Date:	Thu, 2 Jun 2011 03:09:53 -0400 (EDT)
From:	CAI Qian <caiqian@...hat.com>
To:	Andrea Righi <andrea@...terlinux.com>
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,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do_scan()

Hello,

----- Original Message -----
> 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;
Hmm, the reproducer gave something else here but no panic.
$ strace ./test 
execve("./test", ["./test"], [/* 26 vars */]) = 0
brk(0)                                  = 0x220f000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd18ec0a000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=41227, ...}) = 0
mmap(NULL, 41227, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd18ebff000
close(3)                                = 0
open("/lib64/libc.so.6", O_RDONLY)      = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260\355\341n<\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1912928, ...}) = 0
mmap(0x3c6ee00000, 3737768, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3c6ee00000
mprotect(0x3c6ef87000, 2097152, PROT_NONE) = 0
mmap(0x3c6f187000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x187000) = 0x3c6f187000
mmap(0x3c6f18c000, 18600, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x3c6f18c000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd18ebfe000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd18ebfd000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd18ebfc000
arch_prctl(ARCH_SET_FS, 0x7fd18ebfd700) = 0
mprotect(0x3c6f187000, 16384, PROT_READ) = 0
mprotect(0x3c6e81f000, 4096, PROT_READ) = 0
munmap(0x7fd18ebff000, 41227)           = 0
brk(0)                                  = 0x220f000
brk(0x2232000)                          = 0x2232000
madvise(0x2210000, 4096, 0xc /* MADV_??? */) = 0
--- SIGSEGV (Segmentation fault) @ 0 (0) ---
+++ killed by SIGSEGV (core dumped) +++
Segmentation fault (core dumped)

Did I miss anything?

Thanks,
CAI Qian
> 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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