linux-hardening - [PATCH] efi: Replace efi_memory_attributes_table

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Message-Id: <20240710225538.work.224-kees@kernel.org>
Date: Wed, 10 Jul 2024 15:55:42 -0700
From: Kees Cook <kees@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Kees Cook <kees@...nel.org>,
	linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH] efi: Replace efi_memory_attributes_table_t 0-sized array with flexible array

While efi_memory_attributes_table_t::entry isn't used directly as an
array, it is used as a base for pointer arithmetic. The type is wrong
as it's not technically an array of efi_memory_desc_t's; they could be
larger. Regardless, leave the type unchanged and remove the old style
"0" array size. Additionally replace the open-coded entry offset code
with the existing efi_early_memdesc_ptr() helper.

Signed-off-by: Kees Cook <kees@...nel.org>
---
Cc: Ard Biesheuvel <ardb@...nel.org>
Cc: linux-efi@...r.kernel.org
---
 drivers/firmware/efi/memattr.c | 2 +-
 include/linux/efi.h            | 6 +++++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/efi/memattr.c b/drivers/firmware/efi/memattr.c
index ab85bf8e165a..01142604e8df 100644
--- a/drivers/firmware/efi/memattr.c
+++ b/drivers/firmware/efi/memattr.c
@@ -164,7 +164,7 @@ int __init efi_memattr_apply_permissions(struct mm_struct *mm,
 		bool valid;
 		char buf[64];
 
-		valid = entry_is_valid((void *)tbl->entry + i * tbl->desc_size,
+		valid = entry_is_valid(efi_early_memdesc_ptr(tbl->entry, tbl->desc_size, i),
 				       &md);
 		size = md.num_pages << EFI_PAGE_SHIFT;
 		if (efi_enabled(EFI_DBG) || !valid)
diff --git a/include/linux/efi.h b/include/linux/efi.h
index 418e555459da..b06639c4f6a5 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -607,7 +607,11 @@ typedef struct {
 	u32 num_entries;
 	u32 desc_size;
 	u32 flags;
-	efi_memory_desc_t entry[0];
+	/*
+	 * There are @num_entries following, each of size @desc_size bytes,
+	 * including an efi_memory_desc_t header.
+	 */
+	efi_memory_desc_t entry[];
 } efi_memory_attributes_table_t;
 
 typedef struct {
-- 
2.34.1