linux-kernel - Re: [PATCH] ocfs2: mount fails with buffer overflow in strlen

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Date:   Tue, 28 Sep 2021 15:14:50 +0200
From:   Valentin Vidić <vvidic@...entin-vidic.from.hr>
To:     Joseph Qi <joseph.qi@...ux.alibaba.com>
Cc:     Mark Fasheh <mark@...heh.com>, Joel Becker <jlbec@...lplan.org>,
        ocfs2-devel@....oracle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ocfs2: mount fails with buffer overflow in strlen

On Tue, Sep 28, 2021 at 08:05:22PM +0800, Joseph Qi wrote:
> strlcpy in ocfs2_initialize_super() is introduced 8 years ago, so I
> don't understand why you've mentioned that the issues starts from
> v5.11.

v5.11 introduced the overflow checks to string functions so that is
when the mount started to fail.

> osb->osb_cluster_stack and osb->osb_cluster_name is always larger by
> 1 than which in ocfs2_cluster_info, and the input size of strlcpy does
> the same, so I don't see how it overflows.

strlcpy internally calls strlen on the source argument, in this case
that is ci_stack array with size of 4. That array stores the value
"o2cb" so the strlen continues reading into the union until it reaches
a zero byte somewhere. The same would happen with ci_cluster if the
cluster name is long enough.

struct ocfs2_cluster_info {
/*00*/	__u8   ci_stack[OCFS2_STACK_LABEL_LEN];
	union {
		__le32 ci_reserved;
		struct {
			__u8 ci_stackflags;
			__u8 ci_reserved1;
			__u8 ci_reserved2;
			__u8 ci_reserved3;
		};
	};
/*08*/	__u8   ci_cluster[OCFS2_CLUSTER_NAME_LEN];
/*18*/
};

-- 
Valentin