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

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Date:	Thu, 2 Jun 2011 00:20:32 +0200
From:	Andrea Righi <andrea@...terlinux.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	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: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do_scan()

I've just experienced this bug with ksmd:

[   55.837551] BUG: unable to handle kernel NULL pointer dereference at 00000000000000e8
[   55.837598] IP: [<ffffffff810bb9b2>] __lock_acquire+0x62/0x1d70
[   55.837630] PGD 0 
[   55.837643] Oops: 0000 [#1] SMP 
[   55.837663] CPU 2 
[   55.837674] Modules linked in: snd_hda_codec_hdmi snd_hda_codec_conexant rtl8192ce rtl8192c_common rtlwifi mac80211 usbhid hid cfg80211 snd_hda_intel snd_hda_codec psmouse snd_pcm e1000e thinkpad_acpi snd_timer snd_page_alloc snd soundcore nvram
[   55.837816] 
[   55.837824] Pid: 33, comm: ksmd Not tainted 3.0.0-rc1+ #289 LENOVO 4286CTO/4286CTO
[   55.837850] RIP: 0010:[<ffffffff810bb9b2>]  [<ffffffff810bb9b2>] __lock_acquire+0x62/0x1d70
[   55.837878] RSP: 0018:ffff88023d3abc50  EFLAGS: 00010046
[   55.837894] RAX: 0000000000000046 RBX: 00000000000000e8 RCX: 0000000000000001
[   55.837915] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000000000e8
[   55.837936] RBP: ffff88023d3abd40 R08: 0000000000000002 R09: 0000000000000000
[   55.837957] R10: 0000000000000001 R11: 0000000000000000 R12: ffff88023d3a3e00
[   55.837978] R13: 0000000000000000 R14: 0000000000000002 R15: 0000000000000000
[   55.837999] FS:  0000000000000000(0000) GS:ffff88023e280000(0000) knlGS:0000000000000000
[   55.838022] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[   55.838039] CR2: 00000000000000e8 CR3: 00000000016f5000 CR4: 00000000000406e0
[   55.838060] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   55.838081] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[   55.838102] Process ksmd (pid: 33, threadinfo ffff88023d3aa000, task ffff88023d3a3e00)
[   55.838131] Stack:
[   55.838140]  ffff88023d3abce0 0000000000000000 ffffffff81d46810 00000000000012c7
[   55.838168]  000000000000037c ffff88023d3a3e00 0000000000000001 0000000000000000
[   55.838338]  0000000000000000 0000000000000000 00000000001ba37c ffffffff81a22000
[   55.838365] Call Trace:
[   55.838375]  [<ffffffff810be55f>] ? mark_held_locks+0x6f/0xa0
[   55.838394]  [<ffffffff814e3360>] ? _raw_spin_unlock_irqrestore+0x40/0x70
[   55.838416]  [<ffffffff810bdc90>] lock_acquire+0x90/0x110
[   55.838434]  [<ffffffff8114c652>] ? ksm_scan_thread+0x132/0xe20
[   55.838453]  [<ffffffff8112df6c>] ? free_percpu+0x9c/0x130
[   55.838470]  [<ffffffff814e1cbc>] down_read+0x4c/0x70
[   55.838486]  [<ffffffff8114c652>] ? ksm_scan_thread+0x132/0xe20
[   55.838505]  [<ffffffff814e33bb>] ? _raw_spin_unlock+0x2b/0x40
[   55.838523]  [<ffffffff8114c652>] ksm_scan_thread+0x132/0xe20
[   55.838541]  [<ffffffff814df822>] ? schedule+0x3b2/0x960
[   55.838559]  [<ffffffff810a5690>] ? wake_up_bit+0x40/0x40
[   55.838576]  [<ffffffff8114c520>] ? run_store+0x310/0x310
[   55.838593]  [<ffffffff810a5186>] kthread+0x96/0xa0
[   55.838609]  [<ffffffff814e5014>] kernel_thread_helper+0x4/0x10
[   55.838628]  [<ffffffff814e3700>] ? retint_restore_args+0xe/0xe
[   55.838647]  [<ffffffff810a50f0>] ? __init_kthread_worker+0x70/0x70
[   55.838666]  [<ffffffff814e5010>] ? gs_change+0xb/0xb
[   55.838681] Code: b7 00 00 48 89 fb 85 c0 41 89 f5 45 0f 45 f0 8b 05 84 de 68 00 85 c0 0f 84 7b 09 00 00 8b 05 7a 49 7a 00 85 c0 0f 84 c6 01 00 00 
[   55.838780]  8b 03 ba 01 00 00 00 48 3d e0 3c 8c 81 44 0f 44 f2 41 83 fd 
[   55.838830] RIP  [<ffffffff810bb9b2>] __lock_acquire+0x62/0x1d70
[   55.838850]  RSP <ffff88023d3abc50>
[   55.839567] CR2: 00000000000000e8
[   55.895721] ---[ end trace eea0fa5dfa6846f1 ]---

The bug can be easily reproduced using the following 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()?

Probably the following is not the right way to fix it, but if I apply
this the problem disappears. Anyway, I'm posting this information, it
can help you to debug the problem better.

Signed-off-by: Andrea Righi <andrea@...terlinux.com>
---
 mm/ksm.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index d708b3e..f457feb 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1308,6 +1308,8 @@ next_mm:
 	}
 
 	mm = slot->mm;
+	if (unlikely(!mm))
+		return NULL;
 	down_read(&mm->mmap_sem);
 	if (ksm_test_exit(mm))
 		vma = NULL;
--
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