[PATCH v9 5/7] mm/util: Fix possible race condition in kstrdup()

[Yafang Shao <laoar.shao@gmail.com>]

In kstrdup(), it is critical to ensure that the dest string is always
NUL-terminated. However, potential race condition can occur between a
writer and a reader.

Consider the following scenario involving task->comm:

    reader                    writer

  len = strlen(s) + 1;
                             strlcpy(tsk->comm, buf, sizeof(tsk->comm));
  memcpy(buf, s, len);

In this case, there is a race condition between the reader and the
writer. The reader calculates the length of the string `s` based on the
old value of task->comm. However, during the memcpy(), the string `s`
might be updated by the writer to a new value of task->comm.

If the new task->comm is larger than the old one, the `buf` might not be
NUL-terminated. This can lead to undefined behavior and potential
security vulnerabilities.

Let's fix it by explicitly adding a NUL terminator after the memcpy. It
is worth noting that memcpy() is not atomic, so the new string can be
shorter when memcpy() already copied past the new NUL.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alejandro Colomar <alx@kernel.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
---
 mm/util.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index 4f1275023eb7..858a9a2f57e7 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -62,8 +62,15 @@ char *kstrdup(const char *s, gfp_t gfp)
 
 	len = strlen(s) + 1;
 	buf = kmalloc_track_caller(len, gfp);
-	if (buf)
+	if (buf) {
 		memcpy(buf, s, len);
+		/*
+		 * During memcpy(), the string might be updated to a new value,
+		 * which could be longer than the string when strlen() is
+		 * called. Therefore, we need to add a NUL terminator.
+		 */
+		buf[len - 1] = '\0';
+	}
 	return buf;
 }
 EXPORT_SYMBOL(kstrdup);
-- 
2.43.5