linux-kernel - Re: [PATCH v4] fs: clear file privilege bits when mmap writing

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Date:	Wed, 9 Dec 2015 13:49:18 +0100
From:	Jan Kara <jack@...e.cz>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	yalin wang <yalin.wang2010@...il.com>,
	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>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] fs: clear file privilege bits when mmap writing

On Tue 08-12-15 15:28:18, Kees Cook 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). We
> could do this during vm_mmap_pgoff, but that would need coverage in
> mprotect as well, but to check for MAP_SHARED, we'd need to hold mmap_sem
> again.
> 
> Instead, detect the need to clear the bits during the page fault, and
> actually remove the bits during final fput. Since the file was open for
> writing, it wouldn't have been possible to execute it yet.
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> Here's another way? I wonder which of these will actually work. I
> wish we could reject writes if file_remove_privs() fails.

Yeah, the fact that we cannot do anything with file_remove_privs() failure
is rather unfortunate. So open for writing may be the best choice for
file_remove_privs() in the end? It's not perfect but it looks like the
least problematic solution.

Frankly writeable files that have SUID / SGID bits set are IMHO problems on
its own, with IMA attrs which are handled by file_remove_privs() as well
things may be somewhat different.

> diff --git a/fs/file_table.c b/fs/file_table.c
> index ad17e05ebf95..abb537ef4344 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -191,6 +191,14 @@ static void __fput(struct file *file)
>  
>  	might_sleep();
>  
> +	/*
> +	 * XXX: While avoiding mmap_sem, we've already been written to.
> +	 * We must ignore the return value, since we can't reject the
> +	 * write.
> +	 */
> +	if (unlikely(file->f_remove_privs))
> +		file_remove_privs(file);
> +

You're missing i_mutex locking again ;).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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