In the Linux kernel, the following vulnerability has been resolved:
fs: preserve ACL_DONT_CACHE state in forget_cached_acl()
The ACL_DONT_CACHE state is meant to be a constant state for the inode for filesystems that want to opt out of posix acl caching.
Commit facd61053cff1 (fuse: fixes after adapting to new posix acl api) used this facility to opt out of posix acl caching for fuse inodes with fuse server that does not negotiate FUSE_POSIX_ACL (fc->posix_acl).
The commit also takes care to gate the forget_all_cached_acls() call in fuse_set_acl() on fc->posix_acl because there is no need for it, but there are other placed in fuse code which call forget_all_cached_acls() unconditional to fc->posix_acl and those cause the loss of the ACL_DONT_CACHE state.
This is not only a functional bug. Properly timed, a get_acl() from this fuse filesystem can return a stale cached value, as was observed in tests, because set_acl() does not invalidate the unintentional acl cache.
We could fix this in fuse, but it actually makes no sense for the vfs helper forget_cached_acl() to invalidate the ACL_DONT_CACHE state, so let it not do that to fix fuse and future users of ACL_DONT_CACHE.
References
- https://git.kernel.org/stable/c/4b9a5458d02e214ef2b384124ca626e3e381d778
- https://git.kernel.org/stable/c/834ddf899484a2f23129080e8773bc04f4691d07
- https://git.kernel.org/stable/c/a019b074903b3ad0a9726087efd0e8291452023b
- https://git.kernel.org/stable/c/b98fad81f1202b0eb26aacf3ff4cc7a21ed3b5bf
- https://git.kernel.org/stable/c/ca03a7984a34f48085fd013e0d2cf4e6420b4acf