HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

When a UHID_CREATE command is written to the uhid char device, a
copy_from_user() is done from a user pointer embedded in the command.
When the address limit is KERNEL_DS, e.g. as is the case during
sys_sendfile(), this can read from kernel memory.  Alternatively,
information can be leaked from a setuid binary that is tricked to write
to the file descriptor.  Therefore, forbid UHID_CREATE in these cases.

No other commands in uhid_char_write() are affected by this bug and
UHID_CREATE is marked as "obsolete", so apply the restriction to
UHID_CREATE only rather than to uhid_char_write() entirely.

Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
Jann Horn for commit 9da3f2b ("x86/fault: BUG() when uaccess
helpers fault on kernel addresses"), allowing this bug to be found.

Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
Fixes: d365c6c ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
Cc: <stable@vger.kernel.org> # v3.6+
Cc: Jann Horn <jannh@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jann Horn <jannh@google.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/uhid.c

@@ -12,6 +12,7 @@
 
 #include <linux/atomic.h>
 #include <linux/compat.h>
+#include <linux/cred.h>
 #include <linux/device.h>
 #include <linux/fs.h>
 #include <linux/hid.h>
@@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
 
 	switch (uhid->input_buf.type) {
 	case UHID_CREATE:
+		/*
+		 * 'struct uhid_create_req' contains a __user pointer which is
+		 * copied from, so it's unsafe to allow this with elevated
+		 * privileges (e.g. from a setuid binary) or via kernel_write().
+		 */
+		if (file->f_cred != current_cred() || uaccess_kernel()) {
+			pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
+				    task_tgid_vnr(current), current->comm);
+			ret = -EACCES;
+			goto unlock;
+		}
 		ret = uhid_dev_create(uhid, &uhid->input_buf);
 		break;
 	case UHID_CREATE2: