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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Javier Martinez Canillas <javierm@redhat.com>
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Date: Tue, 29 Sep 2020 14:08:55 +0200
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Subject: [PATCH] dl: Only allow unloading modules that are not dependencies
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When a module is attempted to be removed its reference counter is always
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decremented. This means that repeated rmmod invocations will cause the
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module to be unloaded even if another module depends on it.
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This may lead to a use-after-free scenario allowing an attacker to execute
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arbitrary code and by-pass the UEFI Secure Boot protection.
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While being there, add the extern keyword to some function declarations in
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that header file.
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Fixes: CVE-2020-25632
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Reported-by: Chris Coulson <chris.coulson@canonical.com>
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Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
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Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
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---
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grub-core/commands/minicmd.c | 7 +++++--
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grub-core/kern/dl.c | 9 +++++++++
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include/grub/dl.h | 8 +++++---
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3 files changed, 19 insertions(+), 5 deletions(-)
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diff --git a/grub-core/commands/minicmd.c b/grub-core/commands/minicmd.c
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index 6d66b7c45..2bd3ac76f 100644
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--- a/grub-core/commands/minicmd.c
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+++ b/grub-core/commands/minicmd.c
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@@ -140,8 +140,11 @@ grub_mini_cmd_rmmod (struct grub_command *cmd __attribute__ ((unused)),
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if (grub_dl_is_persistent (mod))
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return grub_error (GRUB_ERR_BAD_ARGUMENT, "cannot unload persistent module");
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- if (grub_dl_unref (mod) <= 0)
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- grub_dl_unload (mod);
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+ if (grub_dl_ref_count (mod) > 1)
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+ return grub_error (GRUB_ERR_BAD_ARGUMENT, "cannot unload referenced module");
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+
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+ grub_dl_unref (mod);
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+ grub_dl_unload (mod);
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return 0;
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}
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diff --git a/grub-core/kern/dl.c b/grub-core/kern/dl.c
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index d7a7c8f97..520126bea 100644
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--- a/grub-core/kern/dl.c
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+++ b/grub-core/kern/dl.c
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@@ -621,6 +621,15 @@ grub_dl_unref (grub_dl_t mod)
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return --mod->ref_count;
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}
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+int
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+grub_dl_ref_count (grub_dl_t mod)
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+{
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+ if (mod == NULL)
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+ return 0;
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+
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+ return mod->ref_count;
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+}
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+
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static void
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grub_dl_flush_cache (grub_dl_t mod)
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{
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diff --git a/include/grub/dl.h b/include/grub/dl.h
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index 877821dcb..6a3e251b4 100644
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--- a/include/grub/dl.h
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+++ b/include/grub/dl.h
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@@ -205,9 +205,11 @@ grub_dl_t EXPORT_FUNC(grub_dl_load) (const char *name);
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grub_dl_t grub_dl_load_core (void *addr, grub_size_t size);
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grub_dl_t EXPORT_FUNC(grub_dl_load_core_noinit) (void *addr, grub_size_t size);
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int EXPORT_FUNC(grub_dl_unload) (grub_dl_t mod);
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-void grub_dl_unload_unneeded (void);
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-int EXPORT_FUNC(grub_dl_ref) (grub_dl_t mod);
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-int EXPORT_FUNC(grub_dl_unref) (grub_dl_t mod);
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+extern void grub_dl_unload_unneeded (void);
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+extern int EXPORT_FUNC(grub_dl_ref) (grub_dl_t mod);
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+extern int EXPORT_FUNC(grub_dl_unref) (grub_dl_t mod);
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+extern int EXPORT_FUNC(grub_dl_ref_count) (grub_dl_t mod);
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+
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extern grub_dl_t EXPORT_VAR(grub_dl_head);
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#ifndef GRUB_UTIL
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