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Mon, 07 Oct 2024 07:50:29 -0700 (PDT) Received: from localhost.localdomain ([223.104.210.43]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-71df0d7cf82sm4466432b3a.200.2024.10.07.07.50.20 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Mon, 07 Oct 2024 07:50:29 -0700 (PDT) From: Yafang Shao To: akpm@linux-foundation.org Cc: torvalds@linux-foundation.org, keescook@chromium.org, alx@kernel.org, justinstitt@google.com, ebiederm@xmission.com, alexei.starovoitov@gmail.com, rostedt@goodmis.org, catalin.marinas@arm.com, penguin-kernel@i-love.sakura.ne.jp, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, audit@vger.kernel.org, linux-security-module@vger.kernel.org, selinux@vger.kernel.org, bpf@vger.kernel.org, netdev@vger.kernel.org, dri-devel@lists.freedesktop.org, Yafang Shao , Andy Shevchenko Subject: [PATCH v9 5/7] mm/util: Fix possible race condition in kstrdup() Date: Mon, 7 Oct 2024 22:49:09 +0800 Message-Id: <20241007144911.27693-6-laoar.shao@gmail.com> X-Mailer: git-send-email 2.30.1 (Apple Git-130) In-Reply-To: <20241007144911.27693-1-laoar.shao@gmail.com> References: <20241007144911.27693-1-laoar.shao@gmail.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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 Cc: Andrew Morton Cc: Alejandro Colomar Cc: Andy Shevchenko --- 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);