[SECURITY PATCH 16/30] net/ip: Do IP fragment maths safely

[Daniel Kiper]

From: Daniel Axtens <dja@axtens.net>

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)
        {
-- 
2.11.0