Return-Path: Received: from mail-qk0-f171.google.com ([209.85.220.171]:32806 "EHLO mail-qk0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933455AbcDYUhd (ORCPT ); Mon, 25 Apr 2016 16:37:33 -0400 Received: by mail-qk0-f171.google.com with SMTP id n63so67295515qkf.0 for ; Mon, 25 Apr 2016 13:37:33 -0700 (PDT) Message-ID: <1461616649.4288.35.camel@poochiereds.net> Subject: Re: [RFC PATCH v2 2/3] gssd: using syscalls directly to change thread's identity From: Jeff Layton To: Olga Kornievskaia Cc: Steve Dickson , Jeff Layton , Olga Kornievskaia , linux-nfs Date: Mon, 25 Apr 2016 16:37:29 -0400 In-Reply-To: References: <1461186721-64008-1-git-send-email-kolga@netapp.com> <1461186721-64008-3-git-send-email-kolga@netapp.com> <571A6FB0.8080709@RedHat.com> <571BE5B0.4050608@RedHat.com> <1461468267.12939.9.camel@poochiereds.net> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Sender: linux-nfs-owner@vger.kernel.org List-ID: On Mon, 2016-04-25 at 12:56 -0400, Olga Kornievskaia wrote: > On Sat, Apr 23, 2016 at 11:24 PM, Jeff Layton wrote: > > > > On Sat, 2016-04-23 at 17:14 -0400, Steve Dickson wrote: > > > > > > > > > On 04/22/2016 04:41 PM, Olga Kornievskaia wrote: > > > > > > > > > > > > On Fri, Apr 22, 2016 at 2:38 PM, Steve Dickson > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > On 04/20/2016 05:12 PM, Olga Kornievskaia wrote: > > > > > > > > > > > > > > > > > > For the threaded version we have to set uid,gid per thread > > > > > > instead > > > > > > of per process. glibc setresuid() when called from a thread, > > > > > > it'll > > > > > > send a signal to all other threads to synchronize the uid in > > > > > > all > > > > > > other threads. To bypass this, we have to call syscall() > > > > > > directly. > > > > > > > > > > > > Signed-off-by: Olga Kornievskaia > > > > > > Reviewed-by: Steve Dickson > > > > > > --- > > > > > >  utils/gssd/gssd_proc.c | 10 ++++++++-- > > > > > >  1 file changed, 8 insertions(+), 2 deletions(-) > > > > > > > > > > > > diff --git a/utils/gssd/gssd_proc.c b/utils/gssd/gssd_proc.c > > > > > > index 581a125..5d9a6db 100644 > > > > > > --- a/utils/gssd/gssd_proc.c > > > > > > +++ b/utils/gssd/gssd_proc.c > > > > > > @@ -69,6 +69,7 @@ > > > > > >  #include > > > > > >  #include > > > > > >  #include > > > > > > +#include > > > > > > > > > > > >  #include "gssd.h" > > > > > >  #include "err_util.h" > > > > > > @@ -457,7 +458,12 @@ change_identity(uid_t uid) > > > > > >        * Switch the GIDs. Note that we leave the saved-set-gid > > > > > > alone in an > > > > > >        * attempt to prevent attacks via ptrace() > > > > > >        */ > > > > > > -     if (setresgid(pw->pw_gid, pw->pw_gid, -1) != 0) { > > > > > > +     /* For the threaded version we have to set uid,gid per > > > > > > thread instead > > > > > > +      * of per process. glibc setresuid() when called from a > > > > > > thread, it'll > > > > > > +      * send a signal to all other threads to synchronize the > > > > > > uid in all > > > > > > +      * other threads. To bypass this, we have to call > > > > > > syscall() directly. > > > > > > +      */ > > > > > > +     if (syscall(SYS_setresgid, pw->pw_gid) != 0) { > > > > > >               printerr(0, "WARNING: failed to set gid to > > > > > > %u!\n", pw->pw_gid); > > > > > >               return errno; > > > > > >       } > > > > > > @@ -466,7 +472,7 @@ change_identity(uid_t uid) > > > > > >        * Switch UIDs, but leave saved-set-uid alone to prevent > > > > > > ptrace() by > > > > > >        * other processes running with this uid. > > > > > >        */ > > > > > > -     if (setresuid(uid, uid, -1) != 0) { > > > > > > +     if (syscall(SYS_setresuid, uid) != 0) { > > > > > >               printerr(0, "WARNING: Failed to setuid for user > > > > > > with uid %u\n", > > > > > >                               uid); > > > > > >               return errno; > > > > > > > > > > > We also have to do the same thing to the setgroups() call at the > > > > > top of change_identity(). So add the following diff to this > > > > > patch and we are good to go... > > > > > > > > > > diff --git a/utils/gssd/gssd_proc.c b/utils/gssd/gssd_proc.c > > > > > index e651d71..2f9f8ab 100644 > > > > > --- a/utils/gssd/gssd_proc.c > > > > > +++ b/utils/gssd/gssd_proc.c > > > > > @@ -437,7 +437,7 @@ change_identity(uid_t uid) > > > > >         struct passwd   *pw; > > > > > > > > > >         /* drop list of supplimentary groups first */ > > > > > -       if (setgroups(0, NULL) != 0) { > > > > > +       if (syscall(SYS_setgroups, 0) != 0) { > > > > >                 printerr(0, "WARNING: unable to drop > > > > > supplimentary groups!"); > > > > >                 return errno; > > > > >         } > > > > > > > > > Do you know why we do this before getting the passwd entry? > > > It came in with commit 6b53fc9c which changed the way gssd > > > changed the uid/gid from Jeff Layton... > > > > > > Jeff, what was the thought behind dropping supplementary groups? > > > I believe it was some type of security thing. > > > > > We do that so that the child doesn't run with supplementary groups from > > the main process. > > > > As to why we do that before getting the passwd entry instead of after? > > No real reason for the order that I can recall. If you need to do it > > after the pw entry, I don't think it'll matter. > > > > That said, since this has to run as root anyway, I guess you could drop > > the supplementary groups in the main process. Then all of the children > > should get a zeroed out grouplist, and it's one less syscall... > Thank Jeff, I thought that since we are setting the effective uid/gid > of the process (or with the patches, thread), then the child process > (thread) would have the supplementary groups associated with the given > uid. > No. Calling seteuid should have no effect on the supplementary groups list, AFAIK. Oh, and yeah...the manpage says:        setgroups()  sets  the supplementary group IDs for the calling process.        Appropriate privileges (Linux: the CAP_SETGID capability) are required. So you do need to clean out the groupslist and gids first and only afterward set the uids. > > > > > > > > > > > Doing as you suggest causes problems as in it it fails to drop the > > > > supplementary groups. > > > hmm... what are you seeing that I'm not? > It was my bad. I decided to change the order of when the call is done > (ie., doing it after the setting the uid and gid) and that causes an > error. Perhaps it's becuase once the uid is changed to some user's > uid, then that uid no longer has the ability to drop the supplementary > groups. > > Going to send a new version of patches... > > > > > > > > > > > > steved. > > > > > > -- > > > To unsubscribe from this list: send the line "unsubscribe linux-nfs" > > > in > > > the body of a message to majordomo@vger.kernel.org > > > More majordomo info at  http://vger.kernel.org/majordomo-info.html > > -- > > Jeff Layton > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-nfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at  http://vger.kernel.org/majordomo-info.html -- Jeff Layton