From 3e4817538de828319ba6d59ced2fbb9b5ca13287 Mon Sep 17 00:00:00 2001

From: Daniel Axtens <dja@axtens.net>

Date: Mon, 20 Dec 2021 19:41:21 +1100

Subject: [PATCH] net/ip: Do IP fragment maths safely

We can receive packets with invalid IP fragmentation information. This

can lead to rsm->total_len underflowing and becoming very large.

Then, in grub_netbuff_alloc(), we add to this very large number, which can

cause it to overflow and wrap back around to a small positive number.

The allocation then succeeds, but the resulting buffer is too small and

subsequent operations can write past the end of the buffer.

Catch the underflow here.

Fixes: CVE-2022-28733

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

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

---

 grub-core/net/ip.c | 10 +++++++++-

 1 file changed, 9 insertions(+), 1 deletion(-)

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

index e3d62e97f..3c3d0be0e 100644

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

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

@@ -25,6 +25,7 @@

 #include <grub/net/netbuff.h>

 #include <grub/mm.h>

 #include <grub/priority_queue.h>

+#include <grub/safemath.h>

 #include <grub/time.h>

 struct iphdr {

@@ -512,7 +513,14 @@ grub_net_recv_ip4_packets (struct grub_net_buff *nb,

     {

       rsm->total_len = (8 * (grub_be_to_cpu16 (iph->frags) & OFFSET_MASK)

 			+ (nb->tail - nb->data));

-      rsm->total_len -= ((iph->verhdrlen & 0xf) * sizeof (grub_uint32_t));

+

+      if (grub_sub (rsm->total_len, (iph->verhdrlen & 0xf) * sizeof (grub_uint32_t),

+		    &rsm->total_len))

+	{

+	  grub_dprintf ("net", "IP reassembly size underflow\n");

+	  return GRUB_ERR_NONE;

+	}

+

       rsm->asm_netbuff = grub_netbuff_alloc (rsm->total_len);

       if (!rsm->asm_netbuff)

 	{

-- 

2.33.7