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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@betterlinux.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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