linux-kernel - [PATCH v1 3/5] misc: fastrpc: Restrict untrusted apk to spawn

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Message-ID: <20240508054250.2922-4-quic_ekangupt@quicinc.com>
Date: Wed, 8 May 2024 11:12:46 +0530
From: Ekansh Gupta <quic_ekangupt@...cinc.com>
To: <srinivas.kandagatla@...aro.org>, <linux-arm-msm@...r.kernel.org>
CC: <gregkh@...uxfoundation.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH v1 3/5] misc: fastrpc: Restrict untrusted apk to spawn

Untrusted application can attach to guestOS and staticPD if it can
make root PD, sensors PD or audio PD attach request. This is a
potential security issue as the untrusted application can crash
rootPD or staticPD. Restrict attach to guestOS or staticPD request
if request is being made using non-secure device node.

Also for untrusted dynamic processes, DSP HAL process opens the
device node on behalf of the application. Add a check to restrict
such untrusted applications from offloading to signed PD.

Signed-off-by: Ekansh Gupta <quic_ekangupt@...cinc.com>
---
 drivers/misc/fastrpc.c | 35 +++++++++++++++++++++++++++++------
 1 file changed, 29 insertions(+), 6 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index 258f2b9da039..1f85ce3eb2e2 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -303,6 +303,7 @@ struct fastrpc_user {
 	int pd;
 	bool is_secure_dev;
 	bool is_unsigned_pd;
+	bool untrusted_process;
 	/* Lock for lists */
 	spinlock_t lock;
 	/* lock for allocations */
@@ -1208,20 +1209,24 @@ static int fastrpc_internal_invoke(struct fastrpc_user *fl,  u32 kernel,
 
 static bool is_session_rejected(struct fastrpc_user *fl, bool unsigned_pd_request)
 {
-	/* Check if the device node is non-secure and channel is secure*/
+	/* Check if the device node is non-secure and channel is secure */
 	if (!fl->is_secure_dev && fl->cctx->secure) {
 		/*
 		 * Allow untrusted applications to offload only to Unsigned PD when
 		 * 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");
-			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_err(&fl->cctx->rpdev->dev, "Error: Untrusted application trying to offload to signed PD");
+	return true;
 }
 
 static int fastrpc_mmap_remove_ssr(struct fastrpc_channel_ctx *cctx)
@@ -1274,6 +1279,11 @@ static int fastrpc_init_create_static_process(struct fastrpc_user *fl,
 	} inbuf;
 	u32 sc;
 
+	if (!fl->is_secure_dev) {
+		dev_err(&fl->cctx->rpdev->dev, "untrusted app trying to attach to privileged DSP PD\n");
+		return -EACCES;
+	}
+
 	args = kcalloc(FASTRPC_CREATE_STATIC_PROCESS_NARGS, sizeof(*args), GFP_KERNEL);
 	if (!args)
 		return -ENOMEM;
@@ -1420,11 +1430,19 @@ 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 DSP HAL. 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;
 	}
 
@@ -1688,6 +1706,11 @@ static int fastrpc_init_attach(struct fastrpc_user *fl, int pd)
 	int tgid = fl->tgid;
 	u32 sc;
 
+	if (!fl->is_secure_dev) {
+		dev_err(&fl->cctx->rpdev->dev, "untrusted app trying to attach to privileged DSP PD\n");
+		return -EACCES;
+	}
+
 	args[0].ptr = (u64)(uintptr_t) &tgid;
 	args[0].length = sizeof(tgid);
 	args[0].fd = -1;
-- 
2.43.0