Re: [CFT][PATCH] ptrace: Properly initialize ptracer_cred on fork

From: Takashi Iwai <tiwai@suse.de>
To: ebiederm@xmission.com (Eric W. Biederman)
Cc: linux-kernel@vger.kernel.org
Subject: Re: [CFT][PATCH] ptrace: Properly initialize ptracer_cred on fork
Date: Tue, 23 May 2017 14:50:04 +0200	[thread overview]
Message-ID: <s5ho9ujzqir.wl-tiwai@suse.de> (raw)
In-Reply-To: <87vaorrbj9.fsf@xmission.com>

On Tue, 23 May 2017 14:40:58 +0200,
Eric W. Biederman wrote:
> 
> Takashi Iwai <tiwai@suse.de> writes:
> 
> > On Tue, 23 May 2017 07:47:32 +0200,
> > Takashi Iwai wrote:
> >> 
> >> On Mon, 22 May 2017 23:04:48 +0200,
> >> Eric W. Biederman wrote:
> >> > 
> >> > 
> >> > When I introduced ptracer_cred I failed to consider the weirdness of
> >> > fork where the task_struct copies the old value by default.  This
> >> > winds up leaving ptracer_cred set even when a process forks and
> >> > the child process does not wind up being ptraced.
> >> > 
> >> > Because ptracer_cred is not set on non-ptraced processes whose
> >> > parents were ptraced this has broken the ability of the enlightenment
> >> > window manager to start setuid children.
> >> > 
> >> > Fix this by properly initializing ptracer_cred in ptrace_init_task
> >> > 
> >> > This must be done with a little bit of care to preserve the current value
> >> > of ptracer_cred when ptrace carries through fork.  Re-reading the
> >> > ptracer_cred from the ptracing process at this point is inconsistent
> >> > with how PT_PTRACE_CAP has been maintained all of these years.
> >> > 
> >> > Fixes: 64b875f7ac8a ("ptrace: Capture the ptracer's creds not PT_PTRACE_CAP")
> >> > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> >> > ---
> >> > 
> >> > If I could get some folks to test and verify this fixes the
> >> > enlightenment issue I would really appreciate it.
> >> 
> >> This seems giving a compile warning and it becomes error in the
> >> following:
> >> 
> >> In file included from ./include/linux/mutex.h:13:0,
> >>                  from ./include/linux/kernfs.h:13,
> >>                  from ./include/linux/sysfs.h:15,
> >>                  from ./include/linux/kobject.h:21,
> >>                  from ./include/linux/device.h:17,
> >>                  from drivers/gpu/drm/i915/gvt/kvmgt.c:32:
> >> ./include/linux/ptrace.h: In function ‘ptrace_init_task’:
> >> ./arch/x86/include/asm/current.h:17:17: error: passing argument 3 of ‘__ptrace_link’ discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
> >>  #define current get_current()
> >>                  ^
> >> ./include/linux/ptrace.h:210:41: note: in expansion of macro ‘current’
> >>    __ptrace_link(child, current->parent, current->ptracer_cred);
> >>                                          ^~~~~~~
> >> In file included from ./arch/x86/include/asm/stacktrace.h:10:0,
> >>                  from ./arch/x86/include/asm/perf_event.h:246,
> >>                  from ./include/linux/perf_event.h:24,
> >>                  from ./arch/x86/include/asm/kvm_host.h:24,
> >>                  from ./include/linux/kvm_host.h:37,
> >>                  from drivers/gpu/drm/i915/gvt/kvmgt.c:41:
> >> ./include/linux/ptrace.h:56:13: note: expected ‘struct cred *’ but argument is of type ‘const struct cred *’
> >>  extern void __ptrace_link(struct task_struct *child,
> >>              ^~~~~~~~~~~~~
> >> cc1: all warnings being treated as errors
> >
> > Through a quick test on VM (fixed patch by adding const to
> > __ptrace_link() argument), it seems working fine.
> 
> Thank you.  It seems when I fixed the const issue in my tree I forget
> commit --amend so the fix didn't make it into the patch I sent out.
> 
> > Tested-by: Takashi Iwai <tiwai@suse.de>
> 
> Just to confirm.  You were able to reproduce the enlightenment failure
> and this fixes it?

Yes.  It was reproducible on KVM image.

Takashi