From: Joel Fernandes <joel@joelfernandes.org>
To: Daniel Colascione <dancol@google.com>
Cc: Jann Horn <jannh@google.com>,
kernel list <linux-kernel@vger.kernel.org>,
"jreck@google.com" <jreck@google.com>,
John Stultz <john.stultz@linaro.org>,
Todd Kjos <tkjos@google.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Christoph Hellwig <hch@infradead.org>,
Al Viro <viro@zeniv.linux.org.uk>,
Andrew Morton <akpm@linux-foundation.org>,
Bruce Fields <bfields@fieldses.org>,
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Khalid Aziz <khalid.aziz@oracle.com>,
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Hugh Dickins <hughd@google.com>,
Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
Date: Sat, 10 Nov 2018 09:10:58 -0800 [thread overview]
Message-ID: <20181110171058.GA242356@google.com> (raw)
In-Reply-To: <CAKOZuethC7+YrRyyGciUCfhSSa9cCcAFJ8g_qEw9uh3TBbyOcg@mail.gmail.com>
On Sat, Nov 10, 2018 at 04:26:46AM -0800, Daniel Colascione wrote:
> On Friday, November 9, 2018, Joel Fernandes <joel@joelfernandes.org> wrote:
>
> > On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote:
> > > On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <jannh@google.com> wrote:
> > > > On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> > > > <joel@joelfernandes.org> wrote:
> > > > > Android uses ashmem for sharing memory regions. We are looking
> > forward
> > > > > to migrating all usecases of ashmem to memfd so that we can possibly
> > > > > remove the ashmem driver in the future from staging while also
> > > > > benefiting from using memfd and contributing to it. Note staging
> > drivers
> > > > > are also not ABI and generally can be removed at anytime.
> > > > >
> > > > > One of the main usecases Android has is the ability to create a
> > region
> > > > > and mmap it as writeable, then add protection against making any
> > > > > "future" writes while keeping the existing already mmap'ed
> > > > > writeable-region active. This allows us to implement a usecase where
> > > > > receivers of the shared memory buffer can get a read-only view, while
> > > > > the sender continues to write to the buffer.
> > > > > See CursorWindow documentation in Android for more details:
> > > > > https://developer.android.com/reference/android/database/
> > CursorWindow
> > > > >
> > > > > This usecase cannot be implemented with the existing F_SEAL_WRITE
> > seal.
> > > > > To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE
> > seal
> > > > > which prevents any future mmap and write syscalls from succeeding
> > while
> > > > > keeping the existing mmap active.
> > > >
> > > > Please CC linux-api@ on patches like this. If you had done that, I
> > > > might have criticized your v1 patch instead of your v3 patch...
> > > >
> > > > > The following program shows the seal
> > > > > working in action:
> > > > [...]
> > > > > Cc: jreck@google.com
> > > > > Cc: john.stultz@linaro.org
> > > > > Cc: tkjos@google.com
> > > > > Cc: gregkh@linuxfoundation.org
> > > > > Cc: hch@infradead.org
> > > > > Reviewed-by: John Stultz <john.stultz@linaro.org>
> > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > > > ---
> > > > [...]
> > > > > diff --git a/mm/memfd.c b/mm/memfd.c
> > > > > index 2bb5e257080e..5ba9804e9515 100644
> > > > > --- a/mm/memfd.c
> > > > > +++ b/mm/memfd.c
> > > > [...]
> > > > > @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file,
> > unsigned int seals)
> > > > > }
> > > > > }
> > > > >
> > > > > + if ((seals & F_SEAL_FUTURE_WRITE) &&
> > > > > + !(*file_seals & F_SEAL_FUTURE_WRITE)) {
> > > > > + /*
> > > > > + * The FUTURE_WRITE seal also prevents growing and
> > shrinking
> > > > > + * so we need them to be already set, or requested
> > now.
> > > > > + */
> > > > > + int test_seals = (seals | *file_seals) &
> > > > > + (F_SEAL_GROW | F_SEAL_SHRINK);
> > > > > +
> > > > > + if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
> > > > > + error = -EINVAL;
> > > > > + goto unlock;
> > > > > + }
> > > > > +
> > > > > + spin_lock(&file->f_lock);
> > > > > + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> > > > > + spin_unlock(&file->f_lock);
> > > > > + }
> > > >
> > > > So you're fiddling around with the file, but not the inode? How are
> > > > you preventing code like the following from re-opening the file as
> > > > writable?
> > > >
> > > > $ cat memfd.c
> > > > #define _GNU_SOURCE
> > > > #include <unistd.h>
> > > > #include <sys/syscall.h>
> > > > #include <printf.h>
> > > > #include <fcntl.h>
> > > > #include <err.h>
> > > > #include <stdio.h>
> > > >
> > > > int main(void) {
> > > > int fd = syscall(__NR_memfd_create, "testfd", 0);
> > > > if (fd == -1) err(1, "memfd");
> > > > char path[100];
> > > > sprintf(path, "/proc/self/fd/%d", fd);
> > > > int fd2 = open(path, O_RDWR);
> > > > if (fd2 == -1) err(1, "reopen");
> > > > printf("reopen successful: %d\n", fd2);
> > > > }
> > > > $ gcc -o memfd memfd.c
> > > > $ ./memfd
> > > > reopen successful: 4
> > > > $
> > > >
> > > > That aside: I wonder whether a better API would be something that
> > > > allows you to create a new readonly file descriptor, instead of
> > > > fiddling with the writability of an existing fd.
> > >
> > > My favorite approach would be to forbid open() on memfds, hope that
> > > nobody notices the tiny API break, and then add an ioctl for "reopen
> > > this memfd with reduced permissions" - but that's just my personal
> > > opinion.
> >
> > I did something along these lines and it fixes the issue, but I forbid open
> > of memfd only when the F_SEAL_FUTURE_WRITE seal is in place. So then its
> > not
> > an ABI break because this is a brand new seal. That seems the least
> > intrusive
> > solution and it works. Do you mind testing it and I'll add your and
> > Tested-by
> > to the new fix? The patch is based on top of this series.
> >
>
> Please don't forbid reopens entirely. You're taking a feature that works
> generally (reopens) and breaking it in one specific case (memfd write
> sealed files). The open modes are available in .open in the struct file:
> you can deny *only* opens for write instead of denying reopens generally.
Yes, as we discussed over chat already, I will implement it that way.
Also lets continue to discuss Andy's concerns he raised on the other thread.
thanks,
- Joel
next prev parent reply other threads:[~2018-11-10 17:11 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-08 4:15 [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd Joel Fernandes (Google)
2018-11-08 4:15 ` [PATCH v3 resend 2/2] selftests/memfd: Add tests for F_SEAL_FUTURE_WRITE seal Joel Fernandes (Google)
2018-11-09 8:49 ` [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd Joel Fernandes
2018-11-09 20:36 ` Andrew Morton
2018-11-10 3:54 ` Joel Fernandes
2018-11-09 21:06 ` Jann Horn
2018-11-09 21:19 ` Jann Horn
2018-11-10 3:20 ` Joel Fernandes
2018-11-10 6:05 ` Andy Lutomirski
2018-11-10 18:24 ` Joel Fernandes
2018-11-10 18:45 ` Daniel Colascione
2018-11-10 19:11 ` Daniel Colascione
2018-11-10 19:55 ` Andy Lutomirski
2018-11-10 22:09 ` Joel Fernandes
2018-11-10 22:18 ` Andy Lutomirski
2018-11-11 2:38 ` Joel Fernandes
2018-11-11 3:40 ` Andy Lutomirski
2018-11-11 4:01 ` Joel Fernandes
2018-11-11 8:09 ` Joel Fernandes
2018-11-11 8:30 ` Daniel Colascione
2018-11-11 15:14 ` Andy Lutomirski
2018-11-11 17:36 ` Joel Fernandes
[not found] ` <CAKOZuethC7+YrRyyGciUCfhSSa9cCcAFJ8g_qEw9uh3TBbyOcg@mail.gmail.com>
2018-11-10 17:10 ` Joel Fernandes [this message]
2018-11-09 21:40 ` Andy Lutomirski
2018-11-09 20:02 ` Michael Tirado
2018-11-10 1:49 ` Joel Fernandes
2018-11-09 22:20 ` Daniel Colascione
2018-11-09 22:37 ` Andy Lutomirski
2018-11-09 22:42 ` Daniel Colascione
2018-11-09 23:14 ` Andy Lutomirski
2018-11-10 1:36 ` Joel Fernandes
2018-11-09 23:46 ` Joel Fernandes
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