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.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Aaron Lu <aaron.lu@xxxxxxxxx>
Fixes: 34bbb0009f3b ("x86/boot/compressed: Enable 5-level paging during decompression stage")
---
arch/x86/boot/compressed/ident_map_64.c | 8 +++++
arch/x86/include/asm/boot.h | 47 +++++++++++++++++--------
2 files changed, 40 insertions(+), 15 deletions(-)
diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
index bcc956c17872..08f93b0401bb 100644
--- a/arch/x86/boot/compressed/ident_map_64.c
+++ b/arch/x86/boot/compressed/ident_map_64.c
@@ -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;
diff --git a/arch/x86/include/asm/boot.h b/arch/x86/include/asm/boot.h
index 9191280d9ea3..215d37f7dde8 100644
--- a/arch/x86/include/asm/boot.h
+++ b/arch/x86/include/asm/boot.h
@@ -40,23 +40,40 @@
#ifdef CONFIG_X86_64
# 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.
+ * 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.
*/
-# 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_INIT_PGT_SIZE (6*4096)
+
+/*
+ * 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.
+ */
+# define BOOT_PGT_SIZE_WARN (28*4096)
+# define BOOT_PGT_SIZE (32*4096)
#else /* !CONFIG_X86_64 */
# define BOOT_STACK_SIZE 0x1000
--
2.41.0
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