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 22:23:49 +0200	[thread overview]
Message-ID: <915e2f40-b94a-cef4-7aa7-a402e93dae68@redhat.com> (raw)
In-Reply-To: <CAHk-=wgYAy5ho0Wqx+eri_+a5apYU1Th826UScE7rZRiyhPcGA@mail.gmail.com>

On 09.08.22 22:20, Linus Torvalds wrote:
> On Tue, Aug 9, 2022 at 1:14 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> But as there are two bits, I'm sure somebody ends up touching one and
>> not the other.
> 
> Ugh. The nommu code certainly does odd things here. That just looks wrong.
> 
> And the hugetlb code does something horrible too, but never *sets* it,
> so it looks like some odd subset of VM_SHARED.

mm/mmap.c:do_mmap() contains the magic bit

switch (flags & MAP_TYPE) {
case MAP_SHARED:
...
case MAP_SHARED_VALIDATE:
...
vm_flags |= VM_SHARED | VM_MAYSHARE;
if (!(file->f_mode & FMODE_WRITE))
	vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
fallthrough;

VM_SHARED semantics are confusing.

-- 
Thanks,

David / dhildenb