[RFC PATCH v1 0/2] Add file seal to prevent future exec mappings

Isaac J. Manjarres
2024-12-06T01:09:33Z

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@oracle.com/

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(+)

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