linux-kernel - [PATCH v3 8/9] misc: fastrpc: Restrict untrusted app to spawn signed PD

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Date: Thu, 30 May 2024 15:50:26 +0530
From: Ekansh Gupta <quic_ekangupt@...cinc.com>
To: <srinivas.kandagatla@...aro.org>, <linux-arm-msm@...r.kernel.org>
CC: <gregkh@...uxfoundation.org>, <quic_bkumar@...cinc.com>,
        <linux-kernel@...r.kernel.org>, <quic_chennak@...cinc.com>,
        stable
	<stable@...nel.org>
Subject: [PATCH v3 8/9] misc: fastrpc: Restrict untrusted app to spawn signed PD

Some untrusted applications will not have access to open fastrpc
device nodes and a privileged process can open the device node on
behalf of the application. Add a check to restrict such untrusted
applications from offloading to signed PD.

Fixes: 7f1f481263c3 ("misc: fastrpc: check before loading process to the DSP")
Cc: stable <stable@...nel.org>
Signed-off-by: Ekansh Gupta <quic_ekangupt@...cinc.com>
---
 drivers/misc/fastrpc.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index 73fa0e536cf9..32615ccde7ac 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -328,6 +328,7 @@ struct fastrpc_user {
 	int pd;
 	bool is_secure_dev;
 	bool is_unsigned_pd;
+	bool untrusted_process;
 	char *servloc_name;
 	/* Lock for lists */
 	spinlock_t lock;
@@ -1249,13 +1250,17 @@ static bool is_session_rejected(struct fastrpc_user *fl, bool unsigned_pd_reques
 		 * channel is configured as secure and block untrusted apps on channel
 		 * that does not support unsigned PD offload
 		 */
-		if (!fl->cctx->unsigned_support || !unsigned_pd_request) {
-			dev_err(&fl->cctx->rpdev->dev, "Error: Untrusted application trying to offload to signed PD\n");
-			return true;
-		}
+		if (!fl->cctx->unsigned_support || !unsigned_pd_request)
+			goto reject_session;
 	}
+	/* Check if untrusted process is trying to offload to signed PD */
+	if (fl->untrusted_process && !unsigned_pd_request)
+		goto reject_session;
 
 	return false;
+reject_session:
+	dev_dbg(&fl->cctx->rpdev->dev, "Error: Untrusted application trying to offload to signed PD\n");
+	return true;
 }
 
 static void fastrpc_mmap_remove_pdr(struct fastrpc_static_pd *spd)
@@ -1504,12 +1509,20 @@ static int fastrpc_init_create_process(struct fastrpc_user *fl,
 		goto err;
 	}
 
+	/*
+	 * Third-party apps don't have permission to open the fastrpc device, so
+	 * it is opened on their behalf by a priveleged process. This is detected
+	 * by comparing current PID with the one stored during device open.
+	 */
+	if (current->tgid != fl->tgid)
+		fl->untrusted_process = true;
+
 	if (init.attrs & FASTRPC_MODE_UNSIGNED_MODULE)
 		fl->is_unsigned_pd = true;
 
 
 	if (is_session_rejected(fl, fl->is_unsigned_pd)) {
-		err = -ECONNREFUSED;
+		err = -EACCES;
 		goto err;
 	}
 
-- 
2.43.0