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

[Jann Horn]

On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> <joel@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx
> > Cc: john.stultz@xxxxxxxxxx
> > Cc: tkjos@xxxxxxxxxx
> > Cc: gregkh@xxxxxxxxxxxxxxxxxxx
> > Cc: hch@xxxxxxxxxxxxx
> > Reviewed-by: John Stultz <john.stultz@xxxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> [...]
> > 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.