[PATCH 4/4] security: binder: Add transfer_charge SElinux hook

[T.J. Mercier]

Any process can cause a memory charge transfer to occur to any other
process when transmitting a file descriptor through binder. This should
only be possible for central allocator processes, so a new SELinux
permission is added to restrict which processes are allowed to initiate
these charge transfers.

Signed-off-by: T.J. Mercier <tjmercier at google.com>
---
 drivers/android/binder.c            | 5 +++++
 include/linux/lsm_hook_defs.h       | 2 ++
 include/linux/lsm_hooks.h           | 6 ++++++
 include/linux/security.h            | 2 ++
 security/security.c                 | 6 ++++++
 security/selinux/hooks.c            | 9 +++++++++
 security/selinux/include/classmap.h | 2 +-
 7 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 9830848c8d25..9063db04826d 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2279,6 +2279,11 @@ static int binder_translate_fd(u32 fd, binder_size_t fd_offset, __u32 flags,
 	if (IS_ENABLED(CONFIG_MEMCG) && (flags & BINDER_FD_FLAG_XFER_CHARGE)) {
 		struct dma_buf *dmabuf;
 
+		if (security_binder_transfer_charge(proc->cred, target_proc->cred)) {
+			ret = -EPERM;
+			goto err_security;
+		}
+
 		if (unlikely(!is_dma_buf_file(file))) {
 			binder_user_error(
 				"%d:%d got transaction with XFER_CHARGE for non-dmabuf fd, %d\n",
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index ed6cb2ac55fa..8db2a958557e 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -33,6 +33,8 @@ LSM_HOOK(int, 0, binder_transfer_binder, const struct cred *from,
 	 const struct cred *to)
 LSM_HOOK(int, 0, binder_transfer_file, const struct cred *from,
 	 const struct cred *to, struct file *file)
+LSM_HOOK(int, 0, binder_transfer_charge, const struct cred *from,
+	 const struct cred *to)
 LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child,
 	 unsigned int mode)
 LSM_HOOK(int, 0, ptrace_traceme, struct task_struct *parent)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 0a5ba81f7367..39c40c7bf519 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -1385,6 +1385,12 @@
  *	@file contains the struct file being transferred.
  *	@to contains the struct cred for the receiving process.
  *	Return 0 if permission is granted.
+ * @binder_transfer_charge:
+ *	Check whether @from is allowed to transfer the memory charge for a
+ *	buffer out of its cgroup to @to.
+ *	@from contains the struct cred for the sending process.
+ *	@to contains the struct cred for the receiving process.
+ *	Return 0 if permission is granted.
  *
  * @ptrace_access_check:
  *	Check permission before allowing the current process to trace the
diff --git a/include/linux/security.h b/include/linux/security.h
index 5b67f208f7de..3b7472308430 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -270,6 +270,8 @@ int security_binder_transfer_binder(const struct cred *from,
 				    const struct cred *to);
 int security_binder_transfer_file(const struct cred *from,
 				  const struct cred *to, struct file *file);
+int security_binder_transfer_charge(const struct cred *from,
+				    const struct cred *to);
 int security_ptrace_access_check(struct task_struct *child, unsigned int mode);
 int security_ptrace_traceme(struct task_struct *parent);
 int security_capget(struct task_struct *target,
diff --git a/security/security.c b/security/security.c
index d1571900a8c7..97e1e74d1ff2 100644
--- a/security/security.c
+++ b/security/security.c
@@ -801,6 +801,12 @@ int security_binder_transfer_file(const struct cred *from,
 	return call_int_hook(binder_transfer_file, 0, from, to, file);
 }
 
+int security_binder_transfer_charge(const struct cred *from,
+				    const struct cred *to)
+{
+	return call_int_hook(binder_transfer_charge, 0, from, to);
+}
+
 int security_ptrace_access_check(struct task_struct *child, unsigned int mode)
 {
 	return call_int_hook(ptrace_access_check, 0, child, mode);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 3c5be76a9199..823ef14924bd 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2066,6 +2066,14 @@ static int selinux_binder_transfer_file(const struct cred *from,
 			    &ad);
 }
 
+static int selinux_binder_transfer_charge(const struct cred *from, const struct cred *to)
+{
+	return avc_has_perm(&selinux_state,
+			    cred_sid(from), cred_sid(to),
+			    SECCLASS_BINDER, BINDER__TRANSFER_CHARGE,
+			    NULL);
+}
+
 static int selinux_ptrace_access_check(struct task_struct *child,
 				       unsigned int mode)
 {
@@ -7052,6 +7060,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(binder_transaction, selinux_binder_transaction),
 	LSM_HOOK_INIT(binder_transfer_binder, selinux_binder_transfer_binder),
 	LSM_HOOK_INIT(binder_transfer_file, selinux_binder_transfer_file),
+	LSM_HOOK_INIT(binder_transfer_charge, selinux_binder_transfer_charge),
 
 	LSM_HOOK_INIT(ptrace_access_check, selinux_ptrace_access_check),
 	LSM_HOOK_INIT(ptrace_traceme, selinux_ptrace_traceme),
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index a3c380775d41..2eef180d10d7 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -172,7 +172,7 @@ const struct security_class_mapping secclass_map[] = {
 	{ "tun_socket",
 	  { COMMON_SOCK_PERMS, "attach_queue", NULL } },
 	{ "binder", { "impersonate", "call", "set_context_mgr", "transfer",
-		      NULL } },
+		      "transfer_charge", NULL } },
 	{ "cap_userns",
 	  { COMMON_CAP_PERMS, NULL } },
 	{ "cap2_userns",
-- 
2.39.0.314.g84b9a713c41-goog