Re: [kernel-hardening] [PATCH 1/2] fs: add link restrictions

[Kees Cook]

On Wed, Aug 8, 2012 at 5:19 AM, Vasily Kulikov <segoon@xxxxxxxxxxxx> wrote:
> Hi Kees,
>
> On Wed, Jul 25, 2012 at 17:29 -0700, Kees Cook wrote:
>> +/**
>> + * 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)
>> +{
>> +     umode_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;
>
> We don't want to make hardlinks of SUID files, but we still allow to create
> hardlinks to SUID'ish cap'ed files.  Probably check whether the inode is
> setcap'ed?

Excellent idea. It doesn't look like there is anything "simple" to do
this already. It'd be close to get_file_caps() but without the bprm.
Maybe just get_vfs_caps_from_disk() and a walk of the caps? What would
you recommend?

> Probably we can enhance this further and allow LSMs to define whether this
> particular file is special in LSM's point of view (IOW, it can be able to move
> a process to another security domain which is served by LSM).

Yeah. Perhaps implementing the needed check above with a new security
check and have commoncaps do the vfs fetch with LSMs able to override?

-Kees

-- 
Kees Cook
Chrome OS Security
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