kernel-hardening - Re: [PATCH] fs: hardlink creation restriction cleanup

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Message-ID: <20120222102301.GC30339@elte.hu>
Date: Wed, 22 Feb 2012 11:23:01 +0100
From: Ingo Molnar <mingo@...e.hu>
To: Kees Cook <keescook@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Marcin Slusarz <marcin.slusarz@...il.com>,
	linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...otime.net>,
	Alexander Viro <viro@...iv.linux.org.uk>, linux-doc@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] fs: hardlink creation restriction cleanup


* Kees Cook <keescook@...omium.org> wrote:

> Clean-up of hardlink restriction logic, as suggested by Andrew Morton.
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  fs/namei.c |   59 +++++++++++++++++++++++++++++++++++++++++------------------
>  1 files changed, 41 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 8ed4e00..a4a21a5 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -693,46 +693,69 @@ static inline int may_follow_link(struct path *link)
>  }
>  
>  /**
> + * safe_hardlink_source - Check for safe hardlink conditions
> + * @inode: the source inode to hardlink from
> + *
> + * Return false if at least one of the following conditions:
> + *    - inode is not a regular file
> + *    - inode is setuid
> + *    - inode is setgid and group-exec
> + *    - access failure for read and write
> + *
> + * Otherwise returns true.
> + */
> +static bool safe_hardlink_source(struct inode *inode)
> +{
> +	mode_t mode = inode->i_mode;
> +
> +	/* Special files should not get pinned to the filesystem. */
> +	if (!S_ISREG(mode))
> +		return false;
> +	/* Setuid files should not get pinned to the filesystem. */
> +	if (mode & S_ISUID)
> +		return false;
> +	/* Executable setgid files should not get pinned to the filesystem. */
> +	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
> +		return false;
> +	/* Hardlinking to unreadable or unwritable sources is dangerous. */
> +	if (inode_permission(inode, MAY_READ | MAY_WRITE))
> +		return false;
> +
> +	return true;

A really minor nitpick, could we use this form please:

static bool safe_hardlink_source(struct inode *inode)
{
	mode_t mode = inode->i_mode;

	/* Special files should not get pinned to the filesystem. */
	if (!S_ISREG(mode))
		return false;

	/* Setuid files should not get pinned to the filesystem. */
	if (mode & S_ISUID)
		return false;

	/* Executable setgid files should not get pinned to the filesystem. */
	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
		return false;

	/* Hardlinking to unreadable or unwritable sources is dangerous. */
	if (inode_permission(inode, MAY_READ | MAY_WRITE))
		return false;

	return true;
}

Those separate blocks of comments and conditions stand out much 
nicer this way, making it way easier on the eyes - to my eyes at 
least ;-)

Thanks,

	Ingo

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