[PATCH v5 3/3] binder: use cred instead of task for getsecid

[Todd Kjos <tkjos@google.com>]

Use the 'struct cred' saved at binder_open() to lookup
the security ID via security_cred_getsecid(). This
ensures that the security context that opened binder
is the one used to generate the secctx.

Fixes: ec74136ded79 ("binder: create node flag to request sender's
security context")
Signed-off-by: Todd Kjos <tkjos@google.com>
Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Cc: stable@vger.kernel.org # 5.4+
---
v3: added this patch to series
v4: fix build-break for !CONFIG_SECURITY
v5: set *secid=0 for !CONFIG_SECURITY version of secuirty_cred_getsecid()

 drivers/android/binder.c | 11 +----------
 include/linux/security.h |  5 +++++
 2 files changed, 6 insertions(+), 10 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index bc15325f0579..26382e982c5e 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2722,16 +2722,7 @@ static void binder_transaction(struct binder_proc *proc,
 		u32 secid;
 		size_t added_size;
 
-		/*
-		 * Arguably this should be the task's subjective LSM secid but
-		 * we can't reliably access the subjective creds of a task
-		 * other than our own so we must use the objective creds, which
-		 * are safe to access.  The downside is that if a task is
-		 * temporarily overriding it's creds it will not be reflected
-		 * here; however, it isn't clear that binder would handle that
-		 * case well anyway.
-		 */
-		security_task_getsecid_obj(proc->tsk, &secid);
+		security_cred_getsecid(proc->cred, &secid);
 		ret = security_secid_to_secctx(secid, &secctx, &secctx_sz);
 		if (ret) {
 			return_error = BR_FAILED_REPLY;
diff --git a/include/linux/security.h b/include/linux/security.h
index 6344d3362df7..46a02ce34d00 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -1041,6 +1041,11 @@ static inline void security_transfer_creds(struct cred *new,
 {
 }
 
+static inline void security_cred_getsecid(const struct cred *c, u32 *secid)
+{
+	*secid = 0;
+}
+
 static inline int security_kernel_act_as(struct cred *cred, u32 secid)
 {
 	return 0;
-- 
2.33.0.882.g93a45727a2-goog