[kernel-hardening] Re: [PATCH v3 1/4] x86/mm: Adapt MODULES_END based on Fixmap section size

Borislav Petkov
2017-02-15T13:58:24Z

On Tue, Feb 14, 2017 at 11:42:56AM -0800, Thomas Garnier wrote:
> This patch aligns MODULES_END to the beginning of the Fixmap section.
> It optimizes the space available for both sections. The address is
> pre-computed based on the number of pages required by the Fixmap
> section.
>
> It will allow GDT remapping in the Fixmap section. The current
> MODULES_END static address does not provide enough space for the kernel
> to support a large number of processors.
>
> Signed-off-by: Thomas Garnier <thgarnie@google.com>
> ---
> Based on next-20170213
> ---
> arch/x86/include/asm/fixmap.h | 8 ++++++++
> arch/x86/include/asm/pgtable_64_types.h | 3 ---
> arch/x86/kernel/module.c | 1 +
> arch/x86/mm/dump_pagetables.c | 1 +
> arch/x86/mm/kasan_init_64.c | 1 +
> 5 files changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index 8554f960e21b..20231189e0e3 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -132,6 +132,14 @@ enum fixed_addresses {
>
> extern void reserve_top_address(unsigned long reserve);
>
> +/* On 64-bit, the module sections ends with the start of the fixmap */
> +#ifdef CONFIG_X86_64
> +#define MODULES_VADDR (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
> +#define MODULES_END __fix_to_virt(__end_of_fixed_addresses + 1)
> +#define MODULES_LEN (MODULES_END - MODULES_VADDR)
> +#endif /* CONFIG_X86_64 */

JFYI: so there's another patchset which adds KERNEL_MAPPING_SIZE:

https://lkml.kernel.org/r/1486040077-3719-1-git-send-email-bhe@redhat.com

and makes it a 1G, i.e., the KASLR default. I guess the above will have
to be KERNEL_MAPPING_SIZE then.

And why are you moving those to fixmap.h? What's wrong with
including fixmap.h into pgtable_64_types.h so that you can get
__end_of_fixed_addresses?

FWIW, I didn't even have to add any includes with my .config, i.e., that builds:

---
diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index 3a264200c62f..eda7fa856fa9 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -67,7 +67,7 @@ typedef struct { pteval_t pte; } pte_t;
#endif /* CONFIG_RANDOMIZE_MEMORY */
#define VMALLOC_END (VMALLOC_START + _AC((VMALLOC_SIZE_TB << 40) - 1, UL))
#define MODULES_VADDR (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
-#define MODULES_END _AC(0xffffffffff000000, UL)
+#define MODULES_END __fix_to_virt(__end_of_fixed_addresses + 1)
#define MODULES_LEN (MODULES_END - MODULES_VADDR)
#define ESPFIX_PGD_ENTRY _AC(-2, UL)
#define ESPFIX_BASE_ADDR (ESPFIX_PGD_ENTRY << PGDIR_SHIFT)
---

but I wouldn't be surprised if some strange configuration would need it.

> #define FIXADDR_SIZE (__end_of_permanent_fixed_addresses << PAGE_SHIFT)
> #define FIXADDR_START (FIXADDR_TOP - FIXADDR_SIZE)
>
> diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
> index 3a264200c62f..de8bace10200 100644
> --- a/arch/x86/include/asm/pgtable_64_types.h
> +++ b/arch/x86/include/asm/pgtable_64_types.h
> @@ -66,9 +66,6 @@ typedef struct { pteval_t pte; } pte_t;
> #define VMEMMAP_START __VMEMMAP_BASE
> #endif /* CONFIG_RANDOMIZE_MEMORY */
> #define VMALLOC_END (VMALLOC_START + _AC((VMALLOC_SIZE_TB << 40) - 1, UL))
> -#define MODULES_VADDR (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
> -#define MODULES_END _AC(0xffffffffff000000, UL)

How much of an ABI breakage would that be? See
Documentation/x86/x86_64/mm.txt.

With my .config MODULES_END becomes 0xffffffffff1fe000 and it'll remain
dynamic depending on .config. No idea how much in userspace relies on
MODULES_END being static 0xffffffffff000000...

Hmm.

--
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--