sepolicy: Allow to receive FDs from app_zygote
The primary goal is to have an ashmem region shared between the main app
process in Chrome (=Browser Process) and the app zygote. It can only be
passed from the App Zygote, since there is no communication in the other
direction. Passing of the file descriptor should happen by:
(A) inheriting via fork(2)
(B) using binder IPC
Currently ashmem FDs are sufficiently allowed to be mmap(2)-ed in all
Chrome processes. The mode of mapping (read-only, read-write etc.) is
controlled by the settings of the region itself, not by sepolicy.
This change additionally allows an FD created in the app zygote to be
passed to the 'untrusted_app' domain.
Note: This change allows *any* FD, not just an ashmem one to be passed.
This is on purpose: in the future we will likely want to return to the
memfd story. Other usecases (pipes, sockets) might appear.
The app zygote preload takes the responsibility not to share
capabilities in the form of FDs unintentionally with other app
processes.
Historical note: we tried to enable this for memfd (using additional
rules), but it required a 'write' permission when sending an FD. Reasons
for that are still puzzling, and there seems to be no easy workaround
for it. Decision: use ashmem.
Bug: 184808875
Test: Manual: Build and install Chrome (trichrome_chrome_google_bundle)
from [1]. Make sure FileDescriptorAllowlist allows the FD, like
[2]. Reach a NewTabPage, click on a suggested page, observe no
errors related to binder transactions and selinux violations.
[1] A change in Chrome to create an ashmem region during app zygote
preload and pass it to the browser process:
https://crrev.com/c/2752872/29
[2] Allowlist change in review:
https://android-review.googlesource.com/c/platform/frameworks/base/+/1739393
(Alternatively: Remove gOpenFdTable checks in ForkCommon() in
com_android_internal_os_Zygote.cpp)
Change-Id: Ide085f472c8fb6ae76ab0b094319d6924552fc02
Ignore-AOSP-First: in addition to changes in AOSP, copied to prebuilts
2 files changed
