Subject: Re: [criu] 1M guard page ruined restore
To: Oleg Nesterov <oleg@redhat.com>, Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Hugh Dickins <hughd@google.com>, Andrey Vagin <avagin@openvz.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Pavel Emelyanov <xemul@virtuozzo.com>,
        Andrew Morton <akpm@linuxfoundation.org>,
        Adrian Reber <areber@redhat.com>
References: <20170620075206.GB1909@uranus.lan>
 <alpine.LSU.2.11.1706200259080.11599@eggly.anvils>
 <20170621152256.GC31050@uranus> <20170621155730.GA32554@redhat.com>
 <20170621160410.GF31050@uranus> <20170621170129.GA32752@redhat.com>
From: Dmitry Safonov <dsafonov@virtuozzo.com>
Message-ID: <a747d0be-ddf9-d338-06de-223337a03a67@virtuozzo.com>
Date: Wed, 21 Jun 2017 20:15:33 +0300
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On 06/21/2017 08:01 PM, Oleg Nesterov wrote:
> On 06/21, Cyrill Gorcunov wrote:
>>
>> On Wed, Jun 21, 2017 at 05:57:30PM +0200, Oleg Nesterov wrote:
>>>>
>>>> 	p = fake_grow_down;
>>>> 	*p-- = 'c';
>>>
>>> I guess this works? I mean, *p-- = 'c' should not fail...
>>
>> It fails.
> 
> Hmm. Impossible ;) could you add the additional printf's to re-check?
> 
>> Here is the complete code. It supposed to _extend_ stack but it fails
>> on the latest master + Hugh's [PATCH] mm: fix new crash in unmapped_area_topdown()
>> ---
>> [root@fc2 criu]# ~/st2
>> start_addr 7fe6162a8000
>> start_addr 7fe6163d9000
>> Segmentation fault (core dumped)
>> ---
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <errno.h>
>> #include <stdlib.h>
>> #include <string.h>
>> #include <unistd.h>
>>
>> #include <sys/mman.h>
>>
>> #define PAGE_SIZE 4096
>>
>> int main(int argc, char **argv)
>> {
>> 	char *start_addr, *start_addr1, *fake_grow_down, *test_addr, *grow_down;
>> 	volatile char *p;
>>
>> 	start_addr = mmap(NULL, PAGE_SIZE * 512, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>> 	if (start_addr == MAP_FAILED) {
>> 		printf("Can't mal a new region");
>> 		return 1;
>> 	}
>> 	printf("start_addr %lx\n", start_addr);
>> 	munmap(start_addr, PAGE_SIZE * 512);
>>
>> 	start_addr += PAGE_SIZE * 300;
>>
>> 	fake_grow_down = mmap(start_addr + PAGE_SIZE * 5, PAGE_SIZE,
>> 			 PROT_READ | PROT_WRITE,
>> 			 MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED | MAP_GROWSDOWN, -1, 0);
>> 	if (fake_grow_down == MAP_FAILED) {
>> 		printf("Can't mal a new region");
>> 		return 1;
>> 	}
>> 	printf("start_addr %lx\n", fake_grow_down);
>>
>> 	p = fake_grow_down;
>> 	*p-- = 'c';
> 
> once again, I can't believe this STORE can fail...
> 
>> 	*p = 'b';
> 
> Ah. I forgot about another kernel "feature" ;) not related to the recent guard
> page changes...
> 
> Could you test the patch below?

Well, for me only the second store caused segfault.
With your patch it passes..

> 
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 8ad91a0..edc5d68 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1416,7 +1416,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>   		 * and pusha to work. ("enter $65535, $31" pushes
>   		 * 32 pointers and then decrements %sp by 65535.)
>   		 */
> -		if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
> +if (0)		if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
>   			bad_area(regs, error_code, address);
>   			return;
>   		}
> 

-- 
              Dmitry