Re: [PATCH] mm: mmap: show vm_unmapped_area error log

[Matthew Wilcox <willy@infradead.org>]

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.

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.