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 22:42:06 +0200 [thread overview]
Message-ID: <13f149cc-6535-8b50-04de-70437a679826@redhat.com> (raw)
In-Reply-To: <CAHk-=wh1q7ZSWhDWOyqmVawqjq55sUVkn8ASjE_b2VOcE1vFaA@mail.gmail.com>
On 09.08.22 22:30, Linus Torvalds wrote:
> On Tue, Aug 9, 2022 at 1:20 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> IIUC VM_MAYSHARE is always set in a MAP_SHARED mapping, but for file
>> mappings we only set VM_SHARED if the file allows for writes
>
> Heh.
>
> This is a horrific hack, and probably should go away.
>
> Yeah, we have that
>
> if (!(file->f_mode & FMODE_WRITE))
> vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
>
>
> but I think that's _entirely_ historical.
>
> Long long ago, in a galaxy far away, we didn't handle shared mmap()
> very well. In fact, we used to not handle it at all.
>
> But nntpd would use write() to update the spool file, adn them read it
> through a shared mmap.
>
> And since our mmap() *was* coherent with people doing write() system
> calls, but didn't handle actual dirty shared mmap, what Linux used to
> do was to just say "Oh, you want a read-only shared file mmap? I can
> do that - I'll just downgrade it to a read-only _private_ mapping, and
> it actually ends up with the same semantics".
>
> And here we are, 30 years later, and it still does that, but it leaves
> the VM_MAYSHARE flag so that /proc/<pid>/maps can show that it's a
> shared mapping.
I was suspecting that this code is full of legacy :)
What would make sense to me is to just have VM_SHARED and make it
correspond to MAP_SHARED, that would at least confuse me less. Once I
have some spare cycles I'll see how easy that might be to achieve.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2022-08-09 20:42 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-08 7:32 [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW David Hildenbrand
2022-08-08 16:02 ` David Hildenbrand
2022-08-09 18:27 ` Linus Torvalds
2022-08-09 18:45 ` David Hildenbrand
2022-08-09 18:59 ` Linus Torvalds
2022-08-09 19:07 ` Jason Gunthorpe
2022-08-09 19:21 ` Linus Torvalds
2022-08-09 21:16 ` David Laight
2022-08-11 7:13 ` [PATCH] sched/all: Change BUG_ON() instances to WARN_ON() Ingo Molnar
2022-08-11 20:43 ` Linus Torvalds
2022-08-11 21:28 ` Matthew Wilcox
2022-08-11 23:22 ` Jason Gunthorpe
2022-08-14 1:10 ` John Hubbard
2022-08-12 9:29 ` [PATCH v2] sched/all: Change all BUG_ON() instances in the scheduler to WARN_ON_ONCE() Ingo Molnar
2022-08-15 14:41 ` Mel Gorman
2022-08-15 22:12 ` John Hubbard
2022-08-16 9:40 ` Mel Gorman
2022-08-21 11:28 ` Ingo Molnar
2022-08-22 8:40 ` Mel Gorman
2022-08-12 9:57 ` [tip: sched/urgent] " tip-bot2 for Ingo Molnar
2022-08-09 18:40 ` [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW Linus Torvalds
2022-08-09 18:48 ` Jason Gunthorpe
2022-08-09 18:53 ` David Hildenbrand
2022-08-09 19:07 ` Linus Torvalds
2022-08-09 19:20 ` David Hildenbrand
2022-08-09 18:48 ` Linus Torvalds
2022-08-09 19:09 ` David Hildenbrand
2022-08-09 20:00 ` Linus Torvalds
2022-08-09 20:06 ` David Hildenbrand
2022-08-09 20:07 ` David Hildenbrand
2022-08-09 20:14 ` Linus Torvalds
2022-08-09 20:20 ` David Hildenbrand
2022-08-09 20:30 ` Linus Torvalds
2022-08-09 20:38 ` Linus Torvalds
2022-08-09 20:42 ` David Hildenbrand [this message]
2022-08-09 20:20 ` Linus Torvalds
2022-08-09 20:23 ` David Hildenbrand
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