Date: Sun, 1 Dec 2019 03:24:11 +0300 From: Dmitry Marakasov <amdmi3@amdmi3.ru> To: Konstantin Belousov <kostikbel@gmail.com> Cc: freebsd-stable@freebsd.org Subject: Re: How can kill(-1, 0) return EPERM? Message-ID: <20191201002411.GF4071@hades.panopticon> In-Reply-To: <20191129225834.GY10580@kib.kiev.ua> References: <20191129151606.GD4071@hades.panopticon> <20191129164509.GE4071@hades.panopticon> <20191129225834.GY10580@kib.kiev.ua>
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* Konstantin Belousov (kostikbel@gmail.com) wrote:
> > > I'm helping to investigate some userspace issue [1], where kill(-1, SIGKILL)
> > > fails with EPERM. I've managed to isolate this case in a small program:
> > >
> > >
> > > ```
> > > #include <err.h>
> > > #include <errno.h>
> > > #include <signal.h>
> > > #include <stdio.h>
> > > #include <string.h>
> > > #include <unistd.h>
> > >
> > > int main() {
> > > if (setuid(66) == -1) // uucp, just for the test
> > > err(1, "setuid");
> > >
> > > int res = kill(-1, 0); // <- fails with EPERM
> > > fprintf(stderr, "kill(-1, 0) result=%d, errno=%s\n", res, strerror(errno));
> > >
> > > return 0;
> > > }
> > > ```
> > >
> > > when run from root on 12.1 kill call fails with EPERM. However I cannot
> > > comprehend what it is caused by and how it's even possible: kill(2) manpage
> > > says that with pid=-1 kill should only send (and in this case of sig=0,
> > > /not/ send) signals to the processes belonging to the current uid, so there
> > > should be no permission problems. I've also looked into the kernel code
> > > (sys_kill, killpg1), and it matches to what manpage says, I see no way
> > > for it to return EPERM: sys_kill() should fall through to the switch, call
> > > killpg1() with all=1 and killpg1() if(all) branch may only set `ret` to
> > > either 0 or ESRCH. Am I missing something, or is there a problem somewhere?
> >
> > It looks like I have misread the `else if' path of this core.
> >
> > if (all) {
> > /*
> > * broadcast
> > */
> > sx_slock(&allproc_lock);
> > FOREACH_PROC_IN_SYSTEM(p) {
> > if (p->p_pid <= 1 || p->p_flag & P_SYSTEM ||
> > p == td->td_proc || p->p_state == PRS_NEW) {
> > continue;
> > }
> > PROC_LOCK(p);
> > err = p_cansignal(td, p, sig);
> > if (err == 0) {
> > if (sig)
> > pksignal(p, sig, ksi);
> > ret = err;
> > }
> > else if (ret == ESRCH)
> > ret = err;
> > PROC_UNLOCK(p);
> > }
> > sx_sunlock(&allproc_lock);
> > } ...
> >
> > so it's clear now where EPERM comes from. However it looks like the
> > behavior contradicts the manpage - there are no signs of check that
> > the signalled process has the same uid as the caller.
>
> I am not sure what you mean by 'signs of check'. Look at p_cansignal()
> and cr_cansignal() implementation.
I've meant that according to the manpage
If pid is -1:
If the user has super-user privileges, the signal is sent to all
processes excluding system processes (with P_SYSTEM flag set),
process with ID 1 (usually init(8)), and the process sending the
signal. If the user is not the super user, the signal is sent to
all processes with the same uid as the user excluding the process
sending the signal. No error is returned if any process could be
signaled.
IMO there should be an additional check in this condition:
if (p->p_pid <= 1 || p->p_flag & P_SYSTEM ||
p == td->td_proc || p->p_state == PRS_NEW) {
continue;
}
E.g. something like
if (p->p_pid <= 1 || p->p_flag & P_SYSTEM ||
p == td->td_proc || p->p_state == PRS_NEW ||
(td->td_ucred->cr_ruid != 0 &&
p->td_ucred->cr_ruid != td->td_ucred->cr_ruid) {
continue;
}
e.g. it should not even attempt to signal processes with other uids.
--
Dmitry Marakasov . 55B5 0596 FF1E 8D84 5F56 9510 D35A 80DD F9D2 F77D
amdmi3@amdmi3.ru ..: https://github.com/AMDmi3
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