linux-kernel - [PATCH v4 2/3] binder: use cred instead of task for getsecid

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Message-Id: <20211007004629.1113572-3-tkjos@google.com>
Date:   Wed,  6 Oct 2021 17:46:28 -0700
From:   Todd Kjos <tkjos@...gle.com>
To:     gregkh@...uxfoundation.org, arve@...roid.com, tkjos@...roid.com,
        maco@...roid.com, christian@...uner.io, jmorris@...ei.org,
        serge@...lyn.com, paul@...l-moore.com,
        stephen.smalley.work@...il.com, eparis@...isplace.org,
        keescook@...omium.org, jannh@...gle.com, jeffv@...gle.com,
        zohar@...ux.ibm.com, linux-security-module@...r.kernel.org,
        selinux@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Cc:     joel@...lfernandes.org, kernel-team@...roid.com,
        Todd Kjos <tkjos@...gle.com>,
        kernel test robot <lkp@...el.com>, stable@...r.kernel.org
Subject: [PATCH v4 2/3] binder: use cred instead of task for getsecid

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@...gle.com>
Suggested-by: Stephen Smalley <stephen.smalley.work@...il.com>
Reported-by: kernel test robot <lkp@...el.com>
Cc: stable@...r.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)
+{
+}
+
 static inline int security_kernel_act_as(struct cred *cred, u32 secid)
 {
 	return 0;
-- 
2.33.0.800.g4c38ced690-goog