linux-kernel - [PATCH] Revert "HID: uhid: use strlcpy() instead of strncpy()"

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Message-Id: <20181114131642.21425-1-dh.herrmann@gmail.com>
Date:   Wed, 14 Nov 2018 14:16:42 +0100
From:   David Herrmann <dh.herrmann@...il.com>
To:     linux-input@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, jikos@...nel.org,
        benjamin.tissoires@...hat.com,
        David Herrmann <dh.herrmann@...il.com>
Subject: [PATCH] Revert "HID: uhid: use strlcpy() instead of strncpy()"

This reverts commit 336fd4f5f25157e9e8bd50e898a1bbcd99eaea46.

Please note that `strlcpy()` does *NOT* do what you think it does.
strlcpy() *ALWAYS* reads the full input string, regardless of the
'length' parameter. That is, if the input is not zero-terminated,
strlcpy() will *READ* beyond input boundaries. It does this, because it
always returns the size it *would* copy if the target was big enough,
not the truncated size it actually copied.

The original code was perfectly fine. The hid device is
zero-initialized and the strncpy() functions copied up to n-1
characters. The result is always zero-terminated this way.

This is the third time someone tried to replace strncpy with strlcpy in
this function, and gets it wrong. I now added a comment that should at
least make people reconsider.

Signed-off-by: David Herrmann <dh.herrmann@...il.com>
---
 drivers/hid/uhid.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index fefedc0b4dc6..0dfdd0ac7120 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -496,12 +496,13 @@ static int uhid_dev_create2(struct uhid_device *uhid,
 		goto err_free;
 	}

-	len = min(sizeof(hid->name), sizeof(ev->u.create2.name));
-	strlcpy(hid->name, ev->u.create2.name, len);
-	len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys));
-	strlcpy(hid->phys, ev->u.create2.phys, len);
-	len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq));
-	strlcpy(hid->uniq, ev->u.create2.uniq, len);
+	/* @hid is zero-initialized, strncpy() is correct, strlcpy() not */
+	len = min(sizeof(hid->name), sizeof(ev->u.create2.name)) - 1;
+	strncpy(hid->name, ev->u.create2.name, len);
+	len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys)) - 1;
+	strncpy(hid->phys, ev->u.create2.phys, len);
+	len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq)) - 1;
+	strncpy(hid->uniq, ev->u.create2.uniq, len);

 	hid->ll_driver = &uhid_hid_driver;
 	hid->bus = ev->u.create2.bus;
-- 
2.19.1