[SECURITY PATCH 022/117] kern/efi/mm: Fix possible NULL pointer dereference

[Daniel Kiper]

From: Darren Kenny <darren.kenny@oracle.com>

The model of grub_efi_get_memory_map() is that if memory_map is NULL,
then the purpose is to discover how much memory should be allocated to
it for the subsequent call.

The problem here is that with grub_efi_is_finished set to 1, there is no
check at all that the function is being called with a non-NULL memory_map.

While this MAY be true, we shouldn't assume it.

The solution to this is to behave as expected, and if memory_map is NULL,
then don't try to use it and allow memory_map_size to be filled in, and
return 0 as is done later in the code if the buffer is too small (or NULL).

Additionally, drop unneeded ret = 1.

Fixes: CID 96632

Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/kern/efi/mm.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/grub-core/kern/efi/mm.c b/grub-core/kern/efi/mm.c
index 457772d57..8d380c054 100644
--- a/grub-core/kern/efi/mm.c
+++ b/grub-core/kern/efi/mm.c
@@ -339,16 +339,25 @@ grub_efi_get_memory_map (grub_efi_uintn_t 
*memory_map_size,
   if (grub_efi_is_finished)
     {
       int ret = 1;
-      if (*memory_map_size < finish_mmap_size)
+
+      if (memory_map != NULL)
        {
-         grub_memcpy (memory_map, finish_mmap_buf, *memory_map_size);
+         if (*memory_map_size < finish_mmap_size)
+           {
+             grub_memcpy (memory_map, finish_mmap_buf, *memory_map_size);
+             ret = 0;
+           }
+          else
+           grub_memcpy (memory_map, finish_mmap_buf, finish_mmap_size);
+       }
+      else
+       {
+         /*
+          * Incomplete, no buffer to copy into, same as
+          * GRUB_EFI_BUFFER_TOO_SMALL below.
+          */
          ret = 0;
        }
-      else
-       {
-         grub_memcpy (memory_map, finish_mmap_buf, finish_mmap_size);
-         ret = 1;
-       }
       *memory_map_size = finish_mmap_size;
       if (map_key)
        *map_key = finish_key;
-- 
2.11.0