139
...but c is not valid here in the second iteration of the loop

4  0x0000557bc99eb720 in dns_query_candidate_notify (c=0x557bcaf86890) at ../src/resolve/resolved-dns-query.c:271
c was valid here at entry...

5  0x0000557bc99efe28 in dns_transaction_complete (t=0x557bcac072f0, state=<optimized out>)
    at ../src/resolve/resolved-dns-transaction.c:350
t is a valid transaction (11481 in the backtrace below)

6  0x0000557bc99f1efb in dns_transaction_process_reply (t=0x557bcac072f0, p=<optimized out>)
    at ../src/resolve/resolved-dns-transaction.c:1171
7  0x0000557bc99f2d41 in on_dns_packet (s=<optimized out>, fd=<optimized out>, revents=<optimized out>,
    userdata=0x557bcac072f0) at ../src/resolve/resolved-dns-transaction.c:1223
8  0x00007f6372a25217 in source_dispatch (s=s@entry=0x557bcb162c50) at ../src/libsystemd/sd-event/sd-event.c:3181
9  0x00007f6372a254fd in sd_event_dispatch (e=0x557bcb15b050) at ../src/libsystemd/sd-event/sd-event.c:3620
10 0x00007f6372a267c8 in sd_event_run (e=e@entry=0x557bcb15b050, timeout=timeout@entry=18446744073709551615)
    at ../src/libsystemd/sd-event/sd-event.c:3678
11 0x00007f6372a269ef in sd_event_loop (e=0x557bcb15b050) at ../src/libsystemd/sd-event/sd-event.c:3700
12 0x0000557bc99ddc14 in run (argc=<optimized out>, argv=<optimized out>) at ../src/resolve/resolved.c:92
13 0x0000557bc99d260a in main (argc=<optimized out>, argv=<optimized out>) at ../src/resolve/resolved.c:99

xxx.name.net systemd-resolved[31705]: Got message type=method_call sender=:1.3644 destination=org.freedesktop.resolve1 path=/org/freedesktop/resolve1 interface=org.freedesktop.resolve1.Manager member=ResolveHostname cookie=2 reply_cookie=0 signature=isit error-name=n/a error-message=n/a
xxx.name.net systemd-resolved[31705]: idn2_lookup_u8: xxx → xxx
xxx.name.net systemd-resolved[31705]: Looking up RR for xxx IN A.
xxx.name.net systemd-resolved[31705]: Sent message type=method_call sender=n/a destination=org.freedesktop.DBus path=/org/freedesktop/DBus interface=org.freedesktop.DBus member=AddMatch cookie=1102 reply_cookie=0 signature=s error-name=n/a error-message=n/a
xxx.name.net systemd-resolved[31705]: Sent message type=method_call sender=n/a destination=org.freedesktop.DBus path=/org/freedesktop/DBus interface=org.freedesktop.DBus member=GetNameOwner cookie=1103 reply_cookie=0 signature=s error-name=n/a error-message=n/a
xxx.name.net systemd-resolved[31705]: Got message type=method_return sender=org.freedesktop.DBus destination=:1.3324 path=n/a interface=n/a member=n/a cookie=4294967295 reply_cookie=1103 signature=s error-name=n/a error-message=n/a
xxx.name.net systemd-resolved[31705]: Cache miss for xxx.name.net IN A
xxx.name.net systemd-resolved[31705]: Transaction 11481 for <xxx.name.net IN A> scope dns on enp42s0/*.
xxx.name.net systemd-resolved[31705]: Using feature level UDP for transaction 11481.
xxx.name.net systemd-resolved[31705]: Using DNS server 192.168.1.1 for transaction 11481.
xxx.name.net systemd-resolved[31705]: Sending query packet with id 11481 of size 35.
xxx.name.net systemd-resolved[31705]: Got message type=method_return sender=org.freedesktop.DBus destination=:1.3324 path=n/a interface=n/a member=n/a cookie=4294967295 reply_cookie=1102 signature= error-name=n/a error-message=n/a
xxx.name.net systemd-resolved[31705]: Match type='signal',sender='org.freedesktop.DBus',path='/org/freedesktop/DBus',interface='org.freedesktop.DBus',member='NameOwnerChanged',arg0=':1.3644' successfully installed.
xxx.name.net systemd-resolved[31705]: Processing incoming packet on transaction 11481 (rcode=NXDOMAIN).
xxx.name.net systemd-resolved[31705]: Not caching negative entry without a SOA record: xxx.name.net IN A
xxx.name.net systemd-resolved[31705]: Transaction 11481 for <xxx.name.net IN A> on scope dns on enp42s0/* now complete with <rcode-failure> from network (unsigned).
xxx.name.net systemd-resolved[31705]: Positive cache hit for xxx.lan IN A
xxx.name.net systemd-resolved[31705]: Transaction 64364 for <xxx.lan IN A> on scope dns on enp42s0/* now complete with <success> from cache (unsigned).
xxx.name.net systemd-resolved[31705]: Sent message type=method_return sender=n/a destination=:1.3644 path=n/a interface=n/a member=n/a cookie=1104 reply_cookie=2 signature=a(iiay)st error-name=n/a error-message=n/a
xxx.name.net systemd-resolved[31705]: Sent message type=method_call sender=n/a destination=org.freedesktop.DBus path=/org/freedesktop/DBus interface=org.freedesktop.DBus member=RemoveMatch cookie=1105 reply_cookie=0 signature=s error-name=n/a error-message=n/a
xxx.name.net systemd-resolved[31705]: Freeing transaction 64364.
xxx.name.net systemd[1]: systemd-resolved.service: Main process exited, code=dumped, status=11/SEGV
xxx.name.net systemd[1]: systemd-resolved.service: Failed with result 'core-dump'.

Fixes #16168, https://bugzilla.redhat.com/show_bug.cgi?id=1895937.

(cherry picked from commit 4ea8b44)
  • Loading branch information
@keszybz
keszybz committed Feb 2, 2021
1 parent c97c62e commit 6431710
Show file tree
Hide file tree
Showing 2 changed files with 22 additions and 8 deletions.
14 changes: 12 additions & 2 deletions src/resolve/resolved-dns-query.c
View file
Expand Up @@ -133,23 +133,33 @@ static int dns_query_candidate_go(DnsQueryCandidate *c) {
DnsTransaction *t;
int r;
unsigned n = 0;
bool notify = false;

assert(c);

c->query->block_ready++;

/* Start the transactions that are not started yet */
SET_FOREACH(t, c->transactions) {
if (t->state != DNS_TRANSACTION_NULL)
continue;

r = dns_transaction_go(t);
if (r < 0)
if (r < 0) {
c->query->block_ready--;
return r;
}
if (r == 0)
/* A transaction is complete. */
notify = true;

n++;
}

c->query->block_ready--;

/* If there was nothing to start, then let's proceed immediately */
if (n == 0)
if (n == 0 || notify)
dns_query_candidate_notify(c);

return 0;
Expand Down
16 changes: 10 additions & 6 deletions src/resolve/resolved-dns-transaction.c
View file
Expand Up @@ -152,8 +152,8 @@ bool dns_transaction_gc(DnsTransaction *t) {
static uint16_t pick_new_id(Manager *m) {
uint16_t new_id;

/* Find a fresh, unused transaction id. Note that this loop is bounded because there's a limit on the number of
* transactions, and it's much lower than the space of IDs. */
/* Find a fresh, unused transaction id. Note that this loop is bounded because there's a limit on the
* number of transactions, and it's much lower than the space of IDs. */

assert_cc(TRANSACTIONS_MAX < 0xFFFF);

Expand Down Expand Up @@ -1338,6 +1338,10 @@ static int dns_transaction_prepare(DnsTransaction *t, usec_t ts) {

assert(t);

/* Returns 0 if dns_transaction_complete() has been called. In that case the transaction and query
* candidate objects may have been invalidated and must not be accessed. Returns 1 if the transaction
* has been prepared. */

dns_transaction_stop_timeout(t);

if (!dns_scope_network_good(t->scope)) {
Expand Down Expand Up @@ -1465,7 +1469,6 @@ static int dns_transaction_prepare(DnsTransaction *t, usec_t ts) {
}

static int dns_transaction_make_packet_mdns(DnsTransaction *t) {

_cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
bool add_known_answers = false;
DnsTransaction *other;
Expand Down Expand Up @@ -1633,8 +1636,9 @@ int dns_transaction_go(DnsTransaction *t) {

assert(t);

/* Returns > 0 if the transaction is now pending, returns 0 if could be processed immediately and has finished
* now. */
/* Returns > 0 if the transaction is now pending, returns 0 if could be processed immediately and has
* finished now. In the latter case, the transaction and query candidate objects must not be accessed.
*/

assert_se(sd_event_now(t->scope->manager->event, clock_boottime_or_monotonic(), &ts) >= 0);

Expand Down Expand Up @@ -1692,7 +1696,7 @@ int dns_transaction_go(DnsTransaction *t) {
t->state = DNS_TRANSACTION_PENDING;

log_debug("Delaying %s transaction for " USEC_FMT "us.", dns_protocol_to_string(t->scope->protocol), jitter);
return 0;
return 1;
}

/* Otherwise, we need to ask the network */
Expand Down

0 comments on commit 6431710

Please sign in to comment.