[kernel-hardening] Re: [PATCH 6/7] x86, kaslr: report kernel offset on panic

[HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>]

(2013/10/02 4:37), Kees Cook wrote:
> When the system panics, include the kernel offset in the report to assist
> in debugging.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>   arch/x86/kernel/setup.c |   26 ++++++++++++++++++++++++++
>   1 file changed, 26 insertions(+)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index f0de629..1708862 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -824,6 +824,20 @@ static void __init trim_low_memory_range(void)
>   }
>   	
>   /*
> + * Dump out kernel offset information on panic.
> + */
> +static int
> +dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
> +{
> +	pr_emerg("Kernel Offset: 0x%lx from 0x%lx "
> +		 "(relocation range: 0x%lx-0x%lx)\n",
> +		 (unsigned long)&_text - __START_KERNEL, __START_KERNEL,
> +		 __START_KERNEL_map, MODULES_VADDR-1);

Using phys_base seems better.

> +
> +	return 0;
> +}
> +
> +/*
>    * Determine if we were loaded by an EFI loader.  If so, then we have also been
>    * passed the efi memmap, systab, etc., so we should use these data structures
>    * for initialization.  Note, the efi init code path is determined by the
> @@ -1242,3 +1256,15 @@ void __init i386_reserve_resources(void)
>   }
>   
>   #endif /* CONFIG_X86_32 */
> +
> +static struct notifier_block kernel_offset_notifier = {
> +	.notifier_call = dump_kernel_offset
> +};
> +
> +static int __init register_kernel_offset_dumper(void)
> +{
> +	atomic_notifier_chain_register(&panic_notifier_list,
> +					&kernel_offset_notifier);
> +	return 0;
> +}
> +__initcall(register_kernel_offset_dumper);
> 

Panic notifier is not executed if kdump is enabled. Maybe, Chrome OS doesn't use
kdump? Anyway, kdump related tools now calculate phys_base from memory map
information passed as ELF PT_LOAD entries like below.

$ LANG=C readelf -l vmcore-rhel6up4

Elf file type is CORE (Core file)
Entry point 0x0
There are 5 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  NOTE           0x0000000000000158 0x0000000000000000 0x0000000000000000
                 0x0000000000000b08 0x0000000000000b08         0
  LOAD           0x0000000000000c60 0xffffffff81000000 0x0000000001000000
                 0x000000000103b000 0x000000000103b000  RWE    0
  LOAD           0x000000000103bc60 0xffff880000001000 0x0000000000001000
                 0x000000000009cc00 0x000000000009cc00  RWE    0
  LOAD           0x00000000010d8860 0xffff880000100000 0x0000000000100000
                 0x0000000002f00000 0x0000000002f00000  RWE    0
  LOAD           0x0000000003fd8860 0xffff880013000000 0x0000000013000000
                 0x000000002cffd000 0x000000002cffd000  RWE    0

Each PT_LOAD entry is assigned to virtual and physical address. In this case,
1st PT_LOAD entry belongs to kernel text mapping region, from which we can
calculate phys_base value.

Therefore, we already have phys_base information even in case of kdump, and
as long as using kdump-related tools such as crash utility, we don't need
to see ELF PT_LOAD headers as I explain here because they calculate the process
I explain here automatically.

Another idea is to add phys_base value in VMCOREINFO information that is debugging
information for user-land tools to filter crash dump. This is simple string information
so you can see the values contained in some crash dump by using strings command to it.
For example,

$ LANG=C strings vmcore-rhel6up4
CORE
CORE
CORE
CORE
VMCOREINFO
OSRELEASE=2.6.32-345.el6.x86_64
PAGESIZE=4096
SYMBOL(init_uts_ns)=ffffffff81a8e940
SYMBOL(node_online_map)=ffffffff81c09e20
SYMBOL(swapper_pg_dir)=ffffffff81a85000
...cut...
NUMBER(PG_private)=11
NUMBER(PG_swapcache)=16
SYMBOL(phys_base)=ffffffff81a8d010
SYMBOL(init_level4_pgt)=ffffffff81a85000
SYMBOL(node_data)=ffffffff81c06280
LENGTH(node_data)=512
CRASHTIME=1355815389

The problem is that currently phys_base information is somehow exported as virtual address
assigned to phys_base symbol, not the value of it. User-land tools use this to determine
if they need to calculate phys_base. But obviously, it's simpler to export phys_base information
as its value, not the assigned address. Then, as a side-effect, we can check phys_base value
using strings command to crash dump and this is a little handy.

-- 
Thanks.
HATAYAMA, Daisuke