linux-kernel - Re: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do

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Date:	Wed, 1 Jun 2011 18:53:27 -0700
From:	Chris Wright <chrisw@...s-sol.org>
To:	Andrea Righi <andrea@...terlinux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Chris Wright <chrisw@...s-sol.org>, Mel Gorman <mel@....ul.ie>,
	Izik Eidus <ieidus@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do_scan()

* Andrea Righi (andrea@...terlinux.com) wrote:
> The bug can be easily reproduced using the following testcase:

Thanks for the testcase.

> ========================
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <sys/mman.h>
> 
> #define BUFSIZE getpagesize()
> 
> int main(int argc, char **argv)
> {
> 	void *ptr;
> 
> 	if (posix_memalign(&ptr, getpagesize(), BUFSIZE) < 0) {
> 		perror("posix_memalign");
> 		exit(1);
> 	}
> 	if (madvise(ptr, BUFSIZE, MADV_MERGEABLE) < 0) {
> 		perror("madvise");
> 		exit(1);
> 	}
> 	*(char *)NULL = 0;
> 
> 	return 0;
> }
> ========================
> 
> It seems that when a task segfaults mm_slot->mm becomes NULL, but it's
> still wrongly considered by the ksm scan. Is there a race with
> __ksm_exit()?

Hmm, wonder if khugepaged has the same issue too.  We should be holding
a reference to ->mm, but we seem to have inconsistent serialization w/
mmap_sem.  Hugh mentioned some of these concerns when introducing
ksm_exit.
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