linux-hardening - [PATCH v5 04/27] x86/boot: Increase boot page table size

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Date:   Tue, 14 Mar 2023 13:13:31 +0300
From:   Evgeniy Baskov <baskov@...ras.ru>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Evgeniy Baskov <baskov@...ras.ru>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        "Limonciello, Mario" <mario.limonciello@....com>,
        joeyli <jlee@...e.com>, lvc-project@...uxtesting.org,
        x86@...nel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v5 04/27] x86/boot: Increase boot page table size

Previous upper limit ignored pages implicitly mapped from #PF handler
by code accessing ACPI tables (boot/compressed/{acpi.c,efi.c}),
so theoretical upper limit is higher than it was set.

Using 4KB pages is desirable for better memory protection granularity.
Approximately twice as much memory is required for those.

Increase initial page table size to 64 4KB page tables.

Tested-by: Mario Limonciello <mario.limonciello@....com>
Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
---
 arch/x86/include/asm/boot.h | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/boot.h b/arch/x86/include/asm/boot.h
index 9191280d9ea3..88836067f88c 100644
--- a/arch/x86/include/asm/boot.h
+++ b/arch/x86/include/asm/boot.h
@@ -41,22 +41,26 @@
 # define BOOT_STACK_SIZE	0x4000
 
 # define BOOT_INIT_PGT_SIZE	(6*4096)
-# ifdef CONFIG_RANDOMIZE_BASE
 /*
  * Assuming all cross the 512GB boundary:
  * 1 page for level4
- * (2+2)*4 pages for kernel, param, cmd_line, and randomized kernel
- * 2 pages for first 2M (video RAM: CONFIG_X86_VERBOSE_BOOTUP).
- * Total is 19 pages.
+ * (3+3)*2 pages for param and cmd_line
+ * (2+2+S)*2 pages for kernel and randomized kernel, where S is total number
+ *     of sections of kernel. Explanation: 2+2 are upper level page tables.
+ *     We can have only S unaligned parts of section: 1 at the end of the kernel
+ *     and (S-1) at the section borders. The start address of the kernel is
+ *     aligned, so an extra page table. There are at most S=6 sections in
+ *     vmlinux ELF image.
+ * 3 pages for first 2M (video RAM: CONFIG_X86_VERBOSE_BOOTUP).
+ * Total is 36 pages.
+ *
+ * Some number of page tables is also required for the ACPI and UEFI table
+ * mappings, so we round up 36 to 64. Since ACPI tables are generally getting
+ * allocated in a few contiguous regions of memory, they are very unlikely to be
+ * spread out enough to require more than 28 extra page tables and this
+ * would work fine in all more or less sane environments.
  */
-#  ifdef CONFIG_X86_VERBOSE_BOOTUP
-#   define BOOT_PGT_SIZE	(19*4096)
-#  else /* !CONFIG_X86_VERBOSE_BOOTUP */
-#   define BOOT_PGT_SIZE	(17*4096)
-#  endif
-# else /* !CONFIG_RANDOMIZE_BASE */
-#  define BOOT_PGT_SIZE		BOOT_INIT_PGT_SIZE
-# endif
+# define BOOT_PGT_SIZE		(64*4096)
 
 #else /* !CONFIG_X86_64 */
 # define BOOT_STACK_SIZE	0x1000
-- 
2.39.2