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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Aleksa Sarai <cyphar@cyphar.com>
Cc: linux-ia64@vger.kernel.org,
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Subject: Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution
Date: Wed, 4 Sep 2019 14:35:22 -0700
Message-ID: <CAHk-=wiHRW3Z9xPRiExi9jLjB0cdGhM=3vaW+b80mjuRcbORyw@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wiod1rQMU+6Zew=cLE8uX4tUdf42bM5eKngMnNVS2My7g@mail.gmail.com>

On Wed, Sep 4, 2019 at 2:09 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So you'd have three stages:
>
>  1) ".." always returns -EXDEV
>
>  2) ".." returns -EXDEV if there was a concurrent rename/mount
>
>  3) ".." returns -EXDEV if there was a concurrent rename/mount and we
> reset the sequence numbers and check if you escaped.

In fact, I wonder if this should return -EAGAIN instead - to say that
"retrying may work".

Because then:

> Also, I'm not 100% convinced that (3) is needed at all. I think the
> retry could be done in user space instead, which needs to have a
> fallback anyway. Yes? No?

Any user mode fallback would want to know whether it's a final error
or whether simply re-trying might make it work again.

I think that re-try case is valid for any of the possible "races
happened, we can't guarantee that it's safe", and retrying inside the
kernel (or doing that re-validation) could have latency issues.

Maybe ".." is the only such case. I can't think of any other ones in
your series, but at least conceptually they could happen. For example,
we've had people who wanted pathname lookup without any IO happening,
because if you have to wait for IO you could want to use another
thread etc if you're doing some server in user space..

                     Linus

  reply index

Thread overview: 65+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-04 20:19 [PATCH v12 00/12] namei: openat2(2) path resolution restrictions Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Aleksa Sarai
2019-09-04 20:48   ` [PATCH v12 01/12] lib: introduce copy_struct_{to, from}_user helpers Linus Torvalds
2019-09-04 21:00   ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Randy Dunlap
2019-09-05  7:32   ` Peter Zijlstra
2019-09-05  9:26     ` Aleksa Sarai
2019-09-05  9:43       ` Peter Zijlstra
2019-09-05 10:57         ` Peter Zijlstra
2019-09-11 10:37           ` Aleksa Sarai
2019-09-05 13:35         ` Aleksa Sarai
2019-09-05 17:01         ` Aleksa Sarai
2019-09-05  8:43   ` Rasmus Villemoes
2019-09-05  9:50     ` Aleksa Sarai
2019-09-05 10:45       ` Christian Brauner
2019-09-05  9:09   ` [PATCH v12 01/12] lib: introduce copy_struct_{to, from}_user helpers Andreas Schwab
2019-09-05 10:13     ` Gabriel Paubert
2019-09-05 11:05   ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Christian Brauner
2019-09-05 11:17     ` Rasmus Villemoes
2019-09-05 11:29       ` Christian Brauner
2019-09-05 13:40     ` Aleksa Sarai
2019-09-05 11:09   ` Christian Brauner
2019-09-05 11:27     ` Aleksa Sarai
2019-09-05 11:40       ` Christian Brauner
2019-09-05 18:07   ` Al Viro
2019-09-05 18:23     ` Christian Brauner
2019-09-05 18:28       ` Al Viro
2019-09-05 18:35         ` Christian Brauner
2019-09-05 19:56         ` Aleksa Sarai
2019-09-05 22:31           ` Al Viro
2019-09-06  7:00           ` Christian Brauner
2019-09-05 23:00     ` Aleksa Sarai
2019-09-05 23:49       ` Al Viro
2019-09-06  0:09         ` Aleksa Sarai
2019-09-06  0:14         ` Al Viro
2019-09-04 20:19 ` [PATCH v12 02/12] clone3: switch to copy_struct_from_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 03/12] sched_setattr: switch to copy_struct_{to, from}_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 04/12] perf_event_open: switch to copy_struct_from_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 05/12] namei: obey trailing magic-link DAC permissions Aleksa Sarai
2019-09-17 21:30   ` Jann Horn
2019-09-18 13:51     ` Aleksa Sarai
2019-09-18 15:46       ` Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 06/12] procfs: switch magic-link modes to be more sane Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 07/12] open: O_EMPTYPATH: procfs-less file descriptor re-opening Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 08/12] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 09/12] namei: LOOKUP_IN_ROOT: chroot-like path resolution Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai
2019-09-04 21:09   ` Linus Torvalds
2019-09-04 21:35     ` Linus Torvalds [this message]
2019-09-04 21:36       ` Linus Torvalds
2019-09-04 21:48     ` Aleksa Sarai
2019-09-04 22:16       ` Linus Torvalds
2019-09-04 22:31       ` David Howells
2019-09-04 22:38         ` Linus Torvalds
2019-09-04 23:29           ` Al Viro
2019-09-04 23:44             ` Linus Torvalds
2019-09-04 20:19 ` [PATCH v12 11/12] open: openat2(2) syscall Aleksa Sarai
2019-09-04 21:00   ` Randy Dunlap
2019-09-07 12:40   ` Jeff Layton
2019-09-07 16:58     ` Linus Torvalds
2019-09-07 17:42       ` Andy Lutomirski
2019-09-07 17:45         ` Linus Torvalds
2019-09-07 18:15           ` Andy Lutomirski
2019-09-10  6:35           ` Ingo Molnar
2019-09-08 16:24     ` Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 12/12] selftests: add openat2(2) selftests Aleksa Sarai

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