Re: [PATCH V2 2/2] mm/highmem: Lift memcpy_[to|from]_page to core

[Dan Williams]

On Tue, Dec 8, 2020 at 8:03 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Tue, Dec 08, 2020 at 06:22:50PM -0800, Ira Weiny wrote:
> > Right now we have a mixed bag.  zero_user() [and it's variants, circa 2008]
> > does a BUG_ON.[0]  While the other ones do nothing; clear_highpage(),
> > clear_user_highpage(), copy_user_highpage(), and copy_highpage().
>
> Erm, those functions operate on the entire PAGE_SIZE.  There's nothing
> for them to check.
>
> > While continuing to audit the code I don't see any users who would violating
> > the API with a simple conversion of the code.  The calls which I have worked on
> > [which is many at this point] all have checks in place which are well aware of
> > page boundaries.
>
> Oh good, then this BUG_ON won't trigger.
>
> > Therefore, I tend to agree with Dan that if anything is to be done it should be
> > a WARN_ON() which is only going to throw an error that something has probably
> > been wrong all along and should be fixed but continue running as before.
>
> Silent data corruption is for ever.  Are you absolutely sure nobody has
> done:
>
>         page = alloc_pages(GFP_HIGHUSER_MOVABLE, 3);
>         memcpy_to_page(page, PAGE_SIZE * 2, p, PAGE_SIZE * 2);
>
> because that will work fine if the pages come from ZONE_NORMAL and fail
> miserably if they came from ZONE_HIGHMEM.

...and violently regress with the BUG_ON.

The question to me is: which is more likely that any bad usages have
been covered up by being limited to ZONE_NORMAL / 64-bit only, or that
silent data corruption has been occurring with no ill effects?

> > FWIW I think this is a 'bad BUG_ON' use because we are "checking something that
> > we know we might be getting wrong".[1]  And because, "BUG() is only good for
> > something that never happens and that we really have no other option for".[2]
>
> BUG() is our only option here.  Both limiting how much we copy or
> copying the requested amount result in data corruption or leaking
> information to a process that isn't supposed to see it.

At a minimum I think this should be debated in a follow on patch to
add assertion checking where there was none before. There is no
evidence of a page being overrun in the audit Ira performed.