Commit f530ee95 authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Ingo Molnar
Browse files

x86/boot/compressed: Reserve more memory for page tables 



The decompressor has a hard limit on the number of page tables it can
allocate. This limit is defined at compile-time and will cause boot
failure if it is reached.

The kernel is very strict and calculates the limit precisely for the
worst-case scenario based on the current configuration. However, it is
easy to forget to adjust the limit when a new use-case arises. The
worst-case scenario is rarely encountered during sanity checks.

In the case of enabling 5-level paging, a use-case was overlooked. The
limit needs to be increased by one to accommodate the additional level.
This oversight went unnoticed until Aaron attempted to run the kernel
via kexec with 5-level paging and unaccepted memory enabled.

Update wost-case calculations to include 5-level paging.

To address this issue, let's allocate some extra space for page tables.
128K should be sufficient for any use-case. The logic can be simplified
by using a single value for all kernel configurations.

[ Also add a warning, should this memory run low - by Dave Hansen. ]

Fixes: 34bbb000 ("x86/boot/compressed: Enable 5-level paging during decompression stage")
Reported-by: default avatarAaron Lu <aaron.lu@intel.com>
Signed-off-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230915070221.10266-1-kirill.shutemov@linux.intel.com
parent 7575e5a3

arch/x86/boot/compressed/ident_map_64.c

+8 −0
Original line number Diff line number Diff line
@@ -59,6 +59,14 @@ static void *alloc_pgt_page(void *context) 
		return NULL;

	}



	/* Consumed more tables than expected? */

	if (pages->pgt_buf_offset == BOOT_PGT_SIZE_WARN) {

		debug_putstr("pgt_buf running low in " __FILE__ "\n");

		debug_putstr("Need to raise BOOT_PGT_SIZE?\n");

		debug_putaddr(pages->pgt_buf_offset);

		debug_putaddr(pages->pgt_buf_size);

	}



	entry = pages->pgt_buf + pages->pgt_buf_offset;

	pages->pgt_buf_offset += PAGE_SIZE;

arch/x86/include/asm/boot.h

+31 −14
Original line number Diff line number Diff line
@@ -40,23 +40,40 @@ 
#ifdef CONFIG_X86_64

# define BOOT_STACK_SIZE	0x4000



/*

 * Used by decompressor's startup_32() to allocate page tables for identity

 * mapping of the 4G of RAM in 4-level paging mode:

 * - 1 level4 table;

 * - 1 level3 table;

 * - 4 level2 table that maps everything with 2M pages;

 *

 * The additional level5 table needed for 5-level paging is allocated from

 * trampoline_32bit memory.

 */

# 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.

 * Total number of page tables kernel_add_identity_map() can allocate,

 * including page tables consumed by startup_32().

 *

 * Worst-case scenario:

 *  - 5-level paging needs 1 level5 table;

 *  - KASLR needs to map kernel, boot_params, cmdline and randomized kernel,

 *    assuming all of them cross 256T boundary:

 *    + 4*2 level4 table;

 *    + 4*2 level3 table;

 *    + 4*2 level2 table;

 *  - X86_VERBOSE_BOOTUP needs to map the first 2M (video RAM):

 *    + 1 level4 table;

 *    + 1 level3 table;

 *    + 1 level2 table;

 * Total: 28 tables

 *

 * Add 4 spare table in case decompressor touches anything beyond what is

 * accounted above. Warn if it happens.

 */

#  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_WARN	(28*4096)

# define BOOT_PGT_SIZE		(32*4096)



#else /* !CONFIG_X86_64 */

# define BOOT_STACK_SIZE	0x1000