linux-kernel - Re: [PATCH v3 1/2] mm: Add an F_SEAL_FUTURE

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Message-ID: <118792.1539974951@turing-police.cc.vt.edu>
Date:   Fri, 19 Oct 2018 14:49:11 -0400
From:   valdis.kletnieks@...edu
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        John Reck <jreck@...gle.com>,
        John Stultz <john.stultz@...aro.org>,
        Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Colascione <dancol@...gle.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel@...r.kernel.org,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, marcandre.lureau@...hat.com,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Minchan Kim <minchan@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd

On Fri, 19 Oct 2018 10:57:31 -0700, Joel Fernandes said:
> On Fri, Oct 19, 2018 at 10:32 AM,  <valdis.kletnieks@...edu> wrote:
> > What is supposed to happen if some other process has an already existing R/W
> > mmap of the region?  (For that matter, the test program doesn't seem to
> > actually test that the existing mmap region remains writable?)

> Why would it not remain writable? We don't change anything in the
> mapping that prevents it from being writable, in the patch.

OK, if the meaning here is "if another process races and gets its own R/W mmap
before we seal our mmap, it's OK".  Seems like somewhat shaky security-wise - a
possibly malicious process can fail to get a R/W map because we just sealed it,
but if it had done the attempt a few milliseconds earlier it would have its own
R/W mmap to do as it pleases...

On the other hand, decades of trying have proven that trying to do any sort
of revoke() is a lot harder to do than it looks...

> We do test that existing writable mmaps can continue to exist after
> the seal is set, in a way, because we test that setting of the seal
> succeeds.

Well, if the semantics are "We don't bother trying to deal with existing R/W
maps", then it doesn't really matter - I was thinking along the lines of "If we're
revoking other R/W accesses, we should test that we didn't nuke *this* one in
the bargain"....

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