Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd

[Daniel Colascione <dancol@google.com>]

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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.

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