Quoting Andrew Morgan ([email protected]):
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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?
I don't believe in entitlement :)
> To my eye they seem equivalent.
Not to mine. The one case is classic setuid root, the other exploits
the new file capabilities - even if they don't actually add any
privilege.
> Is there a compelling reason to include the euid==0 check?
Yes, because only setuid root fills your capability sets.
In fact I was wondering whether the check should be under a check
for !SECURE_NOROOT. I think it should, but it only matters after your
per-process securebits patch is reposted.
Would you prefer if the check were under a sysctl, so that those really
using file caps could tutn off the setuid root exception? It also
provies a more graceful path toward eventually getting rid of this
legacy support.
thanks,
-serge
>
> 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 <[email protected]>
> > 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 <[email protected]>
> > ---
> > 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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