Re: [PATCH] mm: dump_page: add debugfs file for dumping page state by pfn

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 25/05/2020 18.33, Matthew Wilcox wrote:
> On Mon, May 25, 2020 at 05:19:11PM +0300, Konstantin Khlebnikov wrote:
>> Tool 'page-types' could list pages mapped by process or file cache pages,
>> but it shows only limited amount of state exported via procfs.
>>
>> Let's employ existing helper dump_page() to reach remaining information:
>> writing pfn into /sys/kernel/debug/dump_page dumps state into kernel log.
>>
>> # echo 0x37c43c > /sys/kernel/debug/dump_page
>> # dmesg | tail -6
>>   page:ffffcb0b0df10f00 refcount:1 mapcount:0 mapping:000000007755d3d9 index:0x30
>>   0xffffffffae4239e0 name:"libGeoIP.so.1.6.9"
>>   flags: 0x200000000020014(uptodate|lru|mappedtodisk)
>>   raw: 0200000000020014 ffffcb0b187fd288 ffffcb0b189e6248 ffff9528a04afe10
>>   raw: 0000000000000030 0000000000000000 00000001ffffffff 0000000000000000
>>   page dumped because: debugfs request
> 
> This makes me deeply uncomfortable.  We're using %px, and %lx
> (for the 'raw' lines) so we actually get to see kernel addresses.
> We've rationalised this in the past as being acceptable because you're
> already in an "assert triggered" kind of situation.  Now you're adding
> a way for any process with CAP_SYS_ADMIN to get kernel addresses dumped
> into the syslog.
> 
> I think we need a different function for this, or we need to re-audit
> dump_page() for exposing kernel pointers, and not expose the raw data
> in struct page.
> 

It's better to add switch for disabling paranoia if bad things happening.
I.e. keep everything safe by default (or whatever sysctl/config set) and
flip the switch when needed.