grub: add a fix for CVE-2020-25632

Fix grub issue with module dereferencing. From the official description
from NVD [1]:

   The rmmod implementation allows the unloading of a module used as
   a dependency without checking if any other dependent module is still
   loaded leading to a use-after-free scenario. This could allow
   arbitrary code to be executed or a bypass of Secure Boot protections.

This patch is a part of a bigger security collection for grub [2].

[1] https://nvd.nist.gov/vuln/detail/CVE-2020-25632
[2] https://lists.gnu.org/archive/html/grub-devel/2021-03/msg00007.html

(From OE-Core rev: d61b9588e5691ef390cfc0f03dc6cb0d142f36de)

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-bsp/grub/files/CVE-2020-25632.patch

@@ -0,0 +1,90 @@
+From 7630ec5397fe418276b360f9011934b8c034936c Mon Sep 17 00:00:00 2001
+From: Javier Martinez Canillas <javierm@redhat.com>
+Date: Tue, 29 Sep 2020 14:08:55 +0200
+Subject: [PATCH] dl: Only allow unloading modules that are not dependencies
+
+When a module is attempted to be removed its reference counter is always
+decremented. This means that repeated rmmod invocations will cause the
+module to be unloaded even if another module depends on it.
+
+This may lead to a use-after-free scenario allowing an attacker to execute
+arbitrary code and by-pass the UEFI Secure Boot protection.
+
+While being there, add the extern keyword to some function declarations in
+that header file.
+
+Fixes: CVE-2020-25632
+
+Reported-by: Chris Coulson <chris.coulson@canonical.com>
+Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
+Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
+
+Upstream-Status: Backport [https://git.savannah.gnu.org/cgit/grub.git/commit/?id=7630ec5397fe418276b360f9011934b8c034936c]
+CVE: CVE-2020-25632
+Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
+---
+ grub-core/commands/minicmd.c | 7 +++++--
+ grub-core/kern/dl.c          | 9 +++++++++
+ include/grub/dl.h            | 8 +++++---
+ 3 files changed, 19 insertions(+), 5 deletions(-)
+
+diff --git a/grub-core/commands/minicmd.c b/grub-core/commands/minicmd.c
+index 6bbce3128..fa498931e 100644
+--- a/grub-core/commands/minicmd.c
++++ b/grub-core/commands/minicmd.c
+@@ -140,8 +140,11 @@ grub_mini_cmd_rmmod (struct grub_command *cmd __attribute__ ((unused)),
+   if (grub_dl_is_persistent (mod))
+     return grub_error (GRUB_ERR_BAD_ARGUMENT, "cannot unload persistent module");
+ 
+-  if (grub_dl_unref (mod) <= 0)
+-    grub_dl_unload (mod);
++  if (grub_dl_ref_count (mod) > 1)
++    return grub_error (GRUB_ERR_BAD_ARGUMENT, "cannot unload referenced module");
++
++  grub_dl_unref (mod);
++  grub_dl_unload (mod);
+ 
+   return 0;
+ }
+diff --git a/grub-core/kern/dl.c b/grub-core/kern/dl.c
+index 48eb5e7b6..48f8a7907 100644
+--- a/grub-core/kern/dl.c
++++ b/grub-core/kern/dl.c
+@@ -549,6 +549,15 @@ grub_dl_unref (grub_dl_t mod)
+   return --mod->ref_count;
+ }
+ 
++int
++grub_dl_ref_count (grub_dl_t mod)
++{
++  if (mod == NULL)
++    return 0;
++
++  return mod->ref_count;
++}
++
+ static void
+ grub_dl_flush_cache (grub_dl_t mod)
+ {
+diff --git a/include/grub/dl.h b/include/grub/dl.h
+index f03c03561..b3753c9ca 100644
+--- a/include/grub/dl.h
++++ b/include/grub/dl.h
+@@ -203,9 +203,11 @@ grub_dl_t EXPORT_FUNC(grub_dl_load) (const char *name);
+ grub_dl_t grub_dl_load_core (void *addr, grub_size_t size);
+ grub_dl_t EXPORT_FUNC(grub_dl_load_core_noinit) (void *addr, grub_size_t size);
+ int EXPORT_FUNC(grub_dl_unload) (grub_dl_t mod);
+-void grub_dl_unload_unneeded (void);
+-int EXPORT_FUNC(grub_dl_ref) (grub_dl_t mod);
+-int EXPORT_FUNC(grub_dl_unref) (grub_dl_t mod);
++extern void grub_dl_unload_unneeded (void);
++extern int EXPORT_FUNC(grub_dl_ref) (grub_dl_t mod);
++extern int EXPORT_FUNC(grub_dl_unref) (grub_dl_t mod);
++extern int EXPORT_FUNC(grub_dl_ref_count) (grub_dl_t mod);
++
+ extern grub_dl_t EXPORT_VAR(grub_dl_head);
+ 
+ #ifndef GRUB_UTIL
+-- 
+2.33.0
+

meta/recipes-bsp/grub/grub2.inc

@@ -45,6 +45,7 @@ SRC_URI = "${GNU_MIRROR}/grub/grub-${PV}.tar.gz \
            file://CVE-2020-27779_5.patch \
            file://CVE-2020-27779_6.patch \
            file://CVE-2020-27779_7.patch \
+           file://CVE-2020-25632.patch \
 "
 SRC_URI[md5sum] = "5ce674ca6b2612d8939b9e6abed32934"
 SRC_URI[sha256sum] = "f10c85ae3e204dbaec39ae22fa3c5e99f0665417e91c2cb49b7e5031658ba6ea"