linux-kernel - Re: [PATCH] mm/gup: protect unpin_user

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Date:   Sat, 19 Sep 2020 21:13:17 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Souptick Joarder <jrdr.linux@...il.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <alex.bou9@...il.com>,
        <gustavoars@...nel.org>, Ira Weiny <ira.weiny@...el.com>,
        <linux-kernel@...r.kernel.org>, <madhuparnabhowmik10@...il.com>,
        <mporter@...nel.crashing.org>, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm/gup: protect unpin_user_pages() against npages==-ERRNO

On 9/19/20 8:03 PM, Souptick Joarder wrote:
> On Thu, Sep 17, 2020 at 1:11 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
>> On Wed, Sep 16, 2020 at 11:57:06PM -0700, John Hubbard wrote:
>>> As suggested by Dan Carpenter, fortify unpin_user_pages() just a bit,
>>> against a typical caller mistake: check if the npages arg is really a
>>> -ERRNO value, which would blow up the unpinning loop: WARN and return.
>>>
>>> If this new WARN_ON() fires, then the system *might* be leaking pages
>>> (by leaving them pinned), but probably not. More likely, gup/pup
>>> returned a hard -ERRNO error to the caller, who erroneously passed it
>>> here.
...
> 
> Do we need a similar check inside unpin_user_pages_dirty_lock(),
> when make_dirty set to false ?

Maybe not. This call is rarely if ever used for error handling, but
rather, for finishing up a successful use of the pages.

There is a balance between protecting against buggy callers and just
fixing any buggy callers. There is also a limit to how much code one can
write in hopes of avoiding bugs in...code that one writes. :)  Which is
why static analysis, unit and regression tests, code reviews are
important too.

Here, I submit that that we're about to cross the line and go too far.
But if you have any examples of buggy callers for
unpin_user_pages_dirty_lock(), that might shift the line.

Or maybe others feel that we haven't gone far enough yet after all?

thanks,
-- 
John Hubbard
NVIDIA