[PATCH 4/4] efi: Fix use-after-free in finish boot services

[Alec Brown]

In grub-core/kern/efi/mm.c, grub_efi_finish_boot_services() has an instance
where the memory for the variable finish_mmap_buf is freed, but on the next
iteration of a while loop, grub_efi_get_memory_map() uses finish_mmap_buf. To
prevent this, we can set finish_mmap_buf to NULL after the free.

Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
---
 grub-core/kern/efi/mm.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/grub-core/kern/efi/mm.c b/grub-core/kern/efi/mm.c
index 3705b8b1b..c74ccbb05 100644
--- a/grub-core/kern/efi/mm.c
+++ b/grub-core/kern/efi/mm.c
@@ -263,6 +263,7 @@ grub_efi_finish_boot_services (grub_efi_uintn_t 
*outbuf_size, void *outbuf,
                                   &finish_desc_size, &finish_desc_version) <= 
0)
        {
          grub_free (finish_mmap_buf);
+         finish_mmap_buf = NULL;
          return grub_error (GRUB_ERR_IO, "couldn't retrieve memory map");
        }
 
@@ -275,10 +276,12 @@ grub_efi_finish_boot_services (grub_efi_uintn_t 
*outbuf_size, void *outbuf,
       if (status != GRUB_EFI_INVALID_PARAMETER)
        {
          grub_free (finish_mmap_buf);
+         finish_mmap_buf = NULL;
          return grub_error (GRUB_ERR_IO, "couldn't terminate EFI services");
        }
 
       grub_free (finish_mmap_buf);
+      finish_mmap_buf = NULL;
       grub_printf ("Trying to terminate EFI services again\n");
     }
   grub_efi_is_finished = 1;
-- 
2.27.0