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Date:   Thu, 14 Sep 2023 22:47:21 +0000
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Message-ID: <20230914-strncpy-drivers-hid-uhid-c-v1-1-18a190060d8d@google.com>
Subject: [PATCH] HID: uhid: refactor deprecated strncpy
From:   Justin Stitt <justinstitt@google.com>
To:     David Rheinsberg <david@readahead.eu>,
        Jiri Kosina <jikos@kernel.org>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc:     linux-input@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        Justin Stitt <justinstitt@google.com>
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Precedence: bulk

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

We should prefer more robust and less ambiguous string interfaces.

A suitable replacement is `strscpy` [2] due to the fact that it
guarantees NUL-termination on the destination buffer without
unnecessarily NUL-padding.

Looking at: Commit 4d26d1d1e806 ("Revert "HID: uhid: use strlcpy() instead of strncpy()"")
we see referenced the fact that many attempts have been made to change
these strncpy's into strlcpy to no success. I think strscpy is an
objectively better interface here as it doesn't unnecessarily NUL-pad
the destination buffer whilst allowing us to drop the `len = min(...)`
pattern as strscpy will implicitly limit the number of bytes copied by
the smaller of its dest and src arguments.

So while the existing code may not have a bug (i.e: overread problems)
we should still favor strscpy due to readability (plus a very slight
performance boost).

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
 drivers/hid/uhid.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 4588d2cd4ea4..00e1566ad37b 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -490,7 +490,7 @@ static int uhid_dev_create2(struct uhid_device *uhid,
 			    const struct uhid_event *ev)
 {
 	struct hid_device *hid;
-	size_t rd_size, len;
+	size_t rd_size;
 	void *rd_data;
 	int ret;
 
@@ -514,13 +514,9 @@ static int uhid_dev_create2(struct uhid_device *uhid,
 		goto err_free;
 	}
 
-	/* @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);
+	strscpy(hid->name, ev->u.create2.name, sizeof(hid->name));
+	strscpy(hid->phys, ev->u.create2.phys, sizeof(hid->phys));
+	strscpy(hid->uniq, ev->u.create2.uniq, sizeof(hid->uniq));
 
 	hid->ll_driver = &uhid_hid_driver;
 	hid->bus = ev->u.create2.bus;

---
base-commit: 3669558bdf354cd352be955ef2764cde6a9bf5ec
change-id: 20230914-strncpy-drivers-hid-uhid-c-a465f3e784dd

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