From c1b7eef9fa4aaefbf7d0507505c3bb2914e1ad6b Mon Sep 17 00:00:00 2001

From: Daniel Axtens <dja@axtens.net>

Date: Thu, 16 Sep 2021 01:29:54 +1000

Subject: [PATCH 2/8] net/dns: Fix double-free addresses on corrupt DNS

 response

grub_net_dns_lookup() takes as inputs a pointer to an array of addresses

("addresses") for the given name, and pointer to a number of addresses

("naddresses"). grub_net_dns_lookup() is responsible for allocating

"addresses", and the caller is responsible for freeing it if

"naddresses" > 0.

The DNS recv_hook will sometimes set and free the addresses array,

for example if the packet is too short:

      if (ptr + 10 >= nb->tail)

	{

	  if (!*data->naddresses)

	    grub_free (*data->addresses);

	  grub_netbuff_free (nb);

	  return GRUB_ERR_NONE;

	}

Later on the nslookup command code unconditionally frees the "addresses"

array. Normally this is fine: the array is either populated with valid

data or is NULL. But in these sorts of error cases it is neither NULL

nor valid and we get a double-free.

Only free "addresses" if "naddresses" > 0.

It looks like the other use of grub_net_dns_lookup() is not affected.

Signed-off-by: Daniel Axtens <dja@axtens.net>

Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

---

 grub-core/net/dns.c | 6 ++++--

 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/grub-core/net/dns.c b/grub-core/net/dns.c

index 27c5f4142..841ede51e 100644

--- a/grub-core/net/dns.c

+++ b/grub-core/net/dns.c

@@ -667,9 +667,11 @@ grub_cmd_nslookup (struct grub_command *cmd __attribute__ ((unused)),

       grub_net_addr_to_str (&addresses[i], buf);

       grub_printf ("%s\n", buf);

     }

-  grub_free (addresses);

   if (naddresses)

-    return GRUB_ERR_NONE;

+    {

+      grub_free (addresses);

+      return GRUB_ERR_NONE;

+    }

   return grub_error (GRUB_ERR_NET_NO_DOMAIN, N_("no DNS record found"));

 }

-- 

2.33.5