Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page tables (core patch)

[Hanjun Guo]

On 2018/1/6 14:28, Hanjun Guo wrote:
> Hi Dave,
> 
> Thank you very much for the quick response! Minor comments inline.
> 
> On 2018/1/6 14:06, Dave Hansen wrote:
>> On 01/05/2018 08:54 PM, Hanjun Guo wrote:
>>> Do you mean NX bit will be brought back later? I'm asking this because
>>> I tested this patch which it fixed the boot panic issue but the system
>>> will hang when rebooting the system, because rebooting will also call efi
>>> then panic as NS bit is set.
>> Wow, you're running a lot of very lighly-used code paths!  You actually
>> found a similar but totally separate issue from what I gather.  Thank
>> you immensely for the quick testing and bug reports!
>>
>> Could you test the attached fix?
>>
>> For those playing along at home, I think this will end up being needed
>> for 4.15 and probably all the backports.  I want to see if it works
>> before I submit it for real, though.
>>
>>
>> pti-tboot-fix.patch
>>
>>
>> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>
>> This is another case similar to what EFI does: create a new set of
>> page tables, map some code at a low address, and jump to it.  PTI
>> mistakes this low address for userspace and mistakenly marks it
>> non-executable in an effort to make it unusable for userspace.  Undo
>> the poison to allow execution.
>>
>> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> Cc: Ning Sun <ning.sun@xxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>> Cc: x86@xxxxxxxxxx
>> Cc: tboot-devel@xxxxxxxxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> ---
>>
>>  b/arch/x86/kernel/tboot.c |    7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff -puN arch/x86/kernel/tboot.c~pti-tboot-fix arch/x86/kernel/tboot.c
>> --- a/arch/x86/kernel/tboot.c~pti-tboot-fix	2018-01-05 21:50:55.755554960 -0800
>> +++ b/arch/x86/kernel/tboot.c	2018-01-05 22:01:51.393553325 -0800
>> @@ -124,6 +124,13 @@ static int map_tboot_page(unsigned long
>>  	pte_t *pte;
>>  
>>  	pgd = pgd_offset(&tboot_mm, vaddr);
>> +	/*
>> +	 * PTI poisons low addresses in the kernel page tables in the
>> +	 * name of making them unusable for userspace.  To execute
>> +	 * code at such a low address, the poison must be cleared.
>> +	 */
>> +	pgd->pgd &= ~_PAGE_NX;
> 
> ...
> 
>> +
>>  	p4d = p4d_alloc(&tboot_mm, pgd, vaddr);
> 
> Seems pgd will be re-set after p4d_alloc(), so should
> we put the code behind (or after pud_alloc())?
> 
>>  	if (!p4d)
>>  		return -1;
> 
>  +	/*
>  +	 * PTI poisons low addresses in the kernel page tables in the
>  +	 * name of making them unusable for userspace.  To execute
>  +	 * code at such a low address, the poison must be cleared.
>  +	 */
>  +	pgd->pgd &= ~_PAGE_NX;
> 
> We will have a try in a minute, and report back later.

And it worksïwe can boot/reboot the system successfully, thank
you all the quick response and debug!

Thanks
Hanjun