linux-kernel - Re: [RFC][PATCH] exec: Use init rlimits for setuid exec

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Message-ID: <87wp7l68r5.fsf@xmission.com>
Date:   Thu, 06 Jul 2017 07:38:06 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>, Willy Tarreau <w@....eu>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Tony Luck <tony.luck@...el.com>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Helge Diller <deller@....de>,
        James Hogan <james.hogan@...tec.com>,
        Laura Abbott <labbott@...hat.com>, Greg KH <greg@...ah.com>,
        "security\@kernel.org" <security@...nel.org>,
        Qualys Security Advisory <qsa@...lys.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ximin Luo <infinity0@...ian.org>
Subject: Re: [RFC][PATCH] exec: Use init rlimits for setuid exec

Kees Cook <keescook@...omium.org> writes:

> In an attempt to provide sensible rlimit defaults for setuid execs, this
> inherits the namespace's init rlimits:
>
> $ ulimit -s
> 8192
> $ ulimit -s unlimited
> $ /bin/sh -c 'ulimit -s'
> unlimited
> $ sudo /bin/sh -c 'ulimit -s'
> 8192
>
> This is modified from Brad Spengler/PaX Team's hard-coded setuid exec
> stack rlimit (8MB) in the last public patch of grsecurity/PaX based on
> my understanding of the code. Changes or omissions from the original
> code are mine and don't reflect the original grsecurity/PaX code.
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> Instead of copying all rlimits, we could also pick specific ones to copy
> (e.g. RLIMIT_STACK, or ones from Andy's list) or exclude from copying
> (probably better to blacklist than whitelist).
>
> I think this is the right way to find the ns init task, but maybe it
> needs locking?
> ---
>  fs/exec.c | 34 ++++++++++++++++++++++++++++++----
>  1 file changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 904199086490..80e8b2bd4284 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1675,6 +1675,12 @@ static int exec_binprm(struct linux_binprm *bprm)
>  	return ret;
>  }
>  
> +static inline bool is_setuid_exec(struct linux_binprm *bprm)
> +{
> +	return (!uid_eq(bprm->cred->euid, current_euid()) ||
> +		!gid_eq(bprm->cred->egid, current_egid()));
> +}

Awesome I can make an executable setuid to myself and get all of roots
rlimits!

Scratch inheritable rlimits as useful for any kind of policy decision.

>  /*
>   * sys_execve() executes a new program.
>   */
> @@ -1687,6 +1693,7 @@ static int do_execveat_common(int fd, struct filename *filename,
>  	struct linux_binprm *bprm;
>  	struct file *file;
>  	struct files_struct *displaced;
> +	struct rlimit saved_rlim[RLIM_NLIMITS];
>  	int retval;
>  
>  	if (IS_ERR(filename))
> @@ -1771,24 +1778,38 @@ static int do_execveat_common(int fd, struct filename *filename,
>  	if (retval < 0)
>  		goto out;
>  
> +	/*
> +	 * From here forward, we've got credentials set up and we're
> +	 * using resources, so do rlimit replacement before we start
> +	 * copying strings. (Note that the RLIMIT_NPROC check has
> +	 * already happened.)
> +	 */
> +	BUILD_BUG_ON(sizeof(saved_rlim) != sizeof(current->signal->rlim));
> +	if (is_setuid_exec(bprm)) {
> +		memcpy(saved_rlim, current->signal->rlim, sizeof(saved_rlim));
> +		memcpy(current->signal->rlim,
> +		       task_active_pid_ns(current)->child_reaper->signal->rlim,
> +		       sizeof(current->signal->rlim));
> +	}
> +

Caerful.  child_reaper can change if you are not holding the tasklist
lock.

It would be better if we could move any rlimit changes after de_thread.
Otherwise there are some really fun races you can play with.

After de_thread is past the point of no return so you would not need to
worry about restoring the rlimits either.

>  	retval = copy_strings_kernel(1, &bprm->filename, bprm);
>  	if (retval < 0)
> -		goto out;
> +		goto out_restore;
>  
>  	bprm->exec = bprm->p;
>  	retval = copy_strings(bprm->envc, envp, bprm);
>  	if (retval < 0)
> -		goto out;
> +		goto out_restore;
>  
>  	retval = copy_strings(bprm->argc, argv, bprm);
>  	if (retval < 0)
> -		goto out;
> +		goto out_restore;
>  
>  	would_dump(bprm, bprm->file);
>  
>  	retval = exec_binprm(bprm);
>  	if (retval < 0)
> -		goto out;
> +		goto out_restore;
>  
>  	/* execve succeeded */
>  	current->fs->in_exec = 0;
> @@ -1802,6 +1823,11 @@ static int do_execveat_common(int fd, struct filename *filename,
>  		put_files_struct(displaced);
>  	return retval;
>  
> +out_restore:
> +	if (is_setuid_exec(bprm)) {
> +		memcpy(current->signal->rlim, saved_rlim, sizeof(saved_rlim));
> +	}
> +
>  out:
>  	if (bprm->mm) {
>  		acct_arg_size(bprm, 0);
> -- 
> 2.7.4

Eric