x86/build: Fix vmlinux size check on 64-bit

Commit

  b4e0409a36f4 ("x86: check vmlinux limits, 64-bit")

added a check that the size of the 64-bit kernel is less than
KERNEL_IMAGE_SIZE.

The check uses (_end - _text), but this is not enough. The initial
PMD used in startup_64() (level2_kernel_pgt) can only map upto
KERNEL_IMAGE_SIZE from __START_KERNEL_map, not from _text, and the
modules area (MODULES_VADDR) starts at KERNEL_IMAGE_SIZE.

The correct check is what is currently done for 32-bit, since
LOAD_OFFSET is defined appropriately for the two architectures. Just
check (_end - LOAD_OFFSET) against KERNEL_IMAGE_SIZE unconditionally.

Note that on 32-bit, the limit is not strict: KERNEL_IMAGE_SIZE is not
really used by the main kernel. The higher the kernel is located, the
less the space available for the vmalloc area. However, it is used by
KASLR in the compressed stub to limit the maximum address of the kernel
to a safe value.

Clean up various comments to clarify that despite the name,
KERNEL_IMAGE_SIZE is not a limit on the size of the kernel image, but a
limit on the maximum virtual address that the image can occupy.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201029161903.2553528-1-nivedita@alum.mit.edu

3 files changed, 17 insertions, 15 deletions

diff --git a/arch/x86/include/asm/page_32_types.h b/arch/x86/include/asm/page_32_types.h
index f462895a33e4..faf9cc1c14bb 100644
--- a/arch/x86/include/asm/page_32_types.h
+++ b/arch/x86/include/asm/page_32_types.h
@@ -53,7 +53,13 @@
 #define STACK_TOP_MAX		STACK_TOP
 
 /*
- * Kernel image size is limited to 512 MB (see in arch/x86/kernel/head_32.S)
+ * In spite of the name, KERNEL_IMAGE_SIZE is a limit on the maximum virtual
+ * address for the kernel image, rather than the limit on the size itself. On
+ * 32-bit, this is not a strict limit, but this value is used to limit the
+ * link-time virtual address range of the kernel, and by KASLR to limit the
+ * randomized address from which the kernel is executed. A relocatable kernel
+ * can be loaded somewhat higher than KERNEL_IMAGE_SIZE as long as enough space
+ * remains for the vmalloc area.
  */
 #define KERNEL_IMAGE_SIZE	(512 * 1024 * 1024)
 
diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
index 3f49dac03617..645bd1d0ee07 100644
--- a/arch/x86/include/asm/page_64_types.h
+++ b/arch/x86/include/asm/page_64_types.h
@@ -98,8 +98,10 @@
 #define STACK_TOP_MAX		TASK_SIZE_MAX
 
 /*
- * Maximum kernel image size is limited to 1 GiB, due to the fixmap living
- * in the next 1 GiB (see level2_kernel_pgt in arch/x86/kernel/head_64.S).
+ * In spite of the name, KERNEL_IMAGE_SIZE is a limit on the maximum virtual
+ * address for the kernel image, rather than the limit on the size itself.
+ * This can be at most 1 GiB, due to the fixmap living in the next 1 GiB (see
+ * level2_kernel_pgt in arch/x86/kernel/head_64.S).
  *
  * On KASLR use 1 GiB by default, leaving 1 GiB for modules once the
  * page tables are fully set up.
diff --git a/arch/x86/include/asm/pgtable_32.h b/arch/x86/include/asm/pgtable_32.h
index d7acae4120d5..7c9c968a42ef 100644
--- a/arch/x86/include/asm/pgtable_32.h
+++ b/arch/x86/include/asm/pgtable_32.h
@@ -57,19 +57,13 @@ do {						\
 #endif
 
 /*
- * This is how much memory in addition to the memory covered up to
- * and including _end we need mapped initially.
- * We need:
- *     (KERNEL_IMAGE_SIZE/4096) / 1024 pages (worst case, non PAE)
- *     (KERNEL_IMAGE_SIZE/4096) / 512 + 4 pages (worst case for PAE)
+ * This is used to calculate the .brk reservation for initial pagetables.
+ * Enough space is reserved to allocate pagetables sufficient to cover all
+ * of LOWMEM_PAGES, which is an upper bound on the size of the direct map of
+ * lowmem.
  *
- * Modulo rounding, each megabyte assigned here requires a kilobyte of
- * memory, which is currently unreclaimed.
- *
- * This should be a multiple of a page.
- *
- * KERNEL_IMAGE_SIZE should be greater than pa(_end)
- * and small than max_low_pfn, otherwise will waste some page table entries
+ * With PAE paging (PTRS_PER_PMD > 1), we allocate PTRS_PER_PGD == 4 pages for
+ * the PMD's in addition to the pages required for the last level pagetables.
  */
 #if PTRS_PER_PMD > 1
 #define PAGE_TABLE_SIZE(pages) (((pages) / PTRS_PER_PMD) + PTRS_PER_PGD)