From 0000000000000000000000000000000000000000 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>
---
 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 b25ca4b9f17..4660a020bda 100644
--- a/grub-core/commands/minicmd.c
+++ b/grub-core/commands/minicmd.c
@@ -137,8 +137,11 @@ grub_mini_cmd_rmmod (struct grub_command *cmd __attribute__ ((unused)),
   if (! mod)
     return grub_error (GRUB_ERR_BAD_ARGUMENT, "no such 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 c45afc64950..fb88894aba5 100644
--- a/grub-core/kern/dl.c
+++ b/grub-core/kern/dl.c
@@ -557,6 +557,15 @@ 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 9562fa6634c..c0e2c55c8c7 100644
--- a/include/grub/dl.h
+++ b/include/grub/dl.h
@@ -202,9 +202,11 @@ 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