[kernel-hardening] Re: [PATCH RFC v6 5/6] fs/proc: Show STACKLEAK metrics in the /proc file system

[Alexander Popov <alex.popov@linux.com>]

Hello Laura and Kees,

[adding Tobin C. Harding]

On 06.12.2017 22:22, Laura Abbott wrote:
> On 12/05/2017 03:33 PM, Alexander Popov wrote:
>> Introduce CONFIG_STACKLEAK_METRICS providing STACKLEAK information about
>> tasks via the /proc file system. In particular, /proc/<pid>/lowest_stack
>> shows the current lowest_stack value and its final value from the previous
>> syscall. That information can be useful for estimating the STACKLEAK
>> performance impact for different workloads.
>>
>> Signed-off-by: Alexander Popov <alex.popov@linux.com>
>> ---
>>   arch/Kconfig                     | 11 +++++++++++
>>   arch/x86/entry/entry_32.S        |  4 ++++
>>   arch/x86/entry/entry_64.S        |  4 ++++
>>   arch/x86/include/asm/processor.h |  3 +++
>>   arch/x86/kernel/asm-offsets.c    |  3 +++
>>   arch/x86/kernel/process_32.c     |  3 +++
>>   arch/x86/kernel/process_64.c     |  3 +++
>>   fs/proc/base.c                   | 14 ++++++++++++++
>>   8 files changed, 45 insertions(+)

[...]

>> +#ifdef CONFIG_STACKLEAK_METRICS
>> +static int proc_lowest_stack(struct seq_file *m, struct pid_namespace *ns,
>> +				struct pid *pid, struct task_struct *task)
>> +{
>> +	seq_printf(m, "prev_lowest_stack: %pK\nlowest_stack: %pK\n",
>> +		   (void *)task->thread.prev_lowest_stack,
>> +		   (void *)task->thread.lowest_stack);
>> +	return 0;
>> +}
>> +#endif /* CONFIG_STACKLEAK_METRICS */
>> +
> 
> This just prints the hashed value with the new pointer leak work.
> I don't think we want to print the fully exposed value via %px so
> it's not clear how valuable this proc file is now.

Yes, I tested that before sending the patch. I was confused when I saw the
hashed values. But setting kptr_restrict to 1 fixed that for me:

root@hobbit:~# cat /proc/2627/lowest_stack
prev_lowest_stack: 00000000ed8ca991
lowest_stack: 0000000040579d76
root@hobbit:~# echo 1 > /proc/sys/kernel/kptr_restrict
root@hobbit:~# cat /proc/2627/lowest_stack
prev_lowest_stack: ffffc9000094fdb8
lowest_stack: ffffc9000094f9e0

However, Documentation/printk-formats.txt and Documentation/sysctl/kernel.txt
don't specify that behaviour.

Best regards,
Alexander