mm/util: fix possible race condition in kstrdup()

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.

Link: https://lkml.kernel.org/r/20241007144911.27693-6-laoar.shao@gmail.com
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Alejandro Colomar <alx@kernel.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@gmail.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Matus Jokay <matus.jokay@stuba.sk>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Ondrej Mosnacek <omosnace@redhat.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Quentin Monnet <qmo@kernel.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Stephen Smalley <stephen.smalley.work@gmail.com>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>