linux-kernel - Re: [PATCH v2] clear file privilege bits when mmap writing

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Message-ID: <CAGXu5jJaY9WeR-NiZXfAu=hM6U7DaPD_d8ZZTAdo_EkS3WDxCw@mail.gmail.com>
Date:	Mon, 7 Dec 2015 14:42:22 -0800
From:	Kees Cook <keescook@...omium.org>
To:	yalin wang <yalin.wang2010@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Willy Tarreau <w@....eu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Chen Gang <gang.chen.5i5j@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] clear file privilege bits when mmap writing

On Thu, Dec 3, 2015 at 5:45 PM, yalin wang <yalin.wang2010@...il.com> wrote:
>
>> On Dec 2, 2015, at 16:03, Kees Cook <keescook@...omium.org> wrote:
>>
>> Normally, when a user can modify a file that has setuid or setgid bits,
>> those bits are cleared when they are not the file owner or a member
>> of the group. This is enforced when using write and truncate but not
>> when writing to a shared mmap on the file. This could allow the file
>> writer to gain privileges by changing a binary without losing the
>> setuid/setgid/caps bits.
>>
>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> during the page fault (due to mmap_sem being held during the fault).
>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> Cc: stable@...r.kernel.org
>> —
>
> is this means mprotect() sys call also need add this check?
> mprotect() can change to PROT_WRITE, then it can write to a
> read only map again , also a secure hole here .

Yes, good point. This needs to be added. I will send a new patch. Thanks!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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