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

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Message-Id: <B4520E53-6DD9-44D7-A064-9F405FBAA793@gmail.com>
Date:	Thu, 3 Dec 2015 17:45:06 -0800
From:	yalin wang <yalin.wang2010@...il.com>
To:	Kees Cook <keescook@...omium.org>
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@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] clear file privilege bits when mmap writing


> 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 .

Thanks

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