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

10 Nov 2018

On Sat, Nov 10, 2018 at 04:26:46AM -0800, Daniel Colascione wrote:
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On Friday, November 9, 2018, Joel Fernandes joel@joelfernandes.org wrote:
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On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote:
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On Fri, Nov 9, 2018 at 10:06 PM Jann Horn jannh@google.com wrote:
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On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
joel@joelfernandes.org wrote:
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Android uses ashmem for sharing memory regions. We are looking
forward
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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
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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
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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
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This usecase cannot be implemented with the existing F_SEAL_WRITE
seal.
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To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE
seal
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which prevents any future mmap and write syscalls from succeeding
while
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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...
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The following program shows the seal
working in action:
[...]
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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

[...]
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diff --git a/mm/memfd.c b/mm/memfd.c
index 2bb5e257080e..5ba9804e9515 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
[...]
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@@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file,
unsigned int seals)
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            }
    }



  if ((seals & F_SEAL_FUTURE_WRITE) &&


      !(*file_seals & F_SEAL_FUTURE_WRITE)) {


          /*


           * The FUTURE_WRITE seal also prevents growing and



shrinking
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           * so we need them to be already set, or requested



now.
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           */


          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

    