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

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Date:	Mon, 7 Dec 2015 16:40:14 -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 Mon, Dec 7, 2015 at 2:42 PM, Kees Cook <keescook@...omium.org> wrote:
> 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!

This continues to look worse and worse.

So... to check this at mprotect time, I have to know it's MAP_SHARED,
but that's in the vma_flags, which I can only see after holding
mmap_sem.

The best I can think of now is to strip the bits at munmap time, since
you can't execute an mmapped file until it closes.

Jan, thoughts on this?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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