Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW

From: David Hildenbrand <david@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	stable@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Axel Rasmussen <axelrasmussen@google.com>,
	Peter Xu <peterx@redhat.com>, Hugh Dickins <hughd@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	John Hubbard <jhubbard@nvidia.com>,
	Jason Gunthorpe <jgg@nvidia.com>
Subject: Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW
Date: Tue, 9 Aug 2022 20:45:33 +0200	[thread overview]
Message-ID: <1a48d71d-41ee-bf39-80d2-0102f4fe9ccb@redhat.com> (raw)
In-Reply-To: <CAHk-=wiEAH+ojSpAgx_Ep=NKPWHU8AdO3V56BXcCsU97oYJ1EA@mail.gmail.com>

On 09.08.22 20:27, Linus Torvalds wrote:
> I'm still reading through this, but
> 
>  STOP DOING THIS
> 
> On Mon, Aug 8, 2022 at 12:32 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> +       VM_BUG_ON(!is_cow_mapping(vma->vm_flags));
> 
> Using BUG_ON() for debugging is simply not ok.
> 
> And saying "but it's just a VM_BUG_ON()" does not change *anything*.
> At least Fedora enables that unconditionally for normal people, it is
> not some kind of "only VM people do this".
> 
> Really. BUG_ON() IS NOT FOR DEBUGGING.

I totally agree with BUG_ON ... but if I get talked to in all-caps on a
Thursday evening and feel like I just touched the forbidden fruit, I
have to ask for details about VM_BUG_ON nowadays.

VM_BUG_ON is only active with CONFIG_DEBUG_VM. ... which indicated some
kind of debugging at least to me. I *know* that Fedora enables it and I
*know* that this will make Fedora crash.

I know why Fedora enables this debug option, but it somewhat destorys
the whole purpose of VM_BUG_ON kind of nowadays?

For this case, this condition will never trigger and I consider it much
more a hint to the reader that we can rest assured that this condition
holds. And on production systems, it will get optimized out.

Should we forbid any new usage of VM_BUG_ON just like we mostly do with
BUG_ON?

> 
> Stop it. Now.
> 
> If you have a condition that must not happen, you either write that
> condition into the code, or - if you are convinced it cannot happen -
> you make it a WARN_ON_ONCE() so that people can report it to you.

I can just turn that into a WARN_ON_ONCE() or even a VM_WARN_ON_ONCE().

-- 
Thanks,

David / dhildenb