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

[Joel Fernandes <joel@joelfernandes.org>]

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.

---8<-----------
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Subject: [PATCH] mm/memfd: Fix possible promotion to writeable of sealed memfd

Jann Horn found that reopening an F_SEAL_FUTURE_WRITE sealed memfd
through /proc/self/fd/N symlink as writeable succeeds. The simplest fix
without causing ABI breakages and ugly VFS hacks is to simply deny all
opens on F_SEAL_FUTURE_WRITE sealed fds.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 mm/shmem.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 446942677cd4..5b378c486b8f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3611,7 +3611,25 @@ static const struct address_space_operations shmem_aops = {
 	.error_remove_page = generic_error_remove_page,
 };
 
+/* Could arrive here for memfds opened through /proc/ */
+int shmem_open(struct inode *inode, struct file *file)
+{
+	struct shmem_inode_info *info = SHMEM_I(inode);
+
+	/*
+	 * memfds for which future writes have been prevented
+	 * should not be reopened, say, through /proc/pid/fd/N
+	 * symlinks otherwise it can cause a sealed memfd to be
+	 * promoted to writable.
+	 */
+	if (info->seals & F_SEAL_FUTURE_WRITE)
+		return -EACCES;
+
+	return 0;
+}
+
 static const struct file_operations shmem_file_operations = {
+	.open		= shmem_open,
 	.mmap		= shmem_mmap,
 	.get_unmapped_area = shmem_get_unmapped_area,
 #ifdef CONFIG_TMPFS
-- 
2.19.1.930.g4563a0d9d0-goog