linux-kernel - [PATCH] usb: ch9: Replace 1-element array with flexible array

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Message-Id: <20230613210400.never.078-kees@kernel.org>
Date:   Tue, 13 Jun 2023 14:04:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        kernel test robot <oliver.sang@...el.com>,
        kernel test robot <lkp@...el.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Jó Ágila Bitsch <jgilab@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH] usb: ch9: Replace 1-element array with flexible array

With "-fstrict-flex-arrays=3" enabled, UBSAN_BOUNDS no longer pretends
1-element arrays are unbounded. Walking wData will trigger a warning,
so make it a proper flexible array. Add a union to keep the struct size
identical for userspace in case anything was depending on the old size.

Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202306102333.8f5a7443-oliver.sang@intel.com
Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: kernel test robot <lkp@...el.com>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: "Jó Ágila Bitsch" <jgilab@...il.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 include/uapi/linux/usb/ch9.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/usb/ch9.h b/include/uapi/linux/usb/ch9.h
index b17e3a21b15f..82ec6af71a1d 100644
--- a/include/uapi/linux/usb/ch9.h
+++ b/include/uapi/linux/usb/ch9.h
@@ -376,7 +376,10 @@ struct usb_string_descriptor {
 	__u8  bLength;
 	__u8  bDescriptorType;
 
-	__le16 wData[1];		/* UTF-16LE encoded */
+	union {
+		__le16 legacy_padding;
+		__DECLARE_FLEX_ARRAY(__le16, wData);	/* UTF-16LE encoded */
+	};
 } __attribute__ ((packed));
 
 /* note that "string" zero is special, it holds language codes that
-- 
2.34.1