In order to avoid concurrency issues around selinuxfs resource
availability
during policy load, we first create new directories out of tree for
reloaded resources, then swap them in, and finally delete the old
versions.
This fix focuses on concurrency in each of the three subtrees
swapped, and
not concurrency across the three trees. This means that it is still
possible
that subsequent reads to eg the booleans directory and the class
directory
during a policy load could see the old state for one and the new for
the other.
The problem of ensuring that policy loads are fully atomic from the
perspective
of userspace is larger than what is dealt with here. This commit
focuses on
ensuring that the directories contents always match either the new or
the old
policy state from the perspective of userspace.
In the previous implementation, on policy load /sys/fs/selinux is
updated
by deleting the previous contents of
/sys/fs/selinux/{class,booleans} and then recreating them. This means
that there is a period of time when the contents of these directories
do not
exist which can cause race conditions as userspace relies on them for
information about the policy. In addition, it means that error
recovery in
the event of failure is challenging.
In order to demonstrate the race condition that this series fixes, you
can use the following commands:
while true; do cat /sys/fs/selinux/class/service/perms/status
/dev/null; done &
while true; do load_policy; done;
In the existing code, this will display errors fairly often as the class
lookup fails. (In normal operation from systemd, this would result in a
permission check which would be allowed or denied based on policy
settings
around unknown object classes.) After applying this patch series you
should expect to no longer see such error messages.
Signed-off-by: Daniel Burgener <dburgener@xxxxxxxxxxxxxxxxxxx>
---
security/selinux/selinuxfs.c | 145 +++++++++++++++++++++++++++++------
1 file changed, 120 insertions(+), 25 deletions(-)
diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
index f09afdb90ddd..d3a19170210a 100644
--- a/security/selinux/selinuxfs.c
+++ b/security/selinux/selinuxfs.c
+ tmp_policycap_dir = sel_make_dir(tmp_parent, POLICYCAP_DIR_NAME,
&fsi->last_ino);
+ if (IS_ERR(tmp_policycap_dir)) {
+ ret = PTR_ERR(tmp_policycap_dir);
+ goto out;
+ }
- return 0;
+ // booleans
+ old_dentry = fsi->bool_dir;
+ lock_rename(tmp_bool_dir, old_dentry);
+ ret = vfs_rename(tmp_parent->d_inode, tmp_bool_dir,
fsi->sb->s_root->d_inode,
+ fsi->bool_dir, NULL, RENAME_EXCHANGE);