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 <redacted>
Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Reported-by: kernel test robot <redacted>
Cc: stable@vger.kernel.org # 5.4+
---
v3: added this patch to series
v4: fix build-break for !CONFIG_SECURITY

 drivers/android/binder.c | 11 +----------
 include/linux/security.h |  4 ++++
 2 files changed, 5 insertions(+), 10 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index ca599ebdea4a..989afd0804ca 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..f02cc0211b10 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h

@@ -1041,6 +1041,10 @@ static inline void security_transfer_creds(struct cred *new,
 {
 }

+static inline void security_cred_getsecid(const struct cred *c, u32 *secid)
+{
+}