Android uses the ashmem driver [1] for creating shared memory regions between processes. The ashmem driver exposes an ioctl command for processes to restrict the permissions an ashmem buffer can be mapped with. Buffers are created with the ability to be mapped as readable, writable, and executable. Processes remove the ability to map some ashmem buffers as executable to ensure that those buffers cannot be exploited to run unintended code. Other buffers retain their ability to be mapped as executable, as these buffers can be used for just-in-time (JIT) compilation. So there is a need to be able to remove the ability to map a buffer as executable on a per-buffer basis. Android is currently trying to migrate towards replacing its ashmem driver usage with memfd. Part of the transition involved introducing a library that serves to abstract away how shared memory regions are allocated (i.e. ashmem vs memfd). This allows clients to use a single interface for restricting how a buffer can be mapped without having to worry about how it is handled for ashmem (through the ioctl command mentioned earlier) or memfd (through file seals). While memfd has support for preventing buffers from being mapped as writable beyond a certain point in time (thanks to F_SEAL_FUTURE_WRITE), it does not have a similar interface to prevent buffers from being mapped as executable beyond a certain point. However, that could be implemented as a file seal (F_SEAL_FUTURE_EXEC) which works similarly to F_SEAL_FUTURE_WRITE. F_SEAL_FUTURE_WRITE was chosen as a template for how this new seal should behave, instead of F_SEAL_WRITE, for the following reasons: 1. Having the new seal behave like F_SEAL_FUTURE_WRITE matches the behavior that was present with ashmem. This aids in seamlessly transitioning clients away from ashmem to memfd. 2. Making the new seal behave like F_SEAL_WRITE would mean that no mappings that could become executable in the future (i.e. via mprotect()) can exist when the seal is applied. However, there are known cases (e.g. CursorWindow [2]) where restrictions are applied on how a buffer can be mapped after a mapping has already been made. That mapping may have VM_MAYEXEC set, which would not allow the seal to be applied successfully. Therefore, the F_SEAL_FUTURE_EXEC seal was designed to have the same semantics as F_SEAL_FUTURE_WRITE. Note: this series depends on Lorenzo's work [3] which allows for a memfd's file seals to be read in do_mmap(). [1] https://cs.android.com/android/kernel/superproject/+/common-android-mainline:common/drivers/staging/android/ashmem.c [2] https://developer.android.com/reference/android/database/CursorWindow [3] https://lore.kernel.org/all/cover.1732804776.git.lorenzo.stoakes@xxxxxxxxxx/ Isaac J. Manjarres (2): mm/memfd: Add support for F_SEAL_FUTURE_EXEC to memfd selftests/memfd: Add tests for F_SEAL_FUTURE_EXEC include/linux/mm.h | 5 ++ include/uapi/linux/fcntl.h | 1 + mm/memfd.c | 1 + mm/mmap.c | 11 +++ tools/testing/selftests/memfd/memfd_test.c | 79 ++++++++++++++++++++++ 5 files changed, 97 insertions(+) -- 2.47.0.338.g60cca15819-goog
