This allows writing to shared memory and then making it read-only before
sharing it to other processes, such that a malicious sandboxed process
cannot regain write access. This is currently available only in the
low-level base::SharedMemory interface.
The freeze operation exposes the common subset of read-only shared
memory that we can implement on all supported OSes: with some APIs
(POSIX shm_open) we can't revoke writeability from existing capabilies,
while for others (Android ashmem) we *must* revoke it. Thus, we require
that the writeable capability not have been duplicated or shared to
another process, and consume it as part of freezing. Also, because in
some backends need special handling at creation time, freezeability must
be explicitly requested.
In particular, this doesn't allow giving an untrusted process read-only
access to memory that the original process can write.
Note that on MacOS before 10.12 this will use temporary files in order to
avoid an OS security bug that allows regaining write access; those OS
versions are no longer supported by Apple (but are supported by Firefox).
Depends on D26742
Differential Revision: https://phabricator.services.mozilla.com/D26743
--HG--
extra : moz-landing-system : lando
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