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Date:   Sat, 21 Oct 2023 00:09:16 +0000
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Message-ID: <20231021-strncpy-drivers-rpmsg-virtio_rpmsg_bus-c-v1-1-8abb919cbe24@google.com>
Subject: [PATCH] rpmsg: virtio: replace deprecated strncpy with strscpy/_pad
From:   Justin Stitt <justinstitt@google.com>
To:     Bjorn Andersson <andersson@kernel.org>,
        Mathieu Poirier <mathieu.poirier@linaro.org>
Cc:     linux-remoteproc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org,
        Justin Stitt <justinstitt@google.com>
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Precedence: bulk

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

This patch replaces 3 callsites of strncpy().

The first two populate the destination buffer `nsm.name` -- which we
expect to be NUL-terminated based on their use with format strings.

Firstly, as I understand it, virtio_rpmsg_announce_create() creates an
rpmsg_ns_msg and sends via:

virtio_rpmsg_bus.c:
336: err = rpmsg_sendto(rpdev->ept, &nsm, sizeof(nsm), RPMSG_NS_ADDR);

... which uses:
virtio_rpmsg_sendto() -> rpmsg_send_offchannel_raw()

... which copies its data into an rpmsg_hdr `msg` in virtio_rpmsg_bus.c
618: memcpy(msg->data, data, len);

... and we end up receiving here via a callback:

rpmsg_ns.c:
30: /* invoked when a name service announcement arrives */
31: static int rpmsg_ns_cb(struct rpmsg_device *rpdev, void *data, int len,
32: 		       void *priv, u32 src)
33: {
34:         struct rpmsg_ns_msg *msg = data;
...
50:         /* don't trust the remote processor for null terminating the name */
51:         msg->name[RPMSG_NAME_SIZE - 1] = '\0';

... which finally leads into the use of `name` within a format string:
rpmsg_ns.c:
57: dev_info(dev, "%sing channel %s addr 0x%x\n",
58:          rpmsg32_to_cpu(rpdev, msg->flags) & RPMSG_NS_DESTROY ?
59:          "destroy" : "creat", msg->name, chinfo.dst);

Taking another look at this comment + NUL-byte assignment:
50: /* don't trust the remote processor for null terminating the name */
51: msg->name[RPMSG_NAME_SIZE - 1] = '\0';

...  we probably _can_ trust that this string is NUL-terminated with the
introduction of strscpy(). However, since there might be some magic
happening between the announcement create and the callback that I don't
understand, I've opted to leave this comment and assignment alone as it
doesn't hurt.

We can also observe that `nsm` is not zero-initialized and as such we
should maintain the NUL-padding behavior that strncpy() provides:

virtio_rpmsg_bus.c:
330: struct rpmsg_ns_msg nsm;

Considering the above, a suitable replacement is `strscpy_pad` due to
the fact that it guarantees both NUL-termination and NUL-padding on the
destination buffer.

Now, for the third and final destination buffer rpdev->id.name we can
just go for strscpy() (not _pad()) as rpdev points to &vch->rpdev:
|       rpdev = &vch->rpdev;

... and vch is zero-allocated:
|       vch = kzalloc(sizeof(*vch), GFP_KERNEL);

... this renders any additional NUL-byte assignments (like the ones
strncpy() or strscpy_pad() does) redundant.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Note: build-tested only.

Found with: $ rg "strncpy\("
---
 drivers/rpmsg/virtio_rpmsg_bus.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
index 905ac7910c98..dc87965f8164 100644
--- a/drivers/rpmsg/virtio_rpmsg_bus.c
+++ b/drivers/rpmsg/virtio_rpmsg_bus.c
@@ -329,7 +329,7 @@ static int virtio_rpmsg_announce_create(struct rpmsg_device *rpdev)
 	    virtio_has_feature(vrp->vdev, VIRTIO_RPMSG_F_NS)) {
 		struct rpmsg_ns_msg nsm;
 
-		strncpy(nsm.name, rpdev->id.name, RPMSG_NAME_SIZE);
+		strscpy_pad(nsm.name, rpdev->id.name, sizeof(nsm.name));
 		nsm.addr = cpu_to_rpmsg32(rpdev, rpdev->ept->addr);
 		nsm.flags = cpu_to_rpmsg32(rpdev, RPMSG_NS_CREATE);
 
@@ -353,7 +353,7 @@ static int virtio_rpmsg_announce_destroy(struct rpmsg_device *rpdev)
 	    virtio_has_feature(vrp->vdev, VIRTIO_RPMSG_F_NS)) {
 		struct rpmsg_ns_msg nsm;
 
-		strncpy(nsm.name, rpdev->id.name, RPMSG_NAME_SIZE);
+		strscpy_pad(nsm.name, rpdev->id.name, sizeof(nsm.name));
 		nsm.addr = cpu_to_rpmsg32(rpdev, rpdev->ept->addr);
 		nsm.flags = cpu_to_rpmsg32(rpdev, RPMSG_NS_DESTROY);
 
@@ -424,7 +424,7 @@ static struct rpmsg_device *__rpmsg_create_channel(struct virtproc_info *vrp,
 	 */
 	rpdev->announce = rpdev->src != RPMSG_ADDR_ANY;
 
-	strncpy(rpdev->id.name, chinfo->name, RPMSG_NAME_SIZE);
+	strscpy(rpdev->id.name, chinfo->name, sizeof(rpdev->id.name));
 
 	rpdev->dev.parent = &vrp->vdev->dev;
 	rpdev->dev.release = virtio_rpmsg_release_device;

---
base-commit: 9c5d00cb7b6bbc5a7965d9ab7d223b5402d1f02c
change-id: 20231020-strncpy-drivers-rpmsg-virtio_rpmsg_bus-c-dba15db4e890

Best regards,
--
Justin Stitt <justinstitt@google.com>