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

[Andy Lutomirski <luto@amacapital.net>]

> On Nov 9, 2018, at 1:06 PM, Jann Horn <jannh@google.com> wrote:
> 
> +linux-api for API addition
> +hughd as FYI since this is somewhat related to mm/shmem
> 
> 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.

Every now and then I try to write a patch to prevent using proc to reopen a file with greater permission than the original open.

I like your idea to have a clean way to reopen a a memfd with reduced permissions. But I would make it a syscall instead and maybe make it only work for memfd at first.  And the proc issue would need to be fixed, too.