Received-SPF: pass (google.com: domain of linux-kernel+bounces-179706-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Date: Wed, 15 May 2024 11:32:34 +0200
Precedence: bulk
Mime-Version: 1.0
Message-ID: <20240515093233.3698988-2-ardb+git@google.com>
Subject: [PATCH] x86/boot: Avoid global attribute in early memory mappings
From: Ard Biesheuvel <ardb+git@google.com>
To: linux-kernel@vger.kernel.org
Cc: x86@kernel.org, Ard Biesheuvel <ardb@kernel.org>
Content-Type: text/plain; charset="UTF-8"

From: Ard Biesheuvel <ardb@kernel.org>

The decompressor uses the __PAGE_KERNEL_LARGE_EXEC template to create
page table entries, and this includes the 'G' global bit. Global
mappings are a performance optimization that requires more elaborate TLB
maintenance, which the decompressor does not implement, and so global
mappings are disabled entirely, by clearing CR4.PGE in the startup code.

Since global mappings are not used, it is better to avoid setting the
bit altogether, so clear it from the 'flag' variable used to populate
page table entries.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/compressed/ident_map_64.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
index dfb9c2deb77c..e24fedf1e36b 100644
--- a/arch/x86/boot/compressed/ident_map_64.c
+++ b/arch/x86/boot/compressed/ident_map_64.c
@@ -121,6 +121,12 @@ void initialize_identity_maps(void *rmode)
 	mapping_info.page_flag = __PAGE_KERNEL_LARGE_EXEC | sme_me_mask;
 	mapping_info.kernpg_flag = _KERNPG_TABLE;
 
+	/*
+	 * Entries with the G bit require more elaborate TLB maintenance,
+	 * so avoid them for these early mappings.
+	 */
+	mapping_info.page_flag &= ~_PAGE_GLOBAL;
+
 	/*
 	 * It should be impossible for this not to already be true,
 	 * but since calling this a second time would rewind the other
-- 
2.45.0.rc1.225.g2a3ae87e7f-goog