Re: [RFC PATCH v2 2/3] gssd: using syscalls directly to change thread's identity

[Jeff Layton <jlayton@poochiereds.net>]

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 <kolga@netapp.com>
> > > > > > Reviewed-by: Steve Dickson <steved@redhat.com>
> > > > > > ---
> > > > > >  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.
> > > 
> > > --
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> > --
> > Jeff Layton <jlayton@poochiereds.net>
> > 
> --
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-- 
Jeff Layton <jlayton@poochiereds.net>