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Date: Wed, 21 Jun 2017 19:01:29 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: 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>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        Andrew Morton <akpm@linuxfoundation.org>,
        Adrian Reber <areber@redhat.com>
Subject: Re: [criu] 1M guard page ruined restore
Message-ID: <20170621170129.GA32752@redhat.com>
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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?

Oleg.


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;
 		}