Re: [PATCH] ksm: fix conflict between mmput and scan_get_next_rmap_item

zhouchengming
2016-05-06T02:56:02Z

On 2016/5/6 5:07, Andrew Morton wrote:
> On Thu, 5 May 2016 20:42:56 +0800 Zhou Chengming<zhouchengming1@huawei.com> wrote:
>
>> A concurrency issue about KSM in the function scan_get_next_rmap_item.
>>
>> task A (ksmd): |task B (the mm's task):
>> |
>> mm = slot->mm; |
>> down_read(&mm->mmap_sem); |
>> |
>> ... |
>> |
>> spin_lock(&ksm_mmlist_lock); |
>> |
>> ksm_scan.mm_slot go to the next slot; |
>> |
>> spin_unlock(&ksm_mmlist_lock); |
>> |mmput() ->
>> | ksm_exit():
>> |
>> |spin_lock(&ksm_mmlist_lock);
>> |if (mm_slot&& ksm_scan.mm_slot != mm_slot) {
>> | if (!mm_slot->rmap_list) {
>> | easy_to_free = 1;
>> | ...
>> |
>> |if (easy_to_free) {
>> | mmdrop(mm);
>> | ...
>> |
>> |So this mm_struct will be freed successfully.
>> |
>> up_read(&mm->mmap_sem); |
>>
>> As we can see above, the ksmd thread may access a mm_struct that already
>> been freed to the kmem_cache.
>> Suppose a fork will get this mm_struct from the kmem_cache, the ksmd thread
>> then call up_read(&mm->mmap_sem), will cause mmap_sem.count to become -1.
>> I changed the scan_get_next_rmap_item function refered to the khugepaged
>> scan function.
>
> Thanks.
>
> We need to decide whether this fix should be backported into earlier
> (-stable) kernels. Can you tell us how easily this is triggered and
> share your thoughts on this?
>
>
> .
>

I write a patch that can easily trigger this bug.
When ksmd go to sleep, if a fork get this mm_struct, BUG_ON
will be triggered.

From eedfdd12eb11858f69ff4a4300acad42946ca260 Mon Sep 17 00:00:00 2001
From: Zhou Chengming <zhouchengming1@huawei.com>
Date: Thu, 5 May 2016 17:49:22 +0800
Subject: [PATCH] ksm: trigger a bug

Signed-off-by: Zhou Chengming <zhouchengming1@huawei.com>
---
mm/ksm.c | 17 +++++++++++++++++
1 files changed, 17 insertions(+), 0 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index ca6d2a0..676368c 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1519,6 +1519,18 @@ static struct rmap_item
*get_next_rmap_item(struct mm_slot *mm_slot,
return rmap_item;
}

+static void trigger_a_bug(struct task_struct *p, struct mm_struct *mm)
+{
+ /* send KILL sig to the task, hope the mm_struct will be freed */
+ do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
+ /* sleep for 5s, the mm_struct will be freed and another fork
+ * will use this mm_struct
+ */
+ schedule_timeout(msecs_to_jiffies(5000));
+ /* the mm_struct owned by another task */
+ BUG_ON(mm->owner != p);
+}
+
static struct rmap_item *scan_get_next_rmap_item(struct page **page)
{
struct mm_struct *mm;
@@ -1526,6 +1538,7 @@ static struct rmap_item
*scan_get_next_rmap_item(struct page **page)
struct vm_area_struct *vma;
struct rmap_item *rmap_item;
int nid;
+ struct task_struct *taskp;

if (list_empty(&ksm_mm_head.mm_list))
return NULL;
@@ -1636,6 +1649,8 @@ next_mm:
remove_trailing_rmap_items(slot, ksm_scan.rmap_list);

spin_lock(&ksm_mmlist_lock);
+ /* get the mm's task now in the ksm_mmlist_lock */
+ taskp = mm->owner;
ksm_scan.mm_slot = list_entry(slot->mm_list.next,
struct mm_slot, mm_list);
if (ksm_scan.address == 0) {
@@ -1651,6 +1666,7 @@ next_mm:
hash_del(&slot->link);
list_del(&slot->mm_list);
spin_unlock(&ksm_mmlist_lock);
+ trigger_a_bug(taskp, mm);

free_mm_slot(slot);
clear_bit(MMF_VM_MERGEABLE, &mm->flags);
@@ -1658,6 +1674,7 @@ next_mm:
mmdrop(mm);
} else {
spin_unlock(&ksm_mmlist_lock);
+ trigger_a_bug(taskp, mm);
up_read(&mm->mmap_sem);
}

--
1.7.7