[PATCH RFC v2 5/8] selinux: Restrict cross-cgroup dma-buf charging

[Albert Esteve]

The security_dma_buf_charge_task() hook allows security modules
to control which processes may charge dma-buf allocations
to another process's cgroup via the charge_fd field of
DMA_HEAP_IOCTL_ALLOC. Without a policy implementation, the
hook is a no-op and the restriction is not enforced.

On SELinux-managed systems any domain with access to a
dma-heap device node can therefore exhaust another cgroup's
memory budget without restriction.

Implement selinux_dma_buf_charge_task() using avc_has_perm() with
a new dma_buf object class and a charge_task permission. Policy
authors can then grant cross-cgroup charging selectively,
for example:

  allow allocator_app_t client_app_t:dma_buf charge_task;

Signed-off-by: Albert Esteve <aesteve at redhat.com>
---
 security/selinux/hooks.c            | 7 +++++++
 security/selinux/include/classmap.h | 1 +
 2 files changed, 8 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 30776b0107f2..8f39d91eabc9 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2204,6 +2204,12 @@ static int selinux_capable(const struct cred *cred, struct user_namespace *ns,
 	return cred_has_capability(cred, cap, opts, ns == &init_user_ns);
 }
 
+static int selinux_dma_buf_charge_task(const struct cred *from, const struct cred *to)
+{
+	return avc_has_perm(cred_sid(from), cred_sid(to),
+			    SECCLASS_DMA_BUF, DMA_BUF__CHARGE_TASK, NULL);
+}
+
 static int selinux_quotactl(int cmds, int type, int id, const struct super_block *sb)
 {
 	const struct cred *cred = current_cred();
@@ -7674,6 +7680,7 @@ static struct security_hook_list selinux_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(capget, selinux_capget),
 	LSM_HOOK_INIT(capset, selinux_capset),
 	LSM_HOOK_INIT(capable, selinux_capable),
+	LSM_HOOK_INIT(dma_buf_charge_task, selinux_dma_buf_charge_task),
 	LSM_HOOK_INIT(quotactl, selinux_quotactl),
 	LSM_HOOK_INIT(quota_on, selinux_quota_on),
 	LSM_HOOK_INIT(syslog, selinux_syslog),
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index 90cb61b16425..b4ebedc6e424 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -181,6 +181,7 @@ const struct security_class_mapping secclass_map[] = {
 	{ "user_namespace", { "create", NULL } },
 	{ "memfd_file",
 	  { COMMON_FILE_PERMS, "execute_no_trans", "entrypoint", NULL } },
+	{ "dma_buf", { "charge_task", NULL } },
 	/* last one */ { NULL, {} }
 };
 

-- 
2.55.0