Re: [PATCH v3 1/9] mm: add overflow and underflow checks for page->_refcount

[Vlastimil Babka]

On 1/26/22 20:22, Pasha Tatashin wrote:
> On Wed, Jan 26, 2022 at 1:59 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>>
>> On Wed, Jan 26, 2022 at 06:34:21PM +0000, Pasha Tatashin wrote:
>> > The problems with page->_refcount are hard to debug, because usually
>> > when they are detected, the damage has occurred a long time ago. Yet,
>> > the problems with invalid page refcount may be catastrophic and lead to
>> > memory corruptions.
>> >
>> > Reduce the scope of when the _refcount problems manifest themselves by
>> > adding checks for underflows and overflows into functions that modify
>> > _refcount.
>>
>> If you're chasing a bug like this, presumably you turn on page
>> tracepoints.  So could we reduce the cost of this by putting the
>> VM_BUG_ON_PAGE parts into __page_ref_mod() et al?  Yes, we'd need to
>> change the arguments to those functions to pass in old & new, but that
>> should be a cheap change compared to embedding the VM_BUG_ON_PAGE.
> 
> This is not only about chasing a bug. This also about preventing
> memory corruption and information leaking that are caused by ref_count
> bugs from happening.

So you mean it like a security hardening feature, not just debugging? To me
it's dubious to put security hardening under CONFIG_DEBUG_VM. I think it's
just Fedora that uses DEBUG_VM in general production kernels?

> Several months ago a memory corruption bug was discovered by accident:
> an engineer was studying a process core from a production system and
> noticed that some memory does not look like it belongs to the original
> process. We tried to manually reproduce that bug but failed. However,
> later analysis by our team, explained that the problem occured due to
> ref_count bug in Linux, and the bug itself was root caused and fixed
> (mentioned in the cover letter).  This work would have prevented
> similar ref_count bugs from yielding to the memory corruption
> situation.
> 
> Pasha
>