linux-kernel - Re: [PATCH] mm: mmap: show vm_unmapped

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Date:   Mon, 9 Mar 2020 07:01:48 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jaewon Kim <jaewon31.kim@...sung.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, walken@...gle.com,
        bp@...e.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        jaewon31.kim@...il.com
Subject: Re: [PATCH] mm: mmap: show vm_unmapped_area error log

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@...sung.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