linux-hardening - [PATCH] seq_file: seq_show_option

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Date:   Wed, 26 Jul 2023 14:59:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Muchun Song <muchun.song@...ux.dev>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH] seq_file: seq_show_option_n() is used for precise sizes

When seq_show_option_n() is used, it is for non-string memory that
happens to be printable bytes. As such, we must use memcpy() to copy the
bytes and then explicitly NUL-terminate the result.

Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Muchun Song <muchun.song@...ux.dev>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 include/linux/seq_file.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index bd023dd38ae6..386ab580b839 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -249,18 +249,19 @@ static inline void seq_show_option(struct seq_file *m, const char *name,
 
 /**
  * seq_show_option_n - display mount options with appropriate escapes
- *		       where @value must be a specific length.
+ *		       where @value must be a specific length (i.e.
+ *		       not NUL-terminated).
  * @m: the seq_file handle
  * @name: the mount option name
  * @value: the mount option name's value, cannot be NULL
- * @length: the length of @value to display
+ * @length: the exact length of @value to display, must be constant expression
  *
  * This is a macro since this uses "length" to define the size of the
  * stack buffer.
  */
 #define seq_show_option_n(m, name, value, length) {	\
 	char val_buf[length + 1];			\
-	strncpy(val_buf, value, length);		\
+	memcpy(val_buf, value, length);			\
 	val_buf[length] = '\0';				\
 	seq_show_option(m, name, val_buf);		\
 }
-- 
2.34.1