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Subject: Re: [PATCH] mm: mmap: show vm_unmapped_area error log
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>, walken@google.com,
        bp@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        jaewon31.kim@gmail.com
From:   Jaewon Kim <jaewon31.kim@samsung.com>
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Date:   Tue, 10 Mar 2020 13:18:40 +0900
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On 2020년 03월 09일 23:01, Matthew Wilcox wrote:
> On Mon, Mar 09, 2020 at 06:12:03PM +0900, Jaewon Kim wrote:
>> On 2020년 03월 08일 21:36, Matthew Wilcox wrote:
>>> On Sun, Mar 08, 2020 at 06:58:47PM +0900, Jaewon Kim wrote:
>>>> On 2020년 03월 08일 10:58, Matthew Wilcox wrote:
>>>>> On Sat, Mar 07, 2020 at 03:47:44PM -0800, Andrew Morton wrote:
>>>>>> On Fri, 6 Mar 2020 15:16:22 +0900 Jaewon Kim <jaewon31.kim@samsung.com> wrote:
>>>>>>> Even on 64 bit kernel, the mmap failure can happen for a 32 bit task.
>>>>>>> Virtual memory space shortage of a task on mmap is reported to userspace
>>>>>>> as -ENOMEM. It can be confused as physical memory shortage of overall
>>>>>>> system.
>>>>> But userspace can trigger this printk.  We don't usually allow printks
>>>>> under those circumstances, even ratelimited.
>>>> Hello thank you your comment.
>>>>
>>>> Yes, userspace can trigger printk, but this was useful for to know why
>>>> a userspace task was crashed. There seems to be still many userspace
>>>> applications which did not check error of mmap and access invalid address.
>>>>
>>>> Additionally in my AARCH64 Android environment, display driver tries to
>>>> get userspace address to map its display memory. The display driver
>>>> report -ENOMEM from vm_unmapped_area and but also from GPU related
>>>> address space.
>>>>
>>>> Please let me know your comment again if this debug is now allowed
>>>> in that userspace triggered perspective.
>>> The scenario that worries us is an attacker being able to fill the log
>>> files and so also fill (eg) the /var partition.  Once it's full, future
>>> kernel messages cannot be stored anywhere and so there will be no traces
>>> of their privilege escalation.
>> Although up to 10 times within 5 secs is not many, I think those log may remove
>> other important log in log buffer if it is the system which produces very few log.
>> In my Android phone device system, there seems to be much more kernel log though.
> I've never seen the logs on my android phone.  All that a ratelimit is
> going to do is make the attacker be more patient.
>
>>> Maybe a tracepoint would be a better idea?  Usually they are disabled,
>>> but they can be enabled by a sysadmin to gain insight into why an
>>> application is crashing.
>> In Android phone device system, we cannot get sysadmin permission if it is built
>> for end user. And it is not easy to reproduce this symptom because it is an user's app.
>>
>> Anyway let me try pr_devel_ratelimited which is disabled by default. I hope this is
>> good enough. Additionally I moved the code from mm.h to mmap.c.
> https://source.android.com/devices/tech/debug/ftrace
I am not sure if an end user can enable a trace point which is not writable.
Anyway I created trace mmap.h file and changed printk to trace_vm_unmapped_area
without ratelimited.

If there is no objection, I am going to resubmit whole patch as v2.
Thank you for your comment.

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -47,6 +47,8 @@
 #include <linux/pkeys.h>
 #include <linux/oom.h>
 #include <linux/sched/mm.h>
+#define CREATE_TRACE_POINTS
+#include <trace/events/mmap.h>
 
 #include <linux/uaccess.h>
 #include <asm/cacheflush.h>
@@ -2061,10 +2063,15 @@ unsigned long unmapped_area_topdown(struct vm_unmapped_area_info *info)
  */
 unsigned long vm_unmapped_area(struct vm_unmapped_area_info *info)
 {
+       unsigned long addr;
+
        if (info->flags & VM_UNMAPPED_AREA_TOPDOWN)
-               return unmapped_area_topdown(info);
+               addr = unmapped_area_topdown(info);
        else
-               return unmapped_area(info);
+               addr = unmapped_area(info);
+
+       trace_vm_unmapped_area(addr, info);
+       return addr;
 }

>