linux-kernel - Re: [PATCH 1/4] mm/mlock: return EINVAL for illegal user memory range in mlock

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Message-Id: <20221228141701.c64add46c4b09aa17f605baf@linux-foundation.org>
Date:   Wed, 28 Dec 2022 14:17:01 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Wupeng Ma <mawupeng1@...wei.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <kuleshovmail@...il.com>, <aneesh.kumar@...ux.ibm.com>,
        <clameter@....com>
Subject: Re: [PATCH 1/4] mm/mlock: return EINVAL for illegal user memory
 range in mlock

On Mon, 5 Dec 2022 11:41:05 +0800 Wupeng Ma <mawupeng1@...wei.com> wrote:

> While testing mlock, we have a problem if the len of mlock is ULONG_MAX.
> The return value of mlock is zero. But nothing will be locked since the
> len in do_mlock overflows to zero due to the following code in mlock:
> 
>   len = PAGE_ALIGN(len + (offset_in_page(start)));
> 
> The same problem happens in munlock.
> 
> Since TASK_SIZE is the maximum user space address. The start or len of
> mlock shouldn't be bigger than this. Function access_ok can be used to
> check this issue, so return -EINVAL if bigger.

What happens if userspace uses a value somewhat smaller than ULONG_MAX?

	mlock(addr, ULONG_MAX - 1000000);

?

Because if the above works successfully and if it no longer works
successfully with this patchset then that could be a
backward-compatibility problem.