Re: [PATCH] file capabilities: don't prevent signaling setuid root programs.

[Andrew Morgan <morgan@kernel.org>]

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Serge,

I still feel a bit uneasy about this. Looking ahead, with filesystem
capabilities, one can simulate this same situation with a setuid
'non-root' program as follows:

[morgan@computer ~]$ cat > test.c
main()
{
    printf("sleeping (%u)\n", getpid());
    sleep(100);
    printf("woke up\n");
}
[morgan@computer ~]$ cc -o test test.c
[morgan@computer ~]$ chmod u+s ./test
[morgan@computer ~]$ ls -ltr test
- -rwsrwxr-x  1 morgan morgan 7090 Nov 26 20:01 test
[morgan@computer ~]$ setcap cap_net_raw+ep ~/test
[morgan@computer ~]$ getcap ~/test
/home/morgan/test = cap_net_raw+ep
[morgan@computer ~]$ su luser
Password:
[luser@computer morgan]$ ./test
sleeping (5935)

<In another shell run by luser>
[luser@computer morgan]$ kill 5935
bash: kill: (5935) - Operation not permitted

Because of the euid=0 test, the piece of code you are adding will behave
differently in this situation. Is the root-behavior deserving of less
protection than this one? To my eye they seem equivalent.

Is there a compelling reason to include the euid==0 check?

Thanks

Andrew

Serge E. Hallyn wrote:
> This patch is needed to preserve legacy behavior when
> CONFIG_SECURITY_FILE_CAPABILITIES=y.  Without this patch, xinit can't
> kill X, so manually starting X in runlevel 3 then exiting your window
> manager will not cause X to exit. 
> 
> thanks,
> -serge
> 
>>From 81a6d780ad570f9a326fc27912ec0e373f5fa14f Mon Sep 17 00:00:00 2001
> From: Serge E. Hallyn <serue@us.ibm.com>
> Date: Tue, 20 Nov 2007 08:47:35 +0000
> Subject: [PATCH] file capabilities: don't prevent signaling setuid root programs.
> 
> An unprivileged process must be able to kill a setuid root
> program started by the same user.  This is legacy behavior
> needed for instance for xinit to kill X when the window manager
> exits.
> 
> When an unprivileged user runs a setuid root program in !SECURE_NOROOT
> mode, fP, fI, and fE are set full on, so pP' and pE' are full on.
> Then cap_task_kill() prevents the user from signaling the setuid root
> task.  This is a change in behavior compared to when
> !CONFIG_SECURITY_FILE_CAPABILITIES.
> 
> This patch introduces a special check into cap_task_kill() just
> to check whether a non-root user is signaling a setuid root
> program started by the same user.  If so, then signal is allowed.
> 
> Changelog:
> 	Nov 26: move test up above CAP_KILL test as per Andrew
> 		Morgan's suggestion.
> 
> Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
> ---
>  security/commoncap.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 302e8d0..5bc1895 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -526,6 +526,15 @@ int cap_task_kill(struct task_struct *p, struct siginfo *info,
>  	if (info != SEND_SIG_NOINFO && (is_si_special(info) || SI_FROMKERNEL(info)))
>  		return 0;
>  
> +	/*
> +	 * Running a setuid root program raises your capabilities.
> +	 * Killing your own setuid root processes was previously
> +	 * allowed.
> +	 * We must preserve legacy signal behavior in this case.
> +	 */
> +	if (p->euid == 0 && p->uid == current->uid)
> +		return 0;
> +
>  	/* sigcont is permitted within same session */
>  	if (sig == SIGCONT && (task_session_nr(current) == task_session_nr(p)))
>  		return 0;
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