net/9p: use memcpy() instead of snprintf() in p9_mount_tag_show()

p9_mount_tag_show() uses '%s' format string to print
non-NULL terminated chan->tag string. This leads
to out of bounds memory read, because format '%s'
implies that string is NULL-terminated.

The length of string is know here, so its simpler and safer
to use memcpy instead of snprintf().

Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 36a1a739ad68..486df019f875 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -504,7 +504,10 @@ static ssize_t p9_mount_tag_show(struct device *dev,
         vdev = dev_to_virtio(dev);
         chan = vdev->priv;
 
-        return snprintf(buf, chan->tag_len + 1, "%s", chan->tag);
+        memcpy(buf, chan->tag, chan->tag_len);
+        buf[chan->tag_len] = 0;
+
+        return chan->tag_len + 1;
 }
 
 static DEVICE_ATTR(mount_tag, 0444, p9_mount_tag_show, NULL);