Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd

On Tue, Oct 9, 2018 at 3:20 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 drop its protection for "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 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_FS_WRITE seal which prevents any future mmap and write syscalls from succeeding while keeping the existing mmap active. The following program shows the seal working in action:

int main() { int ret, fd; void *addr, *addr2, *addr3, *addr1; ret = memfd_create_region("test_region", REGION_SIZE); printf("ret=%d\n", ret); fd = ret;

// Create map
addr = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED)
        printf("map 0 failed\n");
else
        printf("map 0 passed\n");

if ((ret = write(fd, "test", 4)) != 4)
        printf("write failed even though no fs-write seal "
               "(ret=%d errno =%d)\n", ret, errno);
else
        printf("write passed\n");

addr1 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
if (addr1 == MAP_FAILED)
        perror("map 1 prot-write failed even though no seal\n");
else
        printf("map 1 prot-write passed as expected\n");

ret = fcntl(fd, F_ADD_SEALS, F_SEAL_FS_WRITE);
if (ret == -1)
        printf("fcntl failed, errno: %d\n", errno);
else
        printf("fs-write seal now active\n");

if ((ret = write(fd, "test", 4)) != 4)
        printf("write failed as expected due to fs-write seal\n");
else
        printf("write passed (unexpected)\n");

addr2 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
if (addr2 == MAP_FAILED)
        perror("map 2 prot-write failed as expected due to seal\n");
else
        printf("map 2 passed\n");

addr3 = mmap(0, REGION_SIZE, PROT_READ, MAP_SHARED, fd, 0);
if (addr3 == MAP_FAILED)
        perror("map 3 failed\n");
else
        printf("map 3 prot-read passed as expected\n");

}

The output of running this program is as follows: ret=3 map 0 passed write passed map 1 prot-write passed as expected fs-write seal now active write failed as expected due to fs-write seal map 2 prot-write failed as expected due to seal : Permission denied map 3 prot-read passed as expected

Note: This seal will also prevent growing and shrinking of the memfd. This is not something we do in Android so it does not affect us, however I have mentioned this behavior of the seal in the manpage.

Cc: jreck@google.com Cc: john.stultz@linaro.org Cc: tkjos@google.com Cc: gregkh@linuxfoundation.org Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org

Reviewed-by: John Stultz john.stultz@linaro.org

thanks -john